Impact of aerosol optics on vertical distribution of ozone in autumn over Yangtze River Delta

Abstract

Abstract. Tropospheric ozone, an important secondary pollutant, is greatly impacted by aerosols within boundary layer (BL). Previous studies have mainly attributed ozone variation to either aerosol–BL or aerosol–photolysis interactions at the near-surface level. In this study, we analyze the sensitivities of ozone response to aerosol mixing states (e.g., mixing behavior hypothesis of scattering and absorbing components) in the vertical direction and address the effects of aerosol–BL and aerosol–photolysis interactions on ozone profiles in autumn by Weather Research and Forecasting model coupled with Chemistry (WRF-Chem) simulations. The aerosol internal mixing state experiment reasonably reproduces the vertical distribution and time variation in meteorological elements and ozone. Sensitivity experiments show that aerosols lead to turbulent suppression, precursor accumulation, lower-level photolysis reduction, and upper-level photolysis enhancement. Consequently, ozone basically decreases within entire the BL during daytime (08:00–17:00 LT), and the decrease is the least in the external mixing state (2.0 %) when compared with internal (10.5 %) and core shell mixing states (8.6 %). The photolysis enhancement is the most significant in the external mixing state due to its strong scattering ability. By process analysis, lower-level ozone chemical loss is enhanced due to photolysis reduction and NOx accumulation under a volatile organic compound (VOC)-limited regime. Upper-level ozone chemical production is accelerated due to a higher photolysis rate resulting from aerosol backscattering. Therefore, the increased ozone entrainment from BL aloft to the surface induced by the boosted ozone vertical gradient outweighs the decreased ozone entrainment induced by turbulent suppression after 11:00 LT. Additional simulations support the finding that the aerosol effect on precursors, photolysis, and ozone is consistent under different underlying surface and pollution conditions.