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Periodic Report Summary 2 - BACCHUS (Impact of Biogenic versus Anthropogenic emissions on Clouds and Climate: towards a Holistic UnderStanding)

Project Context and Objectives:
The response of clouds to a changing climate and their effect on the radiative budget of the Earth is the most uncertain climate feedback. Aerosol-cloud interactions play a key role in the anthropogenic radiative forcing of the climate system but remain the most uncertain of all forcing agents and are still associated with a low scientific level of understanding.
A major part of the uncertainty in how aerosol and cloud processes respond to changes in anthropogenic and natural emissions is due i) to lack of fundamental understanding about ice-containing clouds and ii) to the incomplete knowledge of the coupling between biosphere and atmosphere. These two areas are explored in greater details in BACCHUS as they may turn out to be key in the climate system. Measurement capabilities of ice nucleating particles (INP) are still insufficient and our understanding of ice formation and ice cloud evolution in different environments is poor. Coupling between the biosphere and the atmosphere resulting from aerosol-cloud interactions may play an important role in regulating climate change via aerosol and cloud formation, particularly in a changing climate as the biosphere responds to warming. Biogenic aerosols, including primary biogenic aerosol particles (PBAPs), act as INP but the importance of PBAPs in the present and changing climate still needs to be quantified.
Understanding the “baseline” natural aerosol loading and its cloud forming potential is essential for quantifying the magnitude of anthropogenic forcing caused by aerosol-cloud interactions, but also for quantifying changes attributable to perturbations of biogenic emissions (particles and volatile organic compounds (VOC)), resulting from global changes due to anthropogenic perturbation. Within BACCHUS these complex aerosol-cloud interactions and feedbacks involving natural aerosols in the present and perturbed climate will be adequately represented in Earth Systems Models (ESMs) to capture the key aerosol-cloud interactions and feedbacks involving natural aerosols in the present and perturbed climate in order to reduce the uncertainty of the impact of biogenic and anthropogenic emissions on clouds and climate in future climate projections.

The core idea of BACCHUS is to quantify key processes controlling clouds and climate and their feedbacks by i) contrasting the processes occurring in climate-relevant environments such as tropical areas and the Arctic and ii) by combining advanced measurements of cloud and aerosol properties with state-of-the-art numerical modeling. Specifically BACCHUS aims to characterize the importance of biogenic versus anthropogenic emissions for cloud formation and climate in regions that are key regulators of climate (tropical rain forests) as well as in regions experiencing the most profound climatic changes, and which may be prone to irreversible transitions, e.g., the Arctic. In addition to contrasting the tropical (Amazon and Barbados) and polar regions (Arctic), we augment the BACCHUS project by using well-established sites at Mace Head (North Atlantic), Hyytiälä (boreal forest) and Jungfraujoch (mainly in free troposphere). These latter sites will enable us to identify relevant parameters through long-term observations and to develop methods to critically test the resulting parameterizations and feedbacks.

This core idea has been developed around two central objectives:
Objective 1: To develop a robust methodology to quantify the influence of anthropogenic aerosol on cloud properties based on the estimate of the background levels of natural aerosols in various environments, the identification of their sources and their role in aerosol-cloud processes. Emphasis is placed on changing cloud properties arising from aerosol-cloud interactions (rather than aerosol-radiation interactions) with a particular focus on the ice-phase as well as the involvement of biogenic and organic aerosols in modifying the properties of cloud condensation nuclei (CCN) and ice nucleating particles (INP).
Objective 2: To characterize and understand the key interactions and feedback mechanisms in the terrestrial and marine biosphere-atmosphere-cloud-climate system by building on advanced in-situ observations, remote sensing, and numerical models operating over a wide spectrum of spatio-temporal scales and complexity. BACCHUS will focus on both the terrestrial and marine biosphere.

(Figure 0.1: BACCHUS objective 1 will quantify the anthropogenic aerosol effect by producing a best estimate of current background levels. Objective 2 focuses on key interactions and feedback mechanisms in the Earth system. IN: Ice Nuclei; CCN: Cloud Condensation Nuclei.)
Project Results:
WP1 successfully implemented an INP database in the first stage of the BACCHUS project. The INP database is currently filled with recent and historical observations. Several campaigns focusing on INP observations have been performed during the second reporting period. CCN data sets collected recently and in past field campaigns have been and will be submitted to the ACTRIS data centre. The CCN and INP data sets come from very different regions around the globe. In this way the geographic differences in CCN and INP are complied. WP1 and WP2 are elaborating the first European long-term data sets in collaboration, linking CCN and chemical properties via experimental and theoretical closure studies and model approaches, focusing on a variety of background conditions, and taking particle sources and atmospheric aging into account. Field data on CCN-chemistry relationships sampled at Central-North Europe, in the Mediterranean, Amazonia, Artic, and Southern Ocean have been analysed.
Vertically resolved field observations of aerosols and clouds in key climate regions with networks of aerosol/cloud lidars and cloud radars, with aircraft, and in the framework of field campaigns have been performed and analysed. The goal is to better understand aerosol-cloud interactions for fresh and aged aerosol mixtures and to support in this way efforts of process, cloud-resolving and regional modeling as well as global climate modeling. Satellite remote sensing with very high horizontal resolution (Visible Infrared Imager Radiometer Suite, VIIRS) on board NASA’s Earth-observing satellite NPP is used to study aerosol-cloud-precipitation interaction on a global scale. The aerosol and cloud algorithm for the spaceborne AATSR/ATSR2 dual view radiometers has been further developed and applied to four regions with the contrasting environments: biomass-burning regions in Amazonia, relatively clean Europe, Saharan dust outbreaks over the Central Atlantic, and polluted China. These data will be used in WP3 for the models/MODIS/AATSR cloud properties intercomparison.

