Impact of aerosol optics on vertical distribution of ozone

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 near surface. In this study, we analyze the sensitivities of ozone response to aerosol mixing states (e.g., mixing behaviour hypothesis of scattering and absorbing components) in the vertical direction and address the effects of aerosol-BL and aerosol-photolysis interactions on ozone profiles by WRF-Chem simulations. The aerosol internal mixing state experiment reasonably reproduces the vertical distribution and time variation of meteorological elements and ozone. Sensitive 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 BL during daytime (08:00~17:00), and the decrease is the least in external mixing state (0.6 %) compared with internal (9.8 %) and core-shell mixing states (7.4 %). The photolysis enhancement is the most significant in 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 VOC-limited regime. Upper-level ozone chemical production is accelerated due to higher photolysis rate resulting from aerosol backscattering. Therefore, the increased ozone entrainment from aloft BL to surface induced by boosted ozone vertical gradient outweighs the decreased ozone entrainment induced by turbulent suppression after 11:00 am. Additional simulations support that aerosol effect on precursor, photolysis and ozone is consistent under different underlying surface and pollution conditions.