Effect of aerosol and boundary-layer interactions on surface-layer concentrations of aerosols and O3 in Baoding during a severely polluted event in July 2014

Aerosols can influence boundary layer through their direct and indirect radiative effects and consequently feedback to surface-layer concentrations of aerosols and ozone (O₃). There were many studies that examined aerosol radiative effects on concentrations of aerosols, but few of them studied such feedback effects on O₃ air quality. Here we use the Weather Research and Forecasting model with online chemistry (WRF-Chem) to quantify aerosol radiative effects (direct and/or indirect effects) on surface concentrations of PM_2.5 and O₃ during a heavily polluted event in Baoding (a city in North China) during 28 July to 3 August 2014. During this pollution episode, the observed concentrations of PM_2.5 and O₃ in Baoding averaged over the studied period were 122.6 μg m^-3 and 58 ppbv, respectively. The WRF-Chem model can reproduce the temporal and spatial variations of PM_2.5, O₃, and meteorological parameters reasonably well. The correlation coefficients between simulated and observed concentrations are 0.7 for PM_2.5 and 0.9 for O₃, and the normal mean biases between simulated and observed concentrations are -6.7% for PM_2.5 and 6.2% for O₃. We find that aerosol radiative effect reduced the surface-layer O₃ concentration, temperature at 2 m, and planetary boundary-layer height by up to 7 ppbv, 0.6℃ and 97.6 m, respectively, but increased relative humidity and surface PM_2.5 concentrations by up to 2.5％ and 6 µg m^-3, respectively. Process analyses are carried out to identify how the radiative effects of aerosols influenced O₃ concentrations in the pollution event through altering boundary layer and hence the chemical production/loss, transport, and deposition of O₃.