Title:Effect of Weakened Diurnal Evolution of Atmospheric Boundary Layer to Air Pollution over Eastern China Associated to Aerosol, Cloud – ABL Feedback

Journal: Atmospheric Environment, 168-179

Authors:QU W. -J. *, J. Wang, X. -Y. Zhang, Y. -Q. Wang, S. -H. Gao, C. -H. Zhao, L.-Y. Sun, Y. Zhou, W. -C. Wang, X. -H. Liu, H. -Q. Hu, and F. Huang

Abstract: Upon the effect of atmospheric stability on air pollution, this study highlights the weakened diurnal evolution of atmospheric boundary layer (ABL), which is crucial to the formation of regional fog-haze and air pollution episodes over eastern China (ECN). The decrease of atmospheric visibility (Vis) during 1973–2012 is found to be related to the more stable ABL with weakened diurnal evolution that is characterized by a concurrent weakening of surface wind, a decrease in ABL height, and a reduction of diurnal temperature range (DTR, – 0.26 and – 0.092 °C/10-years in the winter and summer, respectively). With a general synchronous variation, the increase in both aerosol loading and cloud cover might work in concert to reduce solar radiation reaching the surface, stabilize ABL and weaken the diurnal ABL evolution, thereby weakening the turbulent mixing of pollutants and enhancing air pollution. This conjecture of “aerosol, cloud – ABL – air pollution” feedback, with particular emphasis on the effect of cloud (in addition to the effect of aerosol) on the ABL evolution, is supported by our analysis of 40 years trends in the cloud, Vis and DTR from surface observation, the aerosol optical depth from MODIS, and the validated MERRA-2 reanalysis aerosol, cloud and boundary layer height (by surface observation and routine radiosonde sounding). Regarding the linkage between fog-haze and diurnal evolution of the ABL, the result emphasizes the effect of a persistent high relative humidity (RH), which is found to be important to the formation of regional persistent fog-haze episodes and must be carefully considered in future study. Therefore, a chain of processes is suggested to interpret the occurrence of regional persistent fog-haze episodes over ECN. First, during a polluted day, because of aerosol radiative effects, the high ABL stability and high RH can persist throughout the day, and consequently, favoring the accumulation of pollutants as well as the secondary formation and hygroscopic growth of aerosol. Second, the maintenance of such more stable, humid and polluted ABL is in contrast to a clear day when the ABL is more convective in the afternoon and facilitates the diffusion and dilution of pollutants. Third, under a persistent stable synoptic system, the increase of air pollution associated with the weakened diurnal evolution of ABL due to the “aerosol, cloud – ABL – air pollution” feedback, as described above, can be further strengthened, thereby renders more persistent and severe air pollution events. The DTR variation in relationship with the aerosol, cloud, ABL and air pollution is also discussed.