Impact of the actinic radiative effect of scattering aerosols on ozone vertical distribution in the Pearl River Delta, China

Currently, the Pearl River Delta (PRD) region faces the concurrent issues of aerosol and ozone pollution. Aerosols have a significant impact on the actinic radiative flux, which greatly influences the formation of ozone. This study quantitatively estimates the impact of scattering aerosol actinic radiative effect on the vertical distribution of ozone and secondary pollutants during a double high pollution episode in October 2012 using a localized parameterization formula for aerosol extinction profiles and the Weather Research Forecast–Community Multiscale Air Quality (WRF-CMAQ) model. Scattering aerosols can increase photolysis rates when the Aerosol Optical Depth is low. Aerosol actinic radiative effect reduces ground level ozone and PM_2.5 in the morning. In the core area of the PRD, the reduction in ozone reaches up to 20 ppb, while PM_2.5 decreases by 4–8 μg/m³. But it increases ozone and PM_2.5 levels in the upper and middle layers of the boundary layer during midday and the afternoon, with ozone increasing by 5–20 ppb and PM_2.5 by 1–2 μg/m³. Enhanced oxidizing capacity promotes the generation of sulfate and secondary organic aerosols, further increasing scattering aerosols and forming a positive feedback loop. The acinic radiative effect of aerosols lead to a diurnal variation in the reduction of surface ozone concentration, with a more pronounced effect in the morning. When radiation is weak in the morning and there is sufficient precursor for ozone, changes in radiation have a greater impact on ozone. As radiation becomes stronger, even though aerosols attenuate the radiation, there is still sufficient radiative flux to drive actinic reactions, and the impact of scattering aerosols on surface ozone concentration at midday is not significant. The mechanisms by which aerosol actinic radiative effect influence secondary organic aerosols (SOA) and sulfates are similar to those for ozone described above.