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Regional climate-air quality interactions

Final Report Summary - REQUA (Regional climate-air quality interactions)

The main objective of the REQUA project (Regional climate-air quality interactions) is the strengthening of the research partnership, through staff exchanges and networking activities, between research groups in Europe, the USA and China working in the field of regional climate-air quality interactions.
The climate and the air quality are intimately interactive parts of the earth's system with their feedbacks being amplified in regional scales for the short-lived gas-phase species and aerosols. These interactions are critical for understanding the behavior of the earth system and should be taken into consideration for the design of future effective environmental policies. REQUA was designed to address these complex regional climate - air quality interactions by applying different modeling-based methodologies including:
a) Evaluating and comparing different coupled regional climate-air quality models.
b) Assessing the ability of regional versus global chemistry-climate models to simulate the spatial distribution and temporal evolution of air pollutants with emphasis on tropospheric ozone and particulate matter.
c) Advancing the understanding and quantification of the aerosol and tropospheric ozone effects on climate using coupled climate-air quality atmospheric models.
d) Assessing the impact of past climate variability and future anthropogenic climate change on air pollution levels with emphasis on tropospheric ozone and particulate matter.
During REQUA, there has been 107.81 equivalent months of secondments between European, USA and Chinese scientists. The percentage distribution of the secondments is 46% for Experienced Researchers and 54% for Early Stage Researchers (mainly PhD students). The research work accomplished during these secondments includes a series of regional and global climate studies. Below the most important work performed and the basic scientific findings are summarized.
• The role of dust on the radiative balance of the Mediterranean region was investigated using the online regional model RegCM4. The impact of key parameters (number of aerosol size bins, convective scheme etc) on aerosol load was assessed and the optimization of model performance was accomplished by means of sensitivity studies. Evaluation with satellite data indicated that the model underestimates dust optical depth over the gulf of Guinea and overestimates it over eastern Sahara by 60%. The model simulated adequately the dust extinction profile in the first 5 km, although it overestimated it in higher altitudes.
• With respect to process oriented evaluation, the RegCM4 regional climate model was employed to quantify a stratospheric intrusion event over the greater Athens area in Greece. The dynamical aspects of the event were analyzed and the model sensitivity to vertical resolution was accessed. The results showed that RegCM4 was able to capture the deep stratospheric intrusion event and its impact on tropospheric ozone levels. The same model was able to reproduce successfully the European heat-wave event of summer 2003.
• A number of sensitivity experiments with RegCM4 were performed, aiming to study the regional climate feedback of aerosols over the Mediterranean. Five 10-year simulations were performed including a control, a feedback, an anthropogenic aerosol and two dust runs with different dust parameterizations. The induced changes when including the aerosol feedback were found to be small, in the range of -0.2 to 0.2oC.
• Over the Asian domain, the online-coupled regional climate-chemistry model RegCCMS was employed for the study of the interactions between anthropogenic aerosols and the East Asian summer monsoon. In depth-analysis of the impact of aerosol on the climate system indicates that Black Carbon (BC) induces an enhancement of atmospheric circulation, which can increase local floods in south China, while droughts in north China may worsen in response to the BC semi-direct effect.
• The role of natural and anthropogenic emissions on the radiative balance over China was analyzed with simulations of two regional climate models. Using RegCM4, the anthropogenic secondary organic aerosol direct short wave RF at the surface and at the top of atmosphere was found to be 1.21 and 0.66 W/m2 respectively. RegCCMS was further developed and applied for the study of the sea salt aerosol (SSA) concentrations. The annual direct radiative forcing at the top of atmosphere of SSA in this area was estimated to be -2.35 W.m-2 and 1.17 W.m-2 for clear sky and all sky, respectively.
• The impact of future climate change on atmospheric composition over Europe was studied with two simulations performed under two Representative Concentration Pathway scenarios (RCP4.5 and 8.5). Despite warmer conditions in the future, summer ozone was found to be decreasing, mainly as a result of reduction of emission precursors in the future. A relative enhancement of daily max O3 was seen during autumn.
• Global models were employed for the study of the large-scale interactions of climate and atmospheric composition. More specifically, the global composition climate NASA GISS modelE was used to investigate the impact of biomass burning emissions on several key atmospheric species (carbon monoxide (CO), ozone, hydroxyl radicals (OH), and aerosols). Sensitivity studies showed that for free tropospheric O3, biomass burning is not the dominant cause of variability.
• A model intercomparison study showed that over the United States two global models (HadGEM3-GA4 and GISS ModelE2) have positive bias in sulfates, with GISS-E2 performing relatively better with a mean bias of +10% and HadGEM3-GA4 with +45%. Comparisons over China indicated that HadGEM3-GA4 model has the best skill in capturing total Aerosol Optical Depth (AOD), and specifically sulfur aerosols in the region. The models differed by up to a factor of six in the simulated change in aerosol optical depth (AOD) and shortwave radiative flux over China.
In addition, the following training activities/workshops were organized in the frame of REQUA project:
• RegCM-CHEM experts’ workshop (25-26 May 2016, Trieste, Italy).
• RegCM-CHEM training course (26 and 30 May 2016, Trieste, Italy).
• WRF-CMAQ experts’ workshop (18 and 21 September 2016, Thessaloniki, Greece).
• Final project workshop for the interaction with policy makers (16-17 September 2017, Thessaloniki, Greece).
More details are found at REQUA website