Impact of typhoon periphery on high ozone and high aerosol pollution in the Pearl River Delta region

This paper analyzes observation data in the Pearl River Delta (PRD) region from 2012 to 2013, and explores the impact of typhoon periphery on high ozone and high aerosol pollution episodes (double high episodes). Observation analysis show that severe tropical storms to severe typhoons are mainly located in the range of 10°N-30°N, 116°E-135°E when double high episodes occur. Meanwhile, obvious high temperature, low humidity, low wind speed, high actinic flux, high aerosol optical depth (AOD), and high single scattering albedo (SSA) can be observed in double high episodes. The diurnal cycle of the PM_2.5 is significant in double high episodes, and the average peak concentration in the afternoon can exceed 90 μg/m³. The diurnal cycle of PM_2.5 in non-double high episodes is not significant, and the average value is about 34-39 μg/m³. The ozone peak concentration in double high episodes is 81-103 ppbv, which is about 27-40 ppbv higher than that of non-double high episodes. High correlation can be found between the aerosol and ozone diurnal cycles in double high episodes, and r² reaches 0.76. In double high episodes, black carbon, nitrate, and sea salt decrease while sulfate, ammonium, secondary organic carbon, and total PM_2.5 significantly increase in the afternoon. The growth of PM_2.5 in double high episodes is mainly contributed by scattered fine particles from photochemical processes and transmission. The mechanisms that control the double high episodes in the PRD are described below. Ozone and aerosol begin to accumulate under unfavorable meteorological conditions. Via local photochemical processes and external transport, the scattered aerosol increases and leads to an increase in multiple scattering and actinic radiation, which is in turn more favorable for photochemical reaction and further increases the ozone concentration. Meanwhile, high oxidizability promotes the formation of scattered aerosol, creating positive feedback. In addition, the scattered aerosol increases backscattering, which increases the photolysis rate and ozone concentration in the middle and upper boundary layer. Meanwhile, downdraft and turbulence transports high-concentration ozone to the ground.