Summertime ozone formation in Xi'an and surrounding areas, China

Abstract

In the study, the ozone (O3) formation is investigated in Xi'an and surrounding areas, China using the WRF-CHEM model during the period from 22 to 24 August 2013 corresponding to a heavy air pollution episode with high concentrations of O3 and PM2.5 (particulate matter with aerodynamic diameter less than 2.5 μm). The WRF-CHEM model generally performs well in simulating the surface temperature and relative humidity compared to the observations and also reasonably reproduces the observed temporal variations of the surface wind speed and direction. The convergence formed in Xi'an and surrounding areas is favorable for the accumulation of pollutants, causing high concentrations of O3 and PM2.5. In general, the calculated spatial patterns