Abstract:
Since 2013, Chinese policies have dramatically reduced emissions of particulates and 20 their gas-phase precursors, but the implications of these reductions for aerosol-radiation 21 interactions are unknown. Using a global, coupled chemistry-climate model, we first examine 22 how the radiative impacts of Chinese air pollution in the winter months of 2012 and 2013 affect 23 local meteorology and how these changes may, in turn, influence surface concentrations of PM2.5, 24 particulate matter with diameter less than 2.5 m. We then investigate how decreasing emissions 25 through 2016 and 2017 alter this impact. We find that absorbing aerosols aloft in winter 2012 26 and 2013 heat the mid- and lower troposphere by ~0.5-1° C, reducing cloud liquid water, 27 snowfall, and snow cover. The subsequent decline in surface albedo counteracts the ~15-20 W m -2 28 decrease in shortwave radiation reaching the surface due to attenuation by aerosols overhead. 29 The net result of this novel cloud-snowfall-albedo feedback in winters 2012-13 is a slight 30 increase in surface temperature of ~0.5-1° C in some regions and little change elsewhere. The 31 aerosol heating aloft, however, stabilizes the atmosphere and decreases the seasonal mean 32 planetary boundary layer (PBL) height by ~50 m. In winter 2016 and 2017, the ~20% decrease in 33 mean wintertime PM2.5 weakens the cloud-snowfall-albedo feedback, though it is still evident in 34 western China, where surface temperatures warm by ~0.5-1° C. Regardless of emissions, we find 2 35 that aerosol-radiation interactions enhance mean surface PM2.5 pollution by 10-20% across much 36 of China during all four winters examined, mainly though suppression of PBL heights.
