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Pan-European Gas-AeroSol-climate interaction Study

Final Report Summary - PEGASOS (Pan-European Gas-AeroSol-climate interaction Study)

Executive Summary:
The climate of the Earth is changing and these changes are predominantly caused by anthropogenic emissions of carbon dioxide and other greenhouse gases and aerosol particles. At the same time, air quality has become an issue of global concern as it is estimated to cause more than 2 million premature deaths each year with fine particles and elevated ozone responsible for most of the adverse health effects. Climate change and air pollution are closely linked to the socioeconomic problem of controlling emissions of gaseous air pollutants and aerosols. The Pan-European Gas-AeroSOls-climate interaction Study (PEGASOS) European large scale integrating project brought together several of the leading European research groups, with state-of the-art observational and modeling facilities to:
(1) Quantify the magnitude of regional to global feedbacks between atmospheric chemistry and a changing climate and to reduce the corresponding uncertainty of the major ones.
(2) Identify mitigation strategies and policies to improve air quality while limiting their impact on climate change.

PEGASOS developed new anthropogenic and biogenic air pollutant emission inventories for the past and the future 50 years, discovered new processes linking the biosphere to the atmosphere and climate, performed laboratory studies and field studies using for the first time a Zeppelin platform, continued the development of chemical transport and climate models, and performed policy analysis. Its legacy includes new emission and measurement databases available to the global scientific community, improved tools for the study of air quality and climate interactions, and new insights about strategies that can improve air quality while minimizing climate change.

Some of its major conclusions are:
- A variety of EU air quality policies since 1970 have avoided a dramatic deterioration of air quality in Europe. The EURO norms and fuel quality directives (sulfur) were among the most influential policies. These EU policies increased life expectancy of people in Europe by 5 months. The introduction of EURO standards led to reduction of worldwide ozone levels and a 0.3% increase of crop production.
- There is a range of available policies for further air quality improvement in Europe including some not so obvious options: reductions of ammonia, residential and agricultural biomass burning, etc. Their effect on climate will be limited.
- The global temperature response due to a range of air pollution emission changes will be in the range of +0.3 to -0.2 degrees C on the 20-40 years timescale. The air pollution trajectories in emerging economies will have larger effect on climate. In Europe the warming for global maximum reductions would be 0.5 degrees C. The aerosols effects are not necessarily in the regions of emissions. These impacts are relatively small compared to the effect of carbon dioxide and other greenhouse gases.
- On longer timescales the abatement of methane emissions will reduce global temperature by 0.2 degrees C and also reduce ozone.
- Air quality/climate policies co-benefits depend on type of pollutant: they are important for sulphur dioxide and oxides of nitrogen but less so for volatile organic compounds
- Climate and air pollution policies can reduce ozone concentrations globally
- The feedback from climate change on air quality will be heterogeneous across Europe, seasons, and type of pollution. There will be increases in ozone but also both increases and decreases of PM. There can be potentially significant increases of wildfire emissions in certain regions of Europe. The increased biogenic emissions due to increased temperature will increase ozone in polluted areas. However, ambitious emission controls will have larger impacts than climate change.


Project Context and Objectives:
A rapidly emerging challenge for society is to tackle air pollution and climate change in a common policy framework. Many substances present in our atmosphere due to human activities play a dual role as climate change agents and as air pollutants. However, highly non-linear interactions of anthropogenic emissions with chemical reaction pathways, biosphere-atmosphere exchanges, climate, and pollutant transport make predictions of how this complex system will respond to changes in anthropogenic sources very difficult. A change of a single component can lead to significant and non-linear changes in others and the resulting feedbacks are critical to the behavior of the system as a whole.

There is little doubt that the climate of the Earth is changing and that these changes are predominantly caused by anthropogenic emissions of carbon dioxide and other greenhouse gases and aerosol particles. At the same time, air quality has become an issue of global concern as it is estimated to cause more than 2 million premature deaths each year with fine particles and elevated ozone responsible for most of the adverse health effects. Climate change and air pollution are thus closely linked to the socioeconomic problem of controlling pollutant emissions. The atmosphere is a complex and dynamic system in which trace gases and aerosols play an important role through radiative scattering and absorption, formation of cloud condensation nuclei and chemical reactions transforming one pollutant into another. Furthermore, there are numerous interactions between the atmospheric composition and the biogeochemistry of terrestrial and marine ecosystems as well as physical exchanges related to albedo or surface cover changes, and the surface energy balance.

The Pan-European Gas-AeroSOls-climate interaction Study (PEGASOS) European large scale integrating project has brought together most of the leading European research groups, with state-of-the-art observational and modeling facilities to:
(1) Quantify the magnitude of feedbacks between atmospheric chemistry and a changing climate and to reduce the corresponding uncertainty of the major ones.
(2) Identify mitigation strategies and policies to improve air quality while limiting climate change.

The project has been organized around four science elements:
Theme I: Anthropogenic and biogenic emissions and their response to climate and socio-economy.
Theme II: Atmospheric interactions of chemical and physical processes.
Theme III: Regional and global links between air pollution and climate change.
Theme IV: Air quality in a changing climate: Integration with policy.

PEGASOS has bridged the spatial and temporal scales that connect local surface-air pollutant exchanges, air quality and weather with global atmospheric chemistry and climate. Our major focus for air quality is Europe including effects of changes in pollutant emissions elsewhere and the time horizon for the study has been the next 50 years. During the project we have provided improved process understanding in areas of major uncertainty for better quantification of feedbacks between air quality and a changing climate. We have presented a fully integrated analysis of dynamically changing emissions and deposition, their link to tropospheric chemical interactions and interactions with climate, and emerging feedbacks between chemistry-climate and surface processes. We have targeted both local and regional scales, taking into account chemistry and climate feedbacks on the global scale.

To reach its objectives PEGASOS has addressed the following scientific questions:
(Q1) How has past air quality policy inadvertently affected present day climate and, the corollary, how has climate change over the last decades affected Europe’s ability to meet its air quality targets for ozone, particulate matter, etc.? How will currently planned air quality regulations affect climate?
(Q2) How will emissions (including methane, biomass burning emissions, biogenic hydrocarbons, etc.) respond to a changing climate, shifts in biomes, and land cover/land use changes and what will be the effect of these changes on European air quality (ozone, particulate matter, etc.) and climate?
(Q3) How will climate change affect the atmospheric self-cleansing capacity (hydrogen oxide radicals, HOx radicals budget and cycling), atmospheric aerosol concentrations (both number and mass) and how will this in turn feed back to climate? How will climate change affect the regional accumulation of pollutants (including aerosols) and the resulting air quality and its regulation in Europe on both regional and urban scales?
(Q4) What are the main missing processes in current air quality-climate models and how can these tools be improved for the simulation of multi-scale chemistry-climate interactions including local changes?
(Q5) Which policy-relevant metrics should be used to facilitate the consideration of short-lived species in international treaties dealing with climate-relevant compound regulations and the assessment of air quality and climate policies co-benefits or other interactions?

In order to answer the above questions and achieve our main objectives we planned to perform the following tasks, which combine targeted observations, improvement of model representation, and simulation experiments:
(T1) Develop state-of-the-art emission and exchange models for natural, agricultural, biomass burning, and novel anthropogenic emission inventories of both gas- and particulate-phase pollutants. Use these models and inventories to assess air quality and climate changes for a range of possible future conditions (air quality policy only scenarios, climate policy only scenarios, and combined scenarios).
(T2) Perform field and laboratory experiments of the photo-oxidation of mixtures of anthropogenic and biogenic volatile organic compounds, ammonia and amines to improve our understanding of the corresponding mechanisms focusing on the hydroxyl radical and secondary aerosol formation and aging.
(T3) Characterize simultaneously the complex gas-phase photochemistry and its interactions with aerosol formation and chemical aging over Europe using the combination of a Zeppelin platform with mobile labs and ground monitoring stations.
(T4) Apply and develop a hierarchy of meteorological, climate and chemical transport models that include the relevant processes to perform multi-scale simulations of the air quality-climate system.
(T5) Evaluate these models using data collected in the third Task and by simulating the gas and aerosol concentrations during the last few decades using improved inventories of anthropogenic emissions and reconstructed concentrations from firn air and ice cores, as well as historical changes of land use and land cover.
(T6) Perform a series of sensitivity simulations with the PEGASOS models to identify and quantify the interactions and feedbacks between air quality, climate, and natural emissions at various spatio-temporal scales.
(T7) Investigate the future changes in air quality and climate for the PEGASOS future scenarios using the improved models.
(T8) Identify emission projections that optimise the benefits to air quality while minimising effects on climate and communicated the results to the various stakeholders.