During the second period of the project, WP2 i) improved the organic aerosol representation in the models accounting for organic and inorganic contributions to new particle formation (NPF), growth to CCN and atmospheric ageing and ii) performed simulations to separate the anthropogenic from the natural contributions to CCN/INP.
Simplified parameterizations have been developed and tested, in order to investigate the impact of aerosol origin (natural versus anthropogenic) and chemistry (organic aerosol formation and aerosol ageing) on CCN. Sensitivity modelling studies have been performed to estimate the associated uncertainties (deliverable D2.2). Several new parametrizations for the impact of biogenic and anthropogenic organic compounds on NPF and their importance as CCN/INP have been developed based on both laboratory experiments (CERN cloud chamber) and long-term field observations and the associated uncertainties have been evaluated (deliverables D2.3 and D2.4). These include a physically-based parameterization of CCN activation in the presence of semi-volatile organic carbon co-condensation to implement in a variety of models and an INP parameterization based on field measurements, remote sensing and laboratory studies. The model results are evaluated against the field measurements of INP. Furthermore, a CCN model intercomparison exercise has been initiated to consolidate the uncertainties estimate of the parameterizations of the impact of aerosol origin and chemistry in CCN (see deliverable D2.4). Models diverge with regard to the observed CCN levels and variability. Further analysis of these results will enable the quantification of the importance of the major aerosol components to the CCN levels as well as to uncertainties associated with the emission inventories (to be reported in deliverable D2.5).

WP3 investigates determine the key processes controlling cloud systems in contrasting environments and the relative role of natural vs. anthropogenic aerosol (precursor) emissions in each of them. A joint framework and case study protocol was devised in deliverable D3.1 and has been refined and updated to incorporate links to recent international activities.
Case studies to investigate the relative role of aerosol emissions highlight the complexity of aerosol-cloud interactions in contrasting environments: For the Arctic case study, the results indicate that current, state of the art, high resolution models show significant diversity in simulating key parameters of Arctic clouds, even when cloud droplet/ice crystal number concentration (CDNC/ICNC) are prescribed, and that the model results are further sensitive to uncertain ice crystal properties. This has significant implications for our ability to explain the role of clouds for the sensitive radiation balance of the Arctic. Results from the Amazon case study show that increasing aerosol concentrations in the Amazon areas will enhance the formation of cloud droplets, suppress the formation of rain drops and have little impact on the frozen hydrometeors. However, aerosol perturbations in the Amazon area have a modest effect on precipitation. For the Barbados case study, field measurements and high-resolution simulations have been performed to analyze the interplay of clouds, their meteorological and aerosol environment in regions of shallow convection in the winter trades and near the summer ITCZ.
Traditional bottom-up CDNC/ICNC closure studies were combined (task 3.3) with a novel concept of top-down closures studies to provide constraints on the satellite inferred cloud microphysical properties. Effective radius retrieved from the VIIRS satellite were also compared with in-situ data and modelled to test closure at various different levels and were found to be in reasonable agreement. However, we find that CDNC retrieved from satellite in marine shallow convection can be underestimated and that updraft velocities are a critical component in the subsequent retrieval of CCN.

WP4 has been integrating aerosol and precursor emissions into global climate models (GCMs) to study their climate effects, specifically through clouds. Monoterpene emissions in LPJ-GUESS now include speciation to 8-10 species and wildfires have been implemented using the SIMFIRE-BLAZE model. To allow past and future simulations of BACCHUS ESMs, KIT has generated datasets of BVOC and wildfire emissions for the period 1900-2100. The speciated monoterpenes from LPJ-GUESS are grouped in ECHAM-HAM based on their cyclic structure in order to separate the formation pathways of Extremely Low Volatility Organic Compounds (ELVOCs), This is essential for CCN formation from BVOCs. In the ocean compartment, WP4 has focused on the emissions of marine organic aerosol (MOA) mainly to improve simulations of INPs that also has implications for radiation biases of southern ocean clouds.
The GCMs with the above mentioned improvements have started to assess global scale aerosol-cloud interactions under distinct conditions and scenarios. ETHZ is assessing the impact of retreating sea ice and increasing ship traffic in the future Arctic. UiO has applied NorESM towards the assessment of climate feedback loops with biosphere emissions and aerosol-cloud interactions. MPI applied the Max-Planck Aerosol Climatology to quantify aerosol forcing in ECHAM-HAM. Using the KIT-generated BVOC emission datasets, UHEL quantified potential future changes in Siberian aerosol formation and related climate effects.
Potential Impact:
BACCHUS will strongly enhance our current understanding of aerosol-cloud interactions in the Earth System and as such reduce the uncertainties of current state-of-the-art ESM climate predictions at different scales and in several ways. The expected scientific breakthroughs are:
With its wealth of observational sites, BACCHUS will be able to address the role of biogenic aerosol in climate processes related to clouds, radiative forcing and precipitation.
The observations from the contrasting measurement sites in BACCHUS combined with process modelling will enable us to determine which level of complexity in terms of aerosol-cloud interaction needs to be included in models to reproduce the observations. This, in turn will allow us to assess which models are most reliable for estimating the effective radiative forcing associated with aerosol-cloud interactions. Again, this knowledge combined with a significantly improved understanding of natural aerosols and clouds obtained in BACCHUS will lay the basis for greatly improved estimates of radiative forcing and a significant reduction of the associated uncertainty.
BACCHUS' objective to characterize the importance of biogenic versus anthropogenic emissions for cloud formation and climate will be discussed in an expert meeting and be made available as a community assessment.
These breakthroughs will have a direct impact on decision-making addressing climate change adaptation and mitigation.
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