Increasing wintertime ozone levels and secondary aerosol formation in the Guanzhong basin, central China.

The observed near-surface ozone (O) concentration has been remarkably increasing during recent years in winter in the Guanzhong basin, central China, showing a continuous enhancement of the atmospheric oxidizing capacity (AOC). The impact of such a change in the AOC on secondary aerosol formation, however, has not yet been assessed. In this study, we simulate the formation of O and airborne particles in the atmosphere using the WRF-Chem model, in which the AOC is calculated quantitatively, to understand the responses of secondary aerosols to the AOC increase. Meteorological observations, air pollutants including O, NO, SO, CO, and PM concentrations at ambient monitoring sites, and the main compositions of submicron particulates measured using ACSM are used to constrain the model simulation. The model result shows that the population hourly and postmeridian O (=O + NO) concentrations are good indicators for the wintertime AOC in the basin, suggested by the significantly positive correlations between them. Sensitivity experiments present that the AOC changes may exert important influences on fine particle (PM) concentration with an average rate of 1.94 (μg m)/(10 cm s) for Δ(PM)/Δ(AOC), which is mostly caused by the mass changes in secondary organic aerosol (43%) and nitrate aerosol (40%) and less attributed to the ammonium (11%) and sulfate (6%) components.