Project Results:
PEGASOS was organized around four science elements that include a total of 17 scientific work packages:
(1) Theme I focused on anthropogenic and biogenic emissions and their response to climate and socio-economy. We assessed the changes of biogenic and anthropogenic emissions that are relevant to climate change and air pollution interactions, and the corresponding processes. We also examined the emission/removal-related feedbacks on the state of the atmosphere and ecosystem processes. The new state-of-the-art emission models and emission inventories served as input to the simulations performed in the rest of the project and for interpretation of the PEGASOS campaign measurements.
(2) Theme II addressed the major uncertainties in atmospheric gas-phase (mechanism of non-classical HOx recycling) and aerosol chemistry and physics (formation and aging of organic aerosol and soot, nucleation and growth of fresh particles) related to the air quality-climate change interactions by performing laboratory and field measurements. The laboratory measurements took place in some of the premier European smog chamber facilities (SAPHIR, PSI chamber, Jülich plant chamber), used all the expertise of the PEGASOS consortium and resulted in new parameterizations of HOx production and/or cycling, secondary organic aerosol formation and chemical aging. The field measurements combined airborne measurements in the planetary boundary layer and above it using a Zeppelin platform, a mobile laboratory moving together with the Zeppelin and several ground stations. This novel field measurement approach provided for the first time detailed measurements of HOx, gas-phase pollutants and aerosols throughout the planetary boundary layer over Europe. The observational strategy of PEGASOS complemented information provided by the EUCAARI FP7 project and existing ground-based networks (EUSAAR, EARLINET, EMEP) by focussing on the oxidant/aerosol interactions which have not been addressed in previous projects.
(3) The emission models and inventories produced in Theme I together with the process parameterizations developed in Theme II were used by a comprehensive suite of models in Theme III to quantify the magnitude of feedbacks between atmospheric chemistry and a changing climate (Objective 1 of PEGASOS). The models that were used covered the full range from state-of-the-science zero- and one-dimensional atmospheric chemistry models to Earth System Models. These models were improved based on the findings of the previous Themes, evaluated against PEGASOS and past measurements, intercompared, and then were used in a series of carefully designed studies to elucidate the importance of the various feedbacks. PEGASOS modelling teams have strongly capitalized from current and past EU projects. The database obtained in the EUCAARI project and in particular data from 2008-2009 intensive campaigns in association with EMEP and EUSAAR were used to serve the objectives of Theme III.
(4) The same models were used in Theme IV to identify strategies and policies that improve air quality while limiting climate change (Objective 2 of PEGASOS) and to disseminate these results to the various stakeholders.

The work performed and the major conclusions reached are summarized in the following sections corresponding to the PEGASOS Work packages.

1. TERRESTRIAL TRACE GAS EMISSIONS

1.1 Land-use change effects on emissions of biogenic volatile organic compounds
Isoprene and monoterpenes are emitted by vegetation in large quantities and have a large impact on air quality because they are important precursors for ozone and particle formation. Their emissions from leaves increase strongly with warmer temperatures and for isoprene also with increasing light. Leaves in plants that are grown in above-ambient carbon dioxide concentration emit less isoprene, while the effect of carbon dioxide on emissions of monoterpenes is less certain. In PEGASOS, one of the foci of analysis was the additional aspects of human activities on emissions of biogenic volatile organic compounds. Conversion of natural vegetation into cropland and pastures is expected to decrease isoprene and monoterpene emissions substantially, especially if this conversion is through deforestation. Woody vegetation typically has a much larger emission potential than herbaceous or crop vegetation. Further, some species, like oil palm and fast growing tree species (poplar and willow) used in short-rotation coppice, have very high biogenic volatile organic compound emissions, exceeding those of natural forests. We estimated that taking into account land-use changes over the 20th century natural vegetation isoprene emissions declined. This difference was much smaller for monoterpenes, with only a reduction of 2-3 TgC per over the 20th century. Isoprene emissions are predicted to stabilize in the 21st century and to follow a similar trend as the natural vegetation. Our results suggest that land-use effects will be a dominant factor in the change of biogenic emissions in the future. Figures and associated discussion of the spatial emission patterns are provided in PEGASOS Deliverable 2.4.

1.2 Simulation of future fire emissions and interactions with human population growth
Wildfires are an integral component of ecosystems, but they also represent often a risk for human societies. Understanding how fire patterns might change in future is paramount to diverse questions ranging from health to adaptation of fire-management strategies and urban development, to carbon cycle- climate feedbacks and climate policy.

Based on a new fire model developed in PEGASOS, simulated burned area over the 20th century differed substantially depending on the factors taken into consideration. Mean values ranged from 2.7 to 3.4 million square kilometers per year at the beginning of the 20th century. Neither climate change nor increasing levels of atmospheric carbon dioxide introduced a historical trend in simulated fire frequency over the past century. The main driver of change was increasing population density, leading to declining burned area. In contrast to the past century, the climate change signal becomes very pronounced over the 21st century and a clear increase in burned area is predicted due climate change alone. After adding effects of atmospheric carbon dioxide on vegetation growth to the simulations, the effects of climate change were greatly reduced with simulated burned areas at the end of the 21st century being near or only slightly above present-day levels. Shrub encroachment is predicted to occur in the savanna biome due to enhanced levels of carbon dioxide. As savannas represent a large fraction of total global burned area, fire spread is reduced as shrubs become more dominant at the expense of grass cover. When climate and carbon changes are combined with the future population and urbanization predictions, fire frequency is predicted to decline until the mid-21st century and then to remain approximately constant or to continue declining for some scenarios.

The associated fire emissions in tropical regions are predicted to decline in all land use/ urbanization scenarios considered at the end of the 21st century compared to present-day values. By contrast, in extra-tropical regions, emissions slightly increase. These results confirm the importance of accounting for climate change effects, atmospheric carbon dioxide levels and population conjointly for projections of future fire effects and fire emissions. Details can be found in Knorr et al. (2014).

1.3 Nitrous acid emissions from soil bacteria
Soil biota contributes an important, but poorly understood component of air-quality and climate relevant precursors. There has been little information on the magnitude (and even direction) of nitrous acid exchange with soils and vegetation. In PEGASOS, the contribution of soil nitrous acid emissions to the total reactive nitrogen flux was estimated for various ecosystems around the world, covering a wide range of soil pH, organic matter, and soil nutrient contents. Potential nitrous acid soil emission hot spots include large areas of northern Africa, central/southwestern Asia, and North America as well as some regions around the Mediterranean Sea, covering in total around 20% of the terrestrial surface. This component of the nitrogen cycle constitutes an additional loss term for fixed nitrogen in soils and an important source for reactive nitrogen in the atmosphere. Soil nitrite has emerged as an important, previously unknown, source of nitrous acid, which is estimated to be responsible for about 30% of the hydroxyl radical production (Su et al., 2011). Moreover, cryptogamic covers have been identified as important for the biogeochemical cycle, atmospheric chemistry and climate change by fixing an estimated 3.9 PgC per year (about 7% of global terrestrial net primary production) and accounting for about half of the biological nitrogen fixation on land (Elbert et al., 2012). Details about this important PEGASOS discovery can be found in Oswald et al. (2013) in Science.

2. PAST ANTHROPOGENIC EMISSIONS
Anthropogenic and biomass burning emission inventories covering the period 1960-2010 were developed for use in the hindcast PEGASOS simulations. The so-called MACCity dataset was extended to cover the full period under study, providing the different chemical species considered in the simulations. The evolution of biogenic emissions and their dependency on different factors such as land-use changes was investigated (Sindelarova et al., 2014).

Different issues concerning the changes in anthropogenic emissions were highlighted during PEGASOS, particularly in Asia, where emissions have increased significantly during the past few years. The most recent data on surface emissions were collected, in order to analyze their trends in different emissions datasets. For this analysis, we used the emissions provided by the MACCity inventory (further developed during the first years of PEGASOS), as well as several recently published datasets: ECLIPSE-v4 and ECLIPSE-v5, REAS-v2, HTAP-v2, MEIC, and a few older emissions datasets. The trends in carbon monoxide (CO) emissions among the inventories are consistent, with an increase since the 1980s. In the most recent years, i.e. since 2008, some inventories, i.e. the MEIC and HTAP-v2 datasets, show a stabilization of emissions in China. It should however be noted that HTAPv2 used the MEIC emissions as a basis for its China inventory. The other inventories, i.e. MACCity, ECLIPSE-v4 and ECLIPSE-v5 show a continuous increase in the Chinese emissions. The use of increasing emissions in China in global model simulations leads to an increase in the CO column in this region, which is inconsistent with the observations from the MOPITT satellite. ECLIPSE-v5 provides significantly higher emissions over China (25% more) than MACCity, which are used in the PEGASOS simulations. As a result the anthropogenic emissions in China used in PEGASOS have been reevaluated.

All nitrogen oxide (NOx) anthropogenic emissions show a large increase of a factor of 5-6 from 1980 to 2010. All datasets provide rather similar emissions in 2000. In 2010, the MACCity, ECLIPSE-v4 and ECLIPSE-v5 emissions are quite similar, but the NOx emissions from the REAS-v2, MEIC and HTAPv2 are about 25% higher.

This comparison of different datasets providing anthropogenic emissions has also been performed for Western Europe, Central Europe, the USA, South America, Africa, and South-East Asia. This work serves as a basis for the development of a new historical emissions dataset, which will be used as input for CMIP6 simulations for the next IPCC report, AR6.

