Mesoscale modeling study of the interactions between aerosols and PBL meteorology during a haze episode in China Jing-Jin-Ji and its near surrounding region – Part 2: Aerosols’ radiative feedback effects

This discussion paper is/has been under review for the journal Atmospheric Chemistry and Physics (ACP). Please refer to the corresponding final paper in ACP if available. Abstract Two model experiments, namely a control (CTL) experiment without aerosol-radiation feedbacks and a RAD experiment with online aerosol-radiation interactions, were designed to study the radiative feedback on regional radiation budgets, PBL meteorology and haze formation due to aerosols during haze episodes over China Jing-Jin-Ji and 5 its near surroundings (3JNS Region, for Beijing, Tianjin, Hebei Province, East Shanxi Province, West Shandong Province and North Henan Province) with a two-way atmospheric chemical transport model. The impact of aerosols on solar radiation reaching Earth's surface, outgoing longwave emission at the top of the atmosphere, air temperature , PBL turbulence diffusion, PBL height, wind speeds, air pressure pattern and 10 PM 2.5 has been studied focusing on a haze episode during the period from 7 to 11 July 2008. The results show that the mean solar radiation flux that reaches the ground decreases about 15 % in China 3JNS Region and by 20 to 25 % in the region with the highest AOD during the haze episode. The fact that aerosol cools the PBL atmosphere but warms the atmosphere above it leads to a more stable atmospheric stratification 15 over the region, which causes a decrease in about 52 % of turbulence diffusion and a decrease in about 33 % of the PBL height. This consequently forms a positive feedback on the particle concentration within the PBL and the surface as well as the haze formation. On the other hands, aerosol DRF (direct radiative forcing) increases about 9 % of PBL wind speed, weakens the subtropical high by about 14 hPa, which aids the 20 collapse of haze pollution, resulting in a negative feedback to the haze episode. The synthetic impacts from the two opposite feedbacks result in about a 14 % increase in surface PM 2.5. However, the persistence time of both high PM 2.5 and haze pollution is not effected by the aerosol DRF. On the contrary over offshore China, aerosols heat the PBL atmosphere and cause unstable atmospheric stratification, but the impact and 25 its feedback on the PBLH, turbulence diffusion and wind is weak except its evident impacts on the subtropical high.