Impacts of aerosol-photolysis interaction and aerosol-radiation feedback on surface-layer ozone in North China during multi-pollutant air pollution episodes

We examined the impacts of aerosol-radiation interactions, including the effects of aerosol-photolysis interaction (API) and aerosol-radiation feedback (ARF), on surface-layer ozone (O-3) concentrations during four multi-pollutant air pollution episodes characterized by high O-3 and PM2.5 levels during 28 July to 3 August 2014 (Episode 1), 8-13 July 2015 (Episode2), 5-11 June 2016 (Episode3), and 28 June to 3 July 2017 (Episode4) in North China, by using the Weather Research and Forecasting with Chemistry (WRF-Chem) model embedded with an integrated process analysis scheme. Our results show that API and ARF reduced the daytime shortwave radiative fluxes at the surface by 92.4-102.9 Wm(-2) and increased daytime shortwave radiative fluxes in the atmosphere by 72.8-85.2 Wm(-2), as the values were averaged over the complex air pollution areas (CAPAs) in each of the four episodes. As a result, the stabilized atmosphere decreased the daytime planetary boundary layer height and 10 m wind speed by 129.0-249.0 m and 0.05-0.15 m s(-1), respectively, in CAPAs in the four episodes. Aerosols were simulated to reduce the daytime near-surface photolysis rates of J[NO2] and J[(OD)-D-1] by 1.8 x 10(-3) -2.0 x 10(-3) and 5.7 x 10(-6) -6.4 x 10(-6) s(-1), respectively, in CAPAs in the four episodes. All of the four episodes show the same conclusion, which is that the reduction in 03 by API is larger than that by ARF. API (ARF) was simulated to change daytime surface-layer O-3 concentrations by -8.5 ppb (parts per billion; -2.9 ppb), -10.3 ppb (-1.0 ppb), -9.1 ppb (-0.9 ppb), and -11.4 ppb (+0.7 ppb) in CAPAs of the four episodes, respectively. Process analysis indicated that the weakened O-3 chemical production made the greatest contribution to API effect, while the reduced vertical mixing was the key process for ARF effect. Our conclusions suggest that future PM2.5 reductions may lead to O-3 increases due to the weakened aerosol-radiation interactions, which should be considered in air quality planning.