3. FUTURE ANTHROPOGENIC EMISSIONS RELATED TO AIR POLLUTION AND CLIMATE
There are several important relationships between air pollution and climate change. Air pollutants often originate from the same economic activities as greenhouse gases, e.g. combustion of fossil fuels. This means that air pollution abatement activities may lead to important co-benefits for climate. Many air pollutants also contribute to radiative forcing, either positively (e.g. warming by black carbon and methane) or negatively (e.g. as cooling sulphur aerosols). Climate change can lead to changes in the concentrations of air pollutants through changes in emissions, circulation, formation and removal mechanisms influenced by meteorological factors. Finally, air pollution can influence the functioning of natural systems and agriculture, impacting among others the carbon and nitrogen cycles. These linkages can lead to both benefits and trade-offs in reduction strategies in terms of air pollution of the costs of achieving different policy targets.

Scenarios provide a very useful tool to potential development under different assumptions – but also to look specifically into the possible relationships between climate and air pollution policies. Traditionally, such scenarios focused on specific regions (Europe, Asia, North America). Recently, however, the interest in global air pollution scenarios has significantly increased, given the growing evidence that emissions in various world regions can influence the background concentrations in other regions. The existing global climate scenarios (Representative Concentration Pathways, RCPs), however, only looked into a very limited set of possible trajectories. Therefore, as part of the PEGASOS project, a new set of scenarios was developed using the IMAGE integrated assessment model. For the historical period, these scenarios were made consistent with the EDGAR emission database. For the future, the scenarios combined assumptions on future climate policies following the so-called RCPs with assumptions on future air pollution policies.

The resulting scenarios explore a wide range of future air pollution trajectories, based on different assumptions on climate and air pollution control policy. The scenario results emphasize the importance of co-benefits between climate policy and reduction of air pollutant emissions. The implementation of climate mitigation policies is highly relevant for air pollution control and results in substantial co-benefits for sulphur dioxide and oxides of nitrogen compared to the scenarios with no climate policies. In the absence of climate policies the world would be still highly affected by sulphur dioxide and nitrogen oxide emissions in the middle and even in the end of the 21st century. Even with the most tight air pollution policies in place, without climate policy air pollutant emissions would still be above or equal to the levels of a mitigation scenario with current air pollution measures implemented, meaning a further deterioration of air quality in regions and countries already suffering from high pollution levels.

PEGASOS results also show that policies that go beyond the current legislation scenario are necessary to avoid an increase in air pollutant emissions after 2030. This can be implemented in the form of increasingly stringent air pollution policies, by the introduction of ambitious climate policies or both. To reach ambitious air pollution control targets during the century, strict air pollution control policies are required also with effective greenhouse gas mitigation policies in place. Extending air pollution policies to land-use change and including methane can further improve effectiveness in air pollution control scenarios. The results are described in detail by Braspenning-Radu et al. (2015).

4. LINKING OBSERVATIONS AND LARGE-SCALE MODEL SIMULATIONS
Novel features of the observations of PEGASOS included the availability of detailed reactive compound and aerosol information aloft as a function of height, together with ground measurements covering a number of areas with different characteristics (e.g. industrial pollution, agricultural sources, clean environment with biogenic emissions, etc.). The observations collected by the Zeppelin campaigns have provided valuable information on meteorological and atmospheric composition that covers larger spatial scales than the typical field site observations. However, there is still a major challenge involved in linking such observations to the simulations with regional and global scale chemical transport models.

Two different model systems, deployed in PEGASOS, have been used to link the observations footprint to the scale of the CTM simulations:
(a) a Single Column Model version of the coupled chemistry-climate model ECHAM4 (hereafter referred to as SCM) and
(b) the 1-D Lagrangian transport model PMCAMx-Trj that includes detailed organic and inorganic aerosol modules.
Both models have been used to simulate air quality during southern and northern PEGASOS field campaigns.

Simulations with the SCM assessed the impact of considering the CTM sub-grid scale heterogeneity in land cover and land use on reactive compound concentrations. These simulations revealed a complex response of the system as a function of the sub-grid scale since not only the natural emissions change but also dry deposition, chemistry and boundary layer dynamics change due to changes in micro-meteorology. Especially in the morning, when most of the observations with the Zeppelin were collected, there are large differences (> 50%) especially for hydroxyl radical (OH). On the other hand, later in the day the combined effect of changes in these processes and their interactions result in relatively small simulated daytime changes in the levels of the nitrogen oxides, ozone, OH and isoprene. Differences in ozone are generally very small and mainly reflecting small differences in simulated deposition and turbulent transport. These modelling experiments indicate that differences between the model and the observations might be partly due to the sub-grid scale landscape heterogeneity in biogeophysical properties and exchange regime that has been ignored.

The Lagrangian experiments with PMCAMx-Trj mainly aimed to evaluate the simulations of organic aerosol by comparison with the detailed aerosol observations collected at the PEGASOS field sites. The model was successful in reproducing the average OA concentration and its diurnal profile in the Po Valley suggesting the importance of anthropogenic intermediate volatility compounds as a secondary organic aerosol source. The simulations for the Hyytiala 2013 measurements showed the same satisfactory results but the biogenic vapors were the dominant precursors. The above results suggest that the simple parameterization of the OA chemical aging process currently used in PMCAMx-Trj is consistent with the available measurements from PEGASOS. While the actual processes are without any doubt a lot more complex, this parameterization appears to capture the net average chemical/volatility change of the OA.

The insights gained from the above high spatial resolution simulations have been used in the evaluation of the lower resolution three dimensional chemical transport models used in the rest of the project.

5. QUANTIFYING AND PARAMETERIZING ATMOSPHERE-BIOSPHERE FEEDBACKS
The biosphere-atmosphere-climate interactions play an important role in the behavior of the overall Earth system. A review (PEGASOS Deliverable 5.1) confirmed that the current parameterisations of the deposition process in the PEGASOS models were mostly static, with little response to climate or atmospheric composition. Improved parameterisations were developed and tested in a number of regional models increasing the corresponding sensitivity of the predicted responses to climate.

Emissions of ammonia, an important nitrogen compound and aerosol precursor, are climate dependent and should thus be modelled dynamically within chemical transport models rather than be prescribed in emissions inventories that use static emission factors for different agricultural activities. Our estimates suggest that temperature rise according to the RCP 8.5 scenario would increase ammonia emissions by a further 30% on top of the anticipated increase associated with agricultural intensification (Sutton et al., 2013).

A multi-model study predicts that changes in nitrogen emissions are the main driver for future changes in nitrogen deposition. The anticipated climate induced increase in ammonia emissions is forecast to offset much of the gains in critical loads exceedance reductions from emission abatement for oxides of nitrogen. The lack of sulphur and oxidised nitrogen in the future European atmosphere will lead to more reduced nitrogen to be present as ammonia and reduce transport distances (Simpson et al., 2014). Recent measurements suggest that ammonium nitrate aerosol evaporates during the deposition process, effectively increasing the deposition rate of fine nitrate by more than an order of magnitude. Quantification of this effect in a chemistry and climate model suggests that evaporation reduces European ground-level concentrations of fine nitrate on average by 30% and increases nitrate deposition to more than 1 kg N per hectare locally.

Measurements at the coupled plant-smog chamber at Julich showed that biotic stress (e.g. insect attack) favours emissions of volatile organic compounds with a particularly high yield of secondary organic aerosol, such as methyl salicylate. This may form an important route via which increased stress in a warming climate may result in increased secondary organic loading (Bergström et al., 2014).

A sensitivity analysis using a global climate/chemistry model demonstrated that uncertainties in parameterising the non-stomatal uptake route of ozone alone (using different published algorithms) induces an uncertainty in the predicted ozone concentration of 20%, which is similar to differences between different chemical schemes.

Using a model of the boundary layer several biosphere-climate-chemistry interactions were simulated. The results suggest that the increase in surface temperature will result in an increase in boundary layer height which in turn will increase entrainment of ozone at the top of the boundary layer. At the same time an increase in carbon dioxide will reduce the terrestrial ozone sink by reducing stomatal closure. This effect could enhance ozone levels by 5 – 10 ppb or 25%. In addition, the surface temperature increase will stimulate nitrogen monoxide emissions from soils, which will, however, be more than compensated by increased recapture by overlying vegetation as nitrogen dioxide, partly due to the increased ozone burden.

6. LABORATORY STUDIES
A series of fundamental parameters were investigated during PEGASOS, focusing on the emissions of biogenic volatile organic compounds (BVOCs) and their processing to form secondary organic aerosols (SOA) or radicals. All this information is important for the numerical simulation of air quality-climate interactions at all scales. A series of significant advancements were made especially on the biogenic volatile organic compound emissions patterns under stress and the cycling of HOx radicals (both from homogeneous gas-phase and surface processes).

Formation of SOA from OH initiated oxidation of biotic stress induced BVOC emissions was investigated in detail. Yields of SOA mass formation from sesquiterpenes, methyl salicylate, and C17 alkenes were determined. These emissions were induced by biotic stresses. SOA yields were found to be in the range between 20 and 30%; much higher than those of constitutively emitted monoterpenes. In contrast, emissions of green leaf volatiles suppressed SOA formation most probably due to the suppression of OH concentrations.

The OH-radical induced oxidation of isoprene and the influence of relative humidity and of the pH value of the primary particles on the SOA formation were investigated in a series of chamber experiments. In all the experiments conducted, SOA formation was observed corresponding to SOA yields between 1.3% and 3.8%. SOA yields increased with decreasing pH value of the sulfate seed. In contrast, SOA yields increased with increasing relative humidity. A 10% increase in relative humidity resulted in about 0.1% increase in the SOA yields. The highest SOA yield of 3.8% was observed for highly acidic particles at a relative humidity of 80%.

The recycling of HOx radicals in the photochemical oxidation of isoprene and selected monoterpenes was investigated in the atmospheric simulation chamber SAPHIR. Experiments gave clear evidence for a so far unknown OH recycling pathway in the photochemical oxidation of isoprene suggesting that unimolecular reactions of RO2 radicals from isoprene play a role in the atmosphere.

Following up on previous investigations on photochemically driven particle phase redox processes as a source of radicals and their precursors, photosensitized processes were shown to produce HO2 radicals in citric acid films doped with imidazole-carboxaldehyde, a light-absorbing product of heterogeneous chemistry of glyoxal in ammonium sulphate. A new mechanism based on photosensitized particulate-phase chemistry was suggested.

The radical induced formation of organosulfates as source for aqueous phase SOA production was investigated for methacrolein and methyl vinyl ketone, two main isoprene oxidation products. The results were evaluated against ambient PM10 measurements collected at a rural East German village during the summer 2008.

7. AIRBORNE FIELD STUDIES
The objective of this component of PEGASOS was to explore horizontal and vertical gradients of trace gases and aerosols in the planetary boundary layer in relation to the main atmospheric oxidant, OH radicals. The HOx cycle was explored by direct measurement of HOx concentrations, OH reactivity and (organic) trace gases, with the goal to assess the recently discovered “non-classical OH recycling” over Europe. The objective was achieved by implementing a new research tool, the Zeppelin NT airship, equipped with a top platform carrying the radical measurements and three layouts focusing on radical balance, aerosol formation and properties, and new particle formation respectively. With these layouts on board the Zeppelin NT three campaigns were successfully performed in different parts of Europe: Cabauw, (Netherlands, 25 flight hours), Po-Valley (Italy, 120 flight hours), and Hyytiälä (Finland, 90 flight hours), in order to explore a variety of pollution regimes. The transfer flights of the Zeppelin (24 flight hours to the Netherlands, 40 hours to the Po-Valley and 60 hours to Finland) were also used for measurements of the radical balance, trace gas and aerosol gradients across Europe. A total of about 360 flight hours led to an extensive data set, which is now available for the whole scientific community in the PEGASOS data base.

The major conclusion of the PEGASOS campaigns is that our understanding of atmospheric chemistry in the planetary boundary layer has been strongly biased by the fact that most air pollution measurements have been performed in the surface layer. The Zeppelin missions showed a complex, detailed coupling of dynamics and chemistry in the morning and evening hours. This affected gradients of HOx radicals, new particle formation, and aerosol composition and hygroscopic properties. Moreover, the concentration of reactive organic trace gases above the surface layer was lower than expected, with the consequence that the regime for non-classical HOx recycling - low NOx and high VOC - was not encountered during the missions. OH reactivity in higher layers was mostly determined by oxidized (products of the atmospheric oxidation cycle) and not so much primary VOCs.

Regional new particle formation events were observed to start simultaneously at different altitudes and evolved uniformly inside the mixed boundary layer. Newly formed sub-3 nanometer particles were not observed outside the planetary boundary layer. Sometimes local components to regional new particle formation event bursts were detected.

Vertical profiles and horizontal transect flights in the Po-Valley show that aerosol sulphate is formed on regional scales and transported to the measurement sites while nitrate has clearly only local sources. The organic particulate matter component was characterized by both, long-range transport and local production. In the Netherlands large excess of ammonia was found in sub-micron aerosols, likely bound to organic acids, thus affecting the ammonia budget.

Nitrous acid (HONO) is an important source of OH radicals but its sources have been uncertain. Balancing the HONO budget based on measurements on board the Zeppelin NT indicated a strong source of HONO outside the surface layer. This source was found to be “internal”, i.e. formed from HOx and NOx. This result diminishes the importance of the measured HONO as a radical source.

The PEGASOS campaigns with the Zeppelin NT were a great success. The results are leading to new insights in atmospheric oxidation, new particle formation, and secondary aerosol formation and their couplings to the inherent dynamic of the planetary boundary layer.

8. GROUND-BASED FIELD STUDIES
A number of ground-based intensive campaigns were performed during Zeppelin missions to link the airborne observations with comprehensive observations at the ground. The data analysis from the Po Valley Field Campaign 2012 compared data from stationary sites in Bologna (urban background), San Pietro Capofiume (SCP, rural background in the central valley), Monte Cimone (rural background on the crest of the Apennines) as well as from a mobile platform (the Mosquita mobile lab of Paul Scherer Institute). The observed changes of organic particulate matter during the 2012 campaign could be explained after an in-depth analysis of the results from multiple spectrometric techniques. The continuous particle size distribution measurements revealed that on twenty-eight out of the thirty measurement days new particle formation was observed. A series of isoprene oxidation products were detected both in the gas- and particle samples. Most of these compounds showed higher concentrations during the daytime, indicating that photochemistry serves as the driving force for their formation. The diurnal patterns of glyoxal and methyl-glyoxal, on the other hand, were different from those of other detected SOA precursor compounds, suggesting the presence of different sources and/or loss mechanisms such as photolysis.

In support of the Zeppelin measurements, an intensive spring campaign was performed on the ground at the SMEAR II station, Hyytiälä, Finland from 2 April to 30 June 2013. The major goal was to have a detailed look into the chemical and physical properties of aerosols during new particle formation events. During the 40-day campaign, clear regional new particle formation events occurring during several hours were observed on 11 days in Hyytiälä. Mass spectrometers were used to measure in-situ atmospheric ions and molecules, clusters, and small particles. Besides the oxidation of sulphur dioxide to sulphuric acid and the oxidation of volatile organic compounds to extremely low volatility organic compounds, the importance of stabilized Criegee Intermediates in atmospheric oxidation was highlighted. We were able to quantify their concentrations and production rate as well as estimate their lifetime during this campaign.

Data from the suburbs of two major Greek cities, Patras and Athens, for summer 2012 were also analyzed. The concentration and chemical composition of the non-refractory fine particulate matter and black carbon levels were measured in an effort to better understand the chemical processing of particles in the high photochemical activity environment of the Eastern Mediterranean. The composition of fine particulate matter was surprisingly similar in both areas demonstrating the importance of regional sources for the corresponding pollution levels. Through the use of positive matrix factorization five organic sources could be identified for Patras: 19% very oxygenated OA, 38% moderately oxygenated OA, 21% biogenic oxygenated OA, 7% hydrocarbon-like OA associated with traffic sources and 15% hydrocarbon-like OA related to other primary emissions (including cooking). For Athens the corresponding source contributions were: very oxygenated OA (35%), moderately oxygenated OA (30%), hydrocarbon-like OA associated with traffic sources (18%) and hydrocarbon-like OA related to other primary emissions (17%). In both cities the major component was oxygenated organic aerosol, suggesting that under high photochemical conditions most of the organic aerosol in the Eastern Mediterranean is quite aged. The contribution of the primary sources was important (22% in Patras and 33% in Athens) but not dominant.

9. INTEGRATION AND PARAMETERIZATION OF EXPERIMENTAL RESULTS
9.1 The PEGASOS database
The PEGASOS database design was performed in several steps during the project. The database presently contains approximately 1 Gb of data, with over one thousand files and 753 data objects in the metadata catalogue. There are currently approximately 100 registered users. The produced data sets are available to the whole scientific community. To ensure future availability of the data, the PEGASOS project has agreed to transfer the data sets to permanent storage within the NILU EBAS data system (ebas.nilu.no) in Norway, which already contains many similar aerosol and air quality datasets. The data system has been generated, and the overall user interface has been functional and used in the project. Almost all produced observational datasets are in the system. Overall, the PEGASOS database has been proven to be a successful and useful tool for data sharing within the project.

9.2 Parameterizations of experimental results
The potential impact on organic aerosol formation from biotic stress-induced emissions of organic molecules from forests in Europe has been investigated. The estimated stress-induced emissions in Central and Northern European forests are found to contribute substantially to SOA in large parts of Europe. The emissions of unsaturated C17-BVOC from insect infested vegetation, although episodic and regional, can have a large impact on SOA formation. Our findings suggest that the stress-induced emissions and corresponding SOA are important in large parts of Europe. The inspections of European forests suggest that completely non-infested plants are uncommon and thus some stress is the normal state of vegetation. Including these stress-induced emissions in air quality and modeling study is necessary.

The effects of oxidation of directly tree emitted BVOCs on new particle formation based on measurements at the Jülich Plant Atmosphere Chamber were theoretically investigated. The MALTE model results were in good agreement with chamber observations both for gas phase composition and particle number size evolution. Simulations support the hypothesis that sulphuric acid is one of the critical compounds in nucleation processes. Parameterizations of nucleation involving the oxidation products of BVOCs showed improved agreement with measurements. Results also show that to better reproduce the growth of aerosol particles via condensation, it is necessary to include all condensable vapor precursors with their appropriate saturation vapor pressures.

MODIS Aerosol Optical Depth values during the months of the Zeppelin campaigns were consistent with past observations. This supports that the aerosol conditions during the field campaign were representative of the average conditions in the areas of interest for each campaign.

