Aerosol-Radiation Interactions in China in Winter: Competing Effects of Reduced Shortwave Radiation and Cloud-SnowfallAlbedo Feedbacks Under Rapidly Changing Emissions

Since 2013, Chinese policies have dramatically reduced emissions of particulates and their gas-phase precursors, but the implications of these reductions for aerosol-radiation interactions are unknown. Using a global, coupled chemistry-climate model, we examine how the radiative impacts of Chinese air pollution in the winter months of 2012 and 2013 affect local meteorology and how these changes may, in turn, influence surface concentrations of PM2.5, particulate matter with diameter <2.5 μm. We then investigate how decreasing emissions through 2016 and 2017 alter this impact. We find that absorbing aerosols aloft in winter 2012 and 2013 heat the middle- and lower troposphere by ∼0.5–1 K, reducing cloud liquid water, snowfall, and snow cover. The subsequent decline in surface albedo appears to counteract the ∼15–20 W m −2 decrease in shortwave radiation reaching the surface due to attenuation by aerosols overhead. The net result of this novel cloud-snowfall-albedo feedback in winters 2012–2013 is a slight increase in surface temperature of ∼0.5–1 K in some regions and little change elsewhere. The aerosol heating aloft, however, stabilizes the atmosphere and decreases the seasonal mean planetary boundary layer (PBL) height by ∼50 m. In winter 2016 and 2017, the ∼20% decrease in mean PM2.5 weakens the cloud-snowfall-albedo feedback, though it is still evident in western China, where the feedback again warms the surface by ∼0.5–1 K. Regardless of emissions, we find that aerosolradiation interactions enhance mean surface PM2.5 pollution by 10%–20% across much of China during all four winters examined, mainly though suppression of PBL heights. Plain Language Summary Particulate matter, also called aerosols, influences climate through the absorption and reflection of solar radiation traveling through the atmosphere. Trends in particulate matter can therefore impact local and regional climate, which may in turn exacerbate particulate pollution. Here we embed a detailed atmospheric chemistry scheme within a global climate model to examine aerosol-radiation interactions and their impact on particulate pollution over China in winter 2012–2013 and 2016–2017. We find that surface particulates decrease by 20% between the time periods, a result of stricter regulations. In both time periods, aerosol absorption of sunlight heats the atmosphere aloft, leading to more stable conditions. This in turn amplifies the concentration of surface particulate pollution by 10%–20%, again in both time periods. Aerosol heating aloft also diminishes cloud cover, which then reduces snowfall, especially in 2012–2013. The subsequent loss of snow cover allows more sunlight to be absorbed by the surface, apparently counteracting to some extent the surface cooling from reflection of sunlight by aerosols aloft. This work suggests an important role for absorbing aerosols like soot in affecting local climate; it also demonstrates the potential for climate models with detailed chemistry to shed new light on interactions between aerosols and climate.