Aerosols significantly influence atmospheric environments and climate change. Their direct radiative effect (DRE) involves scattering and absorbing solar/terrestrial radiation. The indirect effect (IRE) alters cloud microphysical properties by acting as cloud condensation nuclei (CCN). The semi-direct effect involves absorbing aerosols heating the atmosphere and evaporating clouds. Feedback effects arise from aerosol impacts on meteorological fields and chemical composition. While aerosols generally cause a net cooling effect, absorbing aerosols like black carbon can warm the atmosphere. Their effect on precipitation is complex, sometimes reducing it and sometimes stimulating convection under specific conditions. China has experienced rapid economic development, industrialization, and urbanization, leading to severe air pollution since the beginning of the 21st century. The severe haze pollution in January 2013 prompted strict government air pollution control strategies, including the "Action Plan on the Prevention and Control of Air Pollution" (2013-2017) and the 'Blue Sky' Clean Air Plan (2018-2020). These plans resulted in significant reductions in anthropogenic emissions and PM2.5 concentrations. The COVID-19 pandemic in 2020 further reduced emissions due to lockdowns. Several studies have examined meteorological and climate responses to these emission reductions, but the impacts of aerosols on heatwaves and their response to emission changes remain largely unexplored. Summer 2022 witnessed unprecedented heatwaves in China, raising public concern. These events were attributed to anomalies in the subtropical high, increased greenhouse gas emissions, and urban expansion. This study uniquely explores the aerosol radiative effects and feedbacks on boundary layer meteorology and PM2.5 concentrations during these heatwaves and their response to emission changes over the past decade using the online coupled regional climate-chemistry-aerosol model (RIEMS-Chem). This provides crucial insights into aerosol-radiation-cloud feedback during heatwaves and informs future emission control strategies.

The literature review section extensively cites previous research on aerosol effects on climate, air pollution in China, and the impact of emission control policies. It highlights existing studies on the direct and indirect radiative effects of aerosols, their influence on precipitation, and the impacts of emission reductions from the Clean Air Plans and the COVID-19 pandemic on air quality and climate. However, it points to a gap in research: the lack of studies exploring the specific effects of aerosols on heatwaves in China and how these effects respond to anthropogenic emission changes. This study aims to fill this gap.

The study utilizes the online coupled regional climate-chemistry-aerosol model RIEMS-Chem. Anthropogenic emissions for 2013 and 2019 are from the Multi-resolution Emission Inventory model for China (MEIC). Emissions for 2022 are derived using a top-down approach combining satellite retrievals (TROPOMI/OMI for NOx and NMVOC, OMI for SO2 and CO) and sensitivity model simulations based on the MEIC 2020 inventory. Emissions outside China are from the MIX inventory, adjusted for COVID-19 impacts. RIEMS-Chem incorporates detailed representations of land surface processes (modified BATS), planetary boundary layer processes (MRF), cumulus convection (Grell scheme), radiation transfer (modified CCM3 radiation package), gas-phase chemistry (updated Carbon-bond mechanism), and aerosol processes (including emissions, transport, multi-phase chemistry, dry and wet deposition, heterogeneous reactions, aerosol hygroscopic growth, and cloud droplet activation). The model includes nine aerosol types: sulfate, nitrate, ammonium, black carbon, primary organic aerosol, secondary organic aerosol, primary PM2.5 and PM10, mineral dust, and sea salt. Three experiments are conducted: BASE (2022 emissions and meteorology), EXP1 (2019 emissions, 2022 meteorology), and EXP2 (2013 emissions, 2022 meteorology). DRE, IRE, and TRE are calculated through sensitivity simulations comparing runs with and without aerosols and with varying levels of aerosol-cloud interaction. Feedback effects on meteorology and PM2.5 are evaluated by comparing these simulations to a reference case (CASE0) without aerosol radiative feedbacks. Model performance is evaluated against observational data from the National Meteorological Information Center, China National Environmental Monitoring Center, AERONET, MERRA-2, and GPM for meteorological variables, PM2.5 concentrations, gas concentrations, and AOD. The study domain covers China, the Korean Peninsula, Japan, parts of India, Nepal, and Southeast Asia, with a 60 km horizontal resolution and 16 vertical sigma layers.

During the August 2022 heatwaves, aerosols exerted mean DRE and IRE of -3.9 Wm⁻² and -2.4 Wm⁻² at the top of the atmosphere (TOA), respectively, leading to a 0.3 °C decrease in average surface air temperature over eastern China. This cooling was accompanied by decreases in PBLH and precipitation and an increase in PM2.5 concentration. Emission reductions since 2013 caused a significant decrease in DRE (27% at TOA) while IRE remained relatively unchanged. This led to a net warming effect (0.14 °C on average over eastern China, exceeding 0.5 °C in BTH) and increased precipitation (2.7 mm on average). The warming trend due to weakened TRE is concerning and could exacerbate heatwaves. Spatial analysis shows stronger DRE in the Beijing-Tianjin-Hebei (BTH) region compared to the Yangtze River Delta (YRD) and Sichuan-Chongqing (SCQ) regions, while IRE was highest in SCQ. In BTH and YRD, DRE dominated TRE, but in SCQ, IRE was comparable to DRE at TOA. Aerosol radiative effects led to significant changes in meteorology. DRE caused surface cooling, decreased wind speed, lowered PBLH, and increased PM2.5 due to weakened turbulence. IRE induced similar changes, particularly affecting precipitation in western China. TRE caused stronger changes than either DRE or IRE alone. The analysis of emission reduction impacts shows that the reduction from 2019 to 2022 (COVID-19) and from 2013 to 2019 (Clean Air Actions) both resulted in anomalous warming, particularly in BTH, increased PBLH, decreased PM2.5, and increased precipitation. The combined effect of emissions reductions from 2013 to 2022 caused considerable warming (maximum exceeding 0.5 °C in BTH and Chongqing), further emphasizing the impact of weakened aerosol cooling.

The findings address the research question by quantifying the direct, indirect, and feedback effects of anthropogenic aerosols on heatwave meteorology in eastern China and their response to emission changes. The significant warming observed after emission reduction highlights the substantial cooling effect of aerosols and the potential implications for future heatwaves. The relatively stable IRE despite DRE reductions suggests a lower sensitivity of cloud properties to aerosol changes at high CCN concentrations. This has important implications for air quality and climate change co-benefits. Regions with high aerosol and cloud amounts (e.g., SCQ) may benefit from further emission reductions, whereas in regions with high cloud cover but moderate-low aerosol levels, prioritizing the control of primary aerosols could be more effective. The study provides new insights into aerosol-radiation-cloud feedback during heatwaves, offering valuable information for designing efficient strategies to mitigate both health and climate change risks. The results are consistent with previous research, yet offer more nuanced findings due to the model's ability to differentiate between DRE and IRE effects and analyze their respective responses to emission changes.

This study demonstrates the significant impact of aerosols on heatwave meteorology in China and how emission reductions affect these impacts. While emission reductions improve air quality, they also lead to warming, particularly in regions with high initial aerosol loading. The increasing relative importance of IRE with emission reductions should be considered in future emission control strategies. Further research should focus on improving aerosol emission inventories and model representations of aerosol-cloud interactions to reduce uncertainties. The findings are valuable for designing efficient strategies for achieving co-benefits for air quality and climate.

Several limitations exist: the 2022 emission inventory relies on a top-down approach with satellite retrievals, introducing uncertainties; the model may underpredict cloud and precipitation in some regions, underestimating IRE; and the study focuses on fast meteorological responses to emission changes, ignoring potential long-term feedback effects on sea surface temperatures.