10. MODEL DEVELOPMENT AND IMPROVEMENT
The PEGASOS regional and global models were refined so that they could be optimally set-up to quantify the relevant AQ-climate interactions. Both gas and aerosol chemistry schemes were improved and evaluated to increase the ability of CTMs to simulate OA formation and chemical aging. The developments and refinements of PEGASOS models focused on the following aspects:
• The gas phase and heterogeneous chemical mechanisms were improved, with a focus on biogenic gases (including isoprene and terpenes) that affect ozone and secondary aerosol and their interaction with anthropogenic pollutants (NOx, hydrocarbons, etc).
• Detailed comparisons of chemical mechanisms were performed using a newly developed one-dimensional chemistry-transport model which allows comparison of different chemistry schemes in a "realistic" setting, i.e. a developing convective boundary layer.
• A reduced multiphase chemistry scheme, including HONO chemistry, suitable for use in global models was developed.
• The two-dimensional volatility basis set (2D-VBS) used for the simulation of OA in CTMs was further developed by introducing more detailed functionalization and fragmentation schemes and by incorporating additional pathways for OA formation (e.g. heterogeneous hydroxyl uptake and aqueous-phase processing).
• Some of the PEGASOS models were further developed as to include missing processes potentially relevant for the investigation of air quality-climate feedbacks such as for example, the coupling between aerosol and gas-phase chemistry, the links between natural emissions and land component, the inclusion of on-line emission fluxes of dimethylsulphide, etc.

11. ASSESSMENT OF MODEL SKILL
The skills of PEGASOS models used for data analysis and for future projections were evaluated by comparison with observations both in term of short-term variability and over longer term periods (e.g. several decades).

In particular, PEGASOS models were evaluated focusing on their ability to reproduce the observed short term variability of trace gases and aerosols. Evaluations have been performed for the years 2008-2009 for global and regional models. Predictions of regional models have been also compared with the new observations from the PEGASOS campaigns. Comparisons were performed as appropriate and tailored to each model type to determine the level of confidence to the model simulations.

The ability of the models to simulate the trends and long-term variability in trace gases and aerosols (e.g. changes in ozone and methane concentrations; changes in sulphate loading, brightening and dimming) has been also tested. Long term observations of ozone, carbon monoxide and aerosols (including individual components) have been compiled and used for the evaluation. This task contributes to the AC&C international project, through a study of the interactions between chemistry and climate during the past three decades.

Comparisons between model results and observations have shown that:
• Best model performances for aerosols are found during spring and worse during the winter period. Similarly, near the surface global model simulations of ozone show stronger seasonal pattern than observed at several studied locations with higher than observed maxima in summer and lower than observed minima in winter.
• Biomass burning OA emissions over Europe are most likely underestimated. At specific locations biomass burning emissions show stronger seasonality than observations.
• Model grid resolution shows modest impact on the results for summer and more significant for winter simulations. Larger differences are expected due to the interpolation of primary emissions.
• The simulation of the conversion of nitric acid vapor to nitrate aerosol has been also identified as an important contributor to model uncertainty. A hybrid that combines the dynamic calculation of mass transfer to coarse-mode particles while maintaining computational efficiency by assuming that the fine mode particles are in equilibrium has improved simulations.

The multiyear hindcast simulations (since 1960 or 1980 until 2005 or 2010) performed by the PEGASOS models show that:
• Several hindcast PEGASOS simulations of CO over the past few decades have shown a general underestimation of CO in the northern mid-latitudes. This underestimation of CO is a common feature in current global models and needs to be analyzed further. The CO trends shown by the models are consistent with the observations.
• Long-term changes in ozone precursors (for example CO and NO2) seen by satellites can show large differences and deserve further investigation.
• European regional aerosol mass concentrations are predicted to have declined throughout the hindcast period. The total aerosol number concentration has also declined over this period, in line with the changes in aerosol mass.
• The PEGASOS models reproduce the long term changes in sulphate mass on a European level relatively well when compared to observations although there are some discrepancies in earlier years. Sulphate seems to be represented by the model relatively well in summer but there is a low bias in winter.
• Secondary organic aerosol is calculated to increase globally, but does not show a clear trend among models over Europe.

12. QUANTIFICATION OF INTERACTIONS IN THE AIR QUALITY-CLIMATE SYSTEM
The drivers of Air Quality – Climate interactions were ranked through expert elicitation. The top process identified was the impact of climate change on biogenic VOC emissions, leading to changes in organic aerosol. Chemical and physical processes such as reaction rates and gas-to-particle partitioning were ranked as low to medium importance. Couplings were studied on the local, European, and global scales. Additional regional scale studies into aerosol-cloud-precipitation feedbacks were performed. The major findings included:
• An increase of temperature by 2 K leads to compensating effects: (i) faster chemical rates and enhanced VOC emissions lead to more PM2.5 (ii) partitioning of ammonium-nitrate shifts to the gas-phase leading to lower PM2.5. Overall, a decrease of average PM2.5 is found.
• Predicted ozone changes for 2100 are complex. The effect of enhanced VOC emissions on ozone depends critically on the NOx levels. A marked increase is calculated for southern Europe. Enhanced SOA formation is predicted, but the amount is small compared to potential effects of anthropogenic emission reductions.
• Aerosol-cloud-precipitation feedback studies with the REMO-HAM modelling system show that (i) during summer the inclusion of feedbacks leads to a lower cloud fraction, with as a consequence more precipitation and higher surface temperatures (ii) In winter, aerosol cloud-couplings lead to a higher cloud cover fraction and lower amounts of precipitation. In contrast, the Enviro-Hirlam simulations calculate a general increase in summertime cloud fractions, highlighting the remaining large uncertainties in aerosol-cloud-precipitation feedbacks.
• Concerning the indirect effect, a study with the GLOMAP model found that an air quality policy that reduced biofuel and biomass burning emissions would have a much smaller effect on climate than reductions in fossil fuel emissions or precursor gases.
• A study into the effect of the Pinatubo eruption on the methane growth rate calculated an important effect of reduced BVOC emissions after the eruption. These reduced emissions enhanced tropospheric OH concentrations and hence reduced the methane lifetime.
• A global ECHAM6-HAM2 simulation, including the effects of Anthropogenic Land Cover Change (ALCC) highlights the need to consider the effect of ALCC in climate change studies, because land cover strongly determines emissions of BVOCs, dust, and other climate active compounds.
• Local scale studies with large eddy simulation and mixed-layer models showed large effects of aerosols on the surface energy balance, and hence on the depth of the planetary boundary layer. Simulations with enhanced carbon dioxide levels show potentially large effects through higher water-use efficiency of vegetation and associated enhanced sensible heat fluxes.
• Global modeling studies showed that the North Atlantic Oscillation (NAO) has a significant contribution to the ozone and particulate matter levels over Europe. The increasing baseline ozone in western and northern Europe during the 1990s could be related to the prevailing positive phase of the NAO in that period. Also extreme NAO phases in the 1990s modulated most of the interannual variability of winter PM concentrations in several European countries.

13. HISTORICAL IMPACTS OF AIR QUALITY-CLIMATE INTERACTIONS
This historical assessment involved quantification of the extent to which climate change over the last 20-30 years has affected Europe’s ability to meet its air quality targets as well as the impact of European air quality legislation on climate.

Three types of simulations were conducted using global models:
1) simulations with constant pollutant emissions;
2) simulations with constant greenhouse gas concentrations; and
3) demography-based pollutant emissions which correspond to no-regulations policy.
The models used were the global climate model ECHAM6, the global aerosol-climate model ECHAM6-HAM2, and the global chemical transport model TM4-ECPL.

The main results can be summarized as follows:
• Air quality legislation over recent decades has resulted in significant reductions in CO, NOx, sulphate and particulate matter over Europe, and has helped to limit the growth of O3 concentrations. The bulk of the reductions have been achieved in the 1980-2000 time period, whereas the rate of improvement has slowed over the last decade.
• The impact of the air quality legislation is more apparent in the reduction of the levels of ozone precursors than for ozone itself.
• After 1980, the effectiveness of air quality legislation is more apparent if one compares a simulation using historical emissions with a simulation with constant emission rates per capita. These improvements far exceed air quality improvements calculated assuming constant emissions after 1980.
• Climate change, either experienced or committed to the Earth system to date, was found to have only small effects on the occurrence of stagnant air conditions favorable for the development of heavy pollution episodes. However, some regions may experience a degradation of the mean air quality due to reduced mean cloud cover and precipitation.
• On the global scale, the growing concentrations of greenhouse gases are the dominant climate forcing and will continue to be the primary agent of anthropogenic climate change. However, reductions in the emission of particulate matter over recent decades show significant influence on regional climate.
• Results from multi-decadal simulations using a fully-coupled earth system model with interactive aerosol indicate that the strongest climate signals resulting from anthropogenic particle emissions are found not in the source regions themselves, but in relatively clean areas further away.

14. MODEL PROJECTIONS OF AIR QUALITY AND CLIMATE CHANGE
PEGASOS used global and regional earth system, climate, and chemistry transport models to address the links between air quality policy, the change in biogenic and anthropogenic emissions and climate, to analyze feedback mechanisms, and to perform additional sensitivity studies using widely available scenarios. Analysis focused on two scenarios from the GAINS and IMAGE models. Additional work was performed evaluating climate feedbacks on natural emissions from sea-salt, pollen, and biomass burning.

14.1 Impacts of air quality regulations on climate, health and ecosystem
The GAINS (ECLIPSE4a) model was used to analyze current legislation and maximum feasible air pollution reduction scenarios with an International Energy Agency 6-degrees energy policy scenario. A ‘matrix’ of climate and air pollution mitigation scenarios (PEGASOS-PBL) was developed using the IMAGE integrated assessment model.

The impact of climate and air quality policy may substantially differ across the world. The differences are largest in developing countries. The IMAGE calculations showed that climate mitigation policies can to a large extent reach similar emission reductions. The most stringent emission reductions can be reached by combining air pollution and climate policies.

FASST calculations of the equilibrium temperature response of the impact of air pollutant emissions on a 40 years timeframe indicate a cooling by -0.4 degrees C between the highest and lowest GAINS and IMAGE scenarios (including methane; excluding aerosol indirect effects). Inclusion of ‘large’, but highly uncertain indirect aerosol effect indicates that emission reductions may induce overall warming, associated with removal of sulphur dioxide, leading to maximum temperature increase of up to 0.5 degrees C in the next decades, but probably less. MAGICC calculations show a net overall global warming due to strong air pollutant emission reductions in the short-term of around 0.2-0.4 degrees Celsius, as the impact of reducing cooling sulphur emissions dominates over the other impacts. Methane emission reductions in the scenarios lead to a net cooling and reduce ozone as well. Health impacts (premature mortality) are expected to decrease under most scenarios in Europe and North America, but will dramatically increase in China and India, if no further climate and air pollution mitigation measures are implemented. In most world regions stagnation or current legislation scenarios would lead to a further deterioration of ozone impacts on crops, whereas it could slightly improve for the maximum feasible reduction climate scenario.

Transient three-member ensemble simulations from 1990-2070 with the NorESM, focusing on aerosol and precursors, show a significant impact on climate of worldwide progressive maximum feasible air pollution reduction controls compared to the current legislation scenario. By 2050 maximum feasible air pollution reduction could lead to an increase in global temperature by 0.3 degrees C, and between 1-2 degrees C in polar regions compared to current legislation. Effects on precipitation vary strongly from region to region, with a tendency to a further drying of regions in Southern Europe and some desert areas (e.g. West Sahara). European PM reductions contribute significantly to the amplification of Arctic warming. The feedbacks between European air quality and Arctic climate should be considered when designing air quality policies, given the vulnerability of the Arctic environment and its link back to the European climate and transport sector.

14.2 Effects of climate on air quality
The ECHAM5-HAMMOZ global climate model was used to analyze the relationship between long-term changes in atmospheric variability and trends and variability of ozone and particulate matter. A substantial part of current long-term variability and trends could be quantified using the North Atlantic Oscillation Index as a proxy. Under climate change conditions the North Atlantic Oscillation in ECHAM5-HAMMOZ tends to become more positive, leading to more stagnant weather conditions in the Mediterranean, and less in the Northern Europe. Changing air pollution emissions can further amplify these changes, driven by aerosol reductions.

The PMCAMx regional model predicts that the effect of expected emission controls of SO2, NH3, NOx, and primary OA will far exceed that of climate change for fine PM for 2050. Future temperature, rain frequency, humidity, and wind speed changes under climate change conditions will all have important effect on PM2.5 concentrations, with effects of similar magnitude but different sign.

14.3 Effects of climate on natural emissions and ozone
Sea-salt emissions are sensitive to climate change, and depending on location and season can become stronger or weaker. Regional SILAM model simulations do not predict major changes off-coast of Europe. Further away, over the open oceans an increasing storm frequency leads to more coarse sea salt aerosols but they have less impact on climate and health issues in Europe. Climate change induced changes in wind speeds in the order of 1 meter per second can cause sea-salt PM response in the order of 5 micrograms per cubic meter across the domain, and about double in Ireland. NorESM simulations suggest small increases in Northern Europe and decreases in Southern Europe, subject to large uncertainty.

For Europe, only a moderate increase in fire emissions is plausible until 2050, although in some European countries, in particular Portugal, wildfires could overtake anthropogenic emissions for CO on an annual basis, and for BC and in some cases even NOx during the fire season.

An ensemble of models predicts for the 2050s (compared to 2000s) an increase of 1 ppb for daily maximum and 2 ppb for the 95th percentile ozone concentrations over the land areas of Southern Europe. The changes are neutral to negative in Northern Europe.

15. INTEGRATED ANALYSIS OF MULTIPLE MODEL PREDICTIONS
Multiple model projections of climate and atmospheric composition were performed including feedbacks between climate change and atmospheric chemistry. Several types of models were used: climate models, chemical transport models, and Earth system models, covering regional to global scales. By using different models, and by applying an ensemble approach (i.e. several simulations with the same model but using different initial conditions), the robustness of results was assessed. In addition, the Norwegian Earth System model, NorESM, provided future climate data to two regional climate models (REMOTE and REMO-HAM) and one global chemical transport model (SILAM) to assess how climate will change in different emission scenarios, and/or how climate change will affect atmospheric composition. The NorESM was run with the GAINS emission scenarios CLE and MFR in 3-member ensembles.

Global climate change as given by NorESM shows a temperature increase between 2010 and 2070 of about 0.9 and 1.2 K for the current legislation and maximum feasible reduction scenarios, respectively. The change in the current legislation case is dominated by increasing carbon dioxide concentrations. The additional warming of about 0.3 degrees C in the maximum feasible reduction scenario is mainly caused by the decreasing sulphur and organic carbon concentrations. Although there is large interannual variability, the result is statistically significant in the global mean, and it is still rather robust even for the regional cases. In particular, temperature changes in the Arctic are statistically significant at the 95% confidence level. By 2050 maximum feasible aerosol air pollution reduction could lead to an increase in temperature by about 1 C in polar regions. However, it must be noted that the carbon dioxide concentration increase alone, assuming the RCP 4.5 scenario, will give about a 3 C warming in the Arctic. Additional simulations with the NorESM calculate a reduction in warming of about 0.2 C on global-mean (and about 0.33 C in the Arctic) achievable by maximum feasible reduction of methane emissions.

REMOTE and REMO-HAM regional climate model simulations forced by NorESM climate data, focused on climate-ozone and aerosol-cloud-precipitation interactions, respectively. REMOTE used chemical data from the TM4 model as boundary conditions to take into account changes in global atmospheric composition change in the current legislation and maximum feasible reduction scenarios. The impact of climate change is seasonally and regionally variable but modest in significance, and balancing out on whole over the European domain. The effect of anthropogenic emissions change is prevalent in central Europe and the Mediterranean region, especially in summer months, and long-range transport have an especially significant effect on simulated ozone in winter time, demonstrating the importance of inter-continental transport on wintertime ozone mixing ratios.

Future climate scenarios were also simulated with the PMCAMx (regional CTM) and ECHAM5-HAM (global GCM) models. The GCM results show a more positive North Atlantic Oscillation (NAO) mean state by 2030 and a blocking frequency over the western Mediterranean. By separating the impacts of changes in aerosol and greenhouse gases, the study suggests that future aerosol abatement may be the primary driver of an increased blocking frequency over the western Mediterranean, leading to more stagnant weather conditions that favor air pollutant accumulation especially in the western Mediterranean sector and less stagnant conditions over Northern and Eastern Europe.

16. COST-OPTIMAL STRATEGIES FOR REDUCING POLLUTANT CONCENTRATIONS
Air pollutants and greenhouse gases are emitted from the same sources; their emissions interact in the atmosphere, and individually or jointly have a wide range of negative impacts on human health, agricultural crops, ecosystems, and global climate. In many cases emission control measures directed at air pollutants affect greenhouse gases, and vice versa. These interactions offer a scope for strategies that result in win-win solutions both for local and regional air pollution as well as for global climate.

The co-benefits of climate mitigation strategies for air pollution in the European Union were analyzed for two energy scenarios. At the global level, the Reference scenario follows up to 2050 the carbon dioxide emissions in the range between the RCP6.0 and the RCP4.5 scenario, while the Low Carbon scenario follows at the global level the emission trajectory of the RCP2.6 scenario. The Low Carbon scenario results in 2030 in 16% lower carbon dioxide than the Reference scenario, 15% lower sulphur dioxide emissions, 11% lower PM2.5 and 9% less NOx. At the same time, costs for implementing current air pollution control legislation would be 13% lower than in the Reference case.

Compared to 2005, the average exposure of the EU population to PM2.5 is predicted to decline from 14.1 to 7.9 micrograms per cubic meter in the Reference scenario. The lower emissions of the Low Carbon scenario will reduce this further to 7.2 micrograms per cubic meter. This will reduce premature mortality, and shortening of statistical life expectancy will decline in the Reference scenario from 8.6 months to 4.8 months, and to 4.4 months in the Low Carbon scenario. This will deliver 17.6 million life years to the European population.

An optimization routine has been developed for the GAINS model that enables the search for cost-effective emission control strategies to simultaneously meet health and climate targets. Two climate impact indicators developed by the FP7 ECLIPSE project have been used, in particular the GWP100 metric (i.e. the impact on global warming integrated over 100 years), and as an alternative the GTP20 (i.e. the impact on temperature change in 20 years). With these indicators, it turns out that the cost-effective emission measures to achieve the health targets of the 2013 Clean Air Policy Package would remove a small fraction of the negative radiative forcing that emerges from the current emissions of short-lived pollutants. Thus, the additional measures that would prevent in the EU-28 58,000 cases of premature deaths annually would result in an additional warming in the coming decades in the order of a few milli-Kelvins.

The new GAINS optimization routine has been used to explore options to ameliorate these negative climate impacts while maintaining the envisaged health benefits. Alternative portfolios of emission reductions measures have been found that would achieve the same health target and would avoid the negative climate impact; however, additional costs of these alternative measures are substantial. For instance, compensation of the negative short-term climate impacts of the proposed Clean Air Policy Package would triple the original emission control costs.

As an alternative approach, health and climate impacts of a low-carbon scenario have been examined. It is found that such decarbonization strategies result, as a consequence of lower fossil fuel use, in lower air pollution emissions and health impacts. At the same time, these lower emissions of short-lived substances compensate some of the climate benefits of the carbon dioxide reduction in the near-term, although the overall balance of such strategies is clearly positive.

17. KNOWLEDGE INTEGRATION AND TRANSFER

17.1 Workshop on policy needs
A workshop was organized in Brussels in June of 2012 to assess the policy needs from EU and national policy makers in the air quality and climate change areas. It was important to understand the policy needs in terms of the requirements of the Directorate General for the Environment (DG-ENV), Directorate General for Climate Action (DG-CLIMA) and national agencies for information of the impacts of AQ &CC. The workshop mapped the policy space in the near to middle term and assessed the requirements in terms of uncertainty of decadal prediction in climate change and the interface to air quality.

PEGASOS participated in the consultation process coordinated by the European Commission providing science-based evidences relevant for the review process of EU Air Quality legislation. The consultation process was kicked off on the 20th of October 2011 in Brussels, where the PEGASOS partners had the opportunity to discuss with the relevant stakeholders: DG RTD, DG JRC, DG ENV, DG SANCO, EEA and WHO. A report was finalized at the end of 2012 and presented at the European Green Week in June 2013.

PEGASOS also participated in a stakeholder dialogue meeting convened in Brussels presenting its major research findings to a broad range of stakeholders from the Air Quality and Climate Change science, policy and industry community. The meeting was held at the Belgian Academy of Sciences in Brussels on Tuesday 2 December 2014. The main discussion points concerned: i) short-lived climate pollutants (SLCP) and CO2; ii) local vs. regional control measures; iii) European vs. hemispheric or global control measures; iv) high-impact weather and the water cycle; v) issues in integrating air quality and climate change policies; vi) possible co-benefits for air quality/climate change integrated policy; vii) integration of natural sciences and social sciences in air quality and climate in air quality/climate change policies. The following points briefly summarize the main points of the discussion:
• climate change and air pollution are two (equally) important global environmental problems for society;
• impacts of air pollution involve human health, ecosystems and on climate;
• 3.5 million people die prematurely globally each year due to air pollution;
• the policy approaches to reduce air pollution and climate change have not been sufficiently connected. Other co-benefits (water, soils, etc.) should be considered;
• air quality is not only an issue for developing countries: it is still a problem for Europe;
• a large number of (local) measures can help to protect climate and air quality; often people ask for immediate measures that favor air quality over long-term climate measures, however, both CO2 and reactive gas emissions need to be mitigated.
• Several regions/environments are particularly vulnerable to climate change and air pollution, especially the Arctic regions for: a) Arctic air degradation (from European and Russian sources); b) transportation and tourism (new routes); c) fisheries; and d) resource extraction.

17.2 Integrated metrics for air quality and climate change
A workshop entitled: “Air and climate metrics integrating air quality and climate change mitigation – Is there a need for new metrics to support decision making?” took place in Copenhagen, 9-10 October 2013 in collaboration with the ECLIPSE project and IASS (Potsdam). The conclusion reached is that there is currently no single “meta-metric” that can deliver all information needed for a meaningful integration of air and climate policies. Instead, a coherent framework that uses a suite of metrics and relates hitherto disconnected pieces of information could be more applicable. Such a framework has been used in PEGASOS.

17.3 Knowledge transfer and training
In order to facilitate the knowledge transfer and dissemination and to train a new generation of young researchers within PEGASOS , two intensive field courses have been planned by the University of Helsinki. The Autumn 2013 School on the Advanced Analysis of Atmosphere-Cryosphere Interactions took place at the Hyytiälä Forestry Field Station over the course of two weeks in November 2013. A total of 16 young researchers from various countries and institutions participated in the course, which focused on the state-of-the-art instrumentation and the consequent analysis of the measured data. The Winter 2013 School on the Advanced Analysis of Atmospheric Processes and Feedbacks and Atmosphere-Biosphere Interactions was given at the same location in March 2014, concentrating on the integration of various datasets within the framework of, amongst other things, aerosol-cloud-climate interactions. The training and knowledge transfer during such courses was accomplished through intensive group work, project management, horizontal learning and collaborative action research.

17.4 Synthesis and Integration
The PEGASOS team has produced a synthesis report (Deliverable 17.6 of the project) addressing the five policy-relevant scientific questions of PEGASOS:
Question 1: How has past air quality policy inadvertently affected present day climate and, the corollary, how has climate change over the last decades affected Europe’s ability to meet its air quality targets for ozone, particulate matter (PM), etc.? How will currently planned air quality regulations affect climate?
Question 2: How will emissions (including methane, biomass burning emissions, biogenic hydrocarbons, etc.) respond to a changing climate, shifts in biomes, and land cover/land use changes and what will be the effect of these changes on European air quality (ozone, PM, etc.) and climate?
Question 3. How will climate change affect the atmospheric self-cleansing capacity (hydrogen oxide radicals, HOx, budget and cycling), atmospheric aerosol concentrations (both number and mass) and how will this in turn feed back to climate? How will climate change affect the regional accumulation of pollutants (including aerosols) and the resulting air quality and its regulation in Europe on both regional and urban scales?
Question 4. What are the main missing processes in current air quality-climate models and how can these tools be improved for the simulation of multi-scale chemistry-climate interactions including local changes?
Question 5: Which policy-relevant metrics should be used to facilitate the consideration of short-lived species in international treaties dealing with climate-relevant compound regulations and the assessment of air quality and climate policies co-benefits or other interactions?
Detailed information can be found in Deliverable 17.6.


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Potential Impact:
A significant fraction of Europe’s population lives in areas where air quality standards are exceeded with particulate matter and ozone pollution posing serious health risks. For example fine particulate matter has been estimated to reduce life expectancy in the EU by more than eight months. At the same time climate change is a major concern at both the global and European scales. PEGASOS has enhanced our understanding of the interactions of climate and atmospheric chemistry in the past, present, and future. This has allowed us to assess the effectiveness of policies in-place and to better design future policy options. Policy making will be influenced by the PEGASOS outcomes; directly by public outreach, and providing relevant results to policy makers, and indirectly by delivering relevant scientific studies to international assessments and organisations that in turn are important in international and European policy making.

1. PEGASOS impacts
The major impacts of PEGASOS include:

(a) Better estimate of air pollution and its impact on climate: PEGASOS started with the state-of-the-art global and regional climate and chemical transport models, improved them with better descriptions of atmospheric free radical formation and recycling, secondary inorganic and organic aerosol formation processes, feedback processes between gases and aerosol, aerosol dynamic modules and new emission inventories. Furthermore, feedbacks on atmospheric chemistry and climate from emissions related to biosphere and land-use changes were developed and included in the improved model systems. There were also improved representations of processes related to the hydrological cycle, and non-linear chemistry related to air pollution emissions. PEGASOS made use of its campaigns but also previous field studies and satellite observations to improve and evaluate the models’ performance for the present. All together these improvements resulted in more reliable calculations for levels of air pollution in Europe and in the world.

PEGASOS also performed hind-cast simulations to test the impact of air pollution on climate in the last decades, and to verify the consistency of forcings by aerosol and gases, changes in atmospheric and observed regional and global temperature responses. PEGASOS has further assessed the future impact of air pollution on climate, by using a suite of models that have broadened and analysed a range of scenarios proposed in the IPCC AR5 report. The models were chosen to be largely complementary, and have therefore provided relatively independent information on the possible development of climate and air quality due to the forcing of long-lived greenhouse gases as well as short lived components such as aerosol and ozone. A particular emphasis has been given to:
a) Feedbacks resulting from changes in vegetation emissions in response to climatic factors.
b) The role of feedbacks between emissions, aerosols and clouds
c) The interactions between air quality and climate policies

Through the long-standing involvement of most of its Partners in the IPCC process and other international assessments, PEGASOS results have had and will have impact beyond Europe, which is important in the climate policy arena.

(b) Support to the EC /Thematic Strategy on Air Pollution and Air Quality regulation.
The European Commission Thematic Strategy on Air pollution and Air Quality regulation has and will further benefit in many ways from the work performed in PEGASOS. The improved process knowledge on the formation of radicals and secondary aerosol coming out of the measurements, the new biogenic and anthropogenic emissions, the improved models, and the better understanding of variability and trends in climate and air pollution, have led to improved understanding on the relationship between emissions and concentrations.

The future projections in PEGASOS have assessed a range of future meteorological and chemical climates which provide the background for regional and urban air pollution conditions. Specifically, worrisome to policy makers, is the possibility that part of the air quality improvements may be counteracted by climate change, reflect by more stable weather conditions in parts of Europe. PEGASOS has analysed this risk for a variety of climate and emission scenarios and shown that it small compared to the improvements due to emission reductions expected in the future. Another important issue in regional air quality modeling is the increasing importance of non-European emissions on European air quality through long range transport. This is particularly true for ozone, but to some extent also for aerosols. PEGASOS has connected the global climate modeling community through coupling and downscaling of climate models to regional air pollution modeling, and in this sense combine the ‘best of two worlds’. Its regional Chemical Transport Models (e.g. PMCAMx) were able to use higher resolution over large urban areas and therefore connected the urban scale to the rest.

(c) Better quantification on regional and global links between air pollution and climate change to underpin mitigation options and other policy initiatives. Air pollution and climate change are linked on regional and global scales in both physical/chemical as well as economical and political ways. PEGASOS has addressed all three aspects. The physical and chemical connections were explored by using global emission inventories and projections, global chemistry transport models, and climate models to assess the impact of major world regions on each other building on experience in e.g. the HTAP (www.htap.org) project. These model results were then scaled down into regional model assessments of climate and air pollution changes. The changes in global emissions are strongly dependent on developments in the world economy, but also in the way these developments are guided by international policy agreements. PEGASOS has addressed some of these policies (e.g. combined versus individual air pollution and climate policies). By looking at specific costs of taking specific policy measures, PEGASOS has also considered this economic coupling in the Earth System. PEGASOS has provided and will continue to provide relevant results to the assessment organized by organizations such as IPCC, UNEP, and WHO which synthesize results on global levels.

1.2 Steps taken to bring about the impacts
To ensure that the PEGASOS research results and methodology development will impact policy, PEGASOS partners have used their involvement in the on-going technical underpinning mechanisms to disseminate the corresponding information. These organizations include:
(a) Technical underpinning mechanisms: EMEP (European Monitoring and Evaluation Programme) including TFMM (Task Force on Measurements and Modelling) and TFHTAP (Task Force on Hemispheric Transport of Air Pollution); GAW (Global Atmosphere Watch); IGAC (International Global Atmospheric Chemistry project) and ILEAPS (Integrated Land Ecosystem Atmosphere Processes Study); ESA (European Space Agency); IIASA (International Institute for Systems Analysis); GMES (Global Monitoring for Environment and Security)
(b) Intergovermental organizations: WMO (World Meteorological Organization); WHO (World Health Organization); IPCC (Intergovermental Panel on Climate Change); IGBP (International Geosphere-Biosphere Programme); UNEP (United Nations Environmental Program)
(c) EU institutions: European Environmental Agency and its Topic Centre on Air Pollution and Climate Change; The EU Commission
(d) Policy frameworks: EU Thematic Strategy on air pollution; CLRTAP (Convention on Long Range Transmission of Air Pollutants); UNFCCC (United Nations’ Framework Convention on Climate Change); Marine Conventions; National and regional mitigation of air pollution (and climate change)
(e) National authorities: National Environmental Agencies and Ministries of Environment.

2. Dissemination of project results
The PEGASOS’ partners have performed a wide range of dissemination activities throughout the course of the project. Research efforts and outcomes have been communicated at the earliest possible stage, in easily accessible formats, both within the consortium as well as to the scientific and industrial communities, to the public and policy makers. The dissemination efforts have used four different channels:
(i) The PEGASOS website (pegasos-eu.gr plus a blog and a Tweeter account)
(ii) peer-reviewed publications,
(iii) participation in scientific congresses and
(iv) organization of workshops.
As a result PEGASOS has improved the public awareness of the role of climate change on air quality.

More than 200 peer-reviewed papers and another 200 publications/presentations in international conferences have resulted so far from PEGASOS. Approximately 150 of the papers have been published and another 50 are in various stages of publication. Most of them are in public access journals. Practically all of them are in high impact journals and there are 14 that have been published in very high impact journals (3 in Nature, 3 in Science, 4 in Nature Geoscience, 3 in the Proceedings of the National Academy of Sciences, and 1 in Nature Communications). A special PEGASOS issue has been published in the leading journal Atmospheric Chemistry and Physics.

Project Partners have participated in the major international conferences on air quality and climate change (e.g. the American Geophysical Union (AGU) the European Geosciences Union (EGU), European Aerosol Conference (EAC) and IGAC, iLEAPS and SOLAS conferences). A special PEGASOS session has been organized in the 2014 EGU conference in Vienna focusing on the presentation, analysis, and synthesis of the results of the PEGASOS campaigns.
A workshop was organized in Brussels in June of 2012 to assess the policy needs from EU and national policy makers in the air quality and climate change areas. It was important to understand the policy needs in terms of the requirements of the Directorate General for the Environment (DG-ENV), Directorate General for Climate Action (DG-CLIMA) and national agencies for information of the impacts of air quality and climate change. The workshop mapped the policy space in the near to middle term and assessed the requirements in terms of uncertainty of decadal prediction in climate change and the interface to air quality.

PEGASOS participated in the consultation process coordinated by the European Commission providing science-based evidences relevant for the review process of EU Air Quality legislation. The consultation process was kicked off on the 20th of October 2011 in Brussels, where the PEGASOS partners had the opportunity to discuss with the relevant stakeholders: DG RTD, DG JRC, DG ENV, DG SANCO, EEA and WHO. A report was finalized at the end of 2012 and presented at the European Green Week in June 2013.

PEGASOS also participated in a stakeholder dialogue meeting convened in Brussels presenting its major research findings to a broad range of stakeholders from the Air Quality and Climate Change science, policy and industry community. The meeting was held at the Belgian Academy of Sciences in Brussels on Tuesday 2 December 2014.

In order to facilitate the knowledge transfer and dissemination and to train a new generation of young researchers within PEGASOS, two intensive field courses were organized by the University of Helsinki. The Autumn 2013 School on the Advanced Analysis of Atmosphere-Cryosphere Interactions took place at the Hyytiälä Forestry Field Station over the course of two weeks in November 2013. A total of 16 young researchers from various countries and institutions participated in the course, which focused on the state-of-the-art instrumentation and the consequent analysis of the measured data. The Winter 2014 School on the Advanced Analysis of Atmospheric Processes and Feedbacks and Atmosphere-Biosphere Interactions was given at the same location in March 2014, concentrating on the integration of various datasets within the framework of, amongst other things, aerosol-cloud-climate interactions.

A number of media briefings took place in Germany, Netherlands, Italy, and Finland during the period of the PEGASOS campaigns. These generated a lot of interest from TV stations, radio stations, newspapers and magazines and tens of articles devoted to PEGASOS were published not only in the above countries but also in France, Switzerland, Greece, Sweden, etc. There were several events organized by PEGASOS for the kick-off of the campaigns in May 2013 but also the arrival of the Zeppelin in Italy, the campaigns in Finland, etc.


These were attended by both local policy makers (including ministers of research and education) and officials of the EU. Overall the PEGASOS Zeppelin campaigns generated more interest from the public, the local policy makers, and the media than most of the corresponding large-scale projects in the air quality and climate areas.

3. The PEGASOS legacy
PEGASOS has and will have a significant impact on air quality and climate research, and on mitigation strategies relating to air pollution – climate change interactions.

Health effects due to air pollution and the potential damage from climate change are probably the two most important environmental problems facing the EU. PEGASOS has quantified the contributions of different anthropogenic and natural sources to the particulate mass and ultrafine particle concentrations. Similarly scenarios have been developed for co-beneficial and effective air quality and climate change mitigation.

A new set of scenarios was developed using the IMAGE integrated assessment model. For the historical period, these scenarios were made consistent with the EDGAR emission database. The historical emissions dataset of PEGASOS will be used as input for simulations for the next IPCC report, AR6. For the future, the scenarios combined assumptions on future climate policies following the so-called RCPs with assumptions on future air pollution policies. The resulting scenarios explore a wide range of future air pollution trajectories, based on different assumptions on climate and air pollution control policy. The scenario results emphasize the importance of co-benefits between climate policy and reduction of air pollutant emissions.

Data produced within PEGASOS will be rapidly transferred to existing EU data centers in Europe, such as EBAS. The PEGASOS campaign datasets will be a focus point for future atmospheric chemistry research at both the European but also at the global scale.

PEGASOS has developed new parametrizations for dry deposition of ammonium nitrate, new modelling schemes for organic aerosol atmospheric processing, The improved PEGASOS models will be available for use both for the design of strategies of air quality improvement and also climate change.

In PEGASOS we have provided more informed tools, compared to this previously existing, to perform a more accurate pollution-impact assessment with a particular emphasis on atmospheric aerosols.

Finally, in terms of knowledge transfer, PEGASOS activities included extensive workshops, seminars, winter and summer schools as well as daily mentoring of students and post doctoral researchers.

List of Websites:
pegasos-eu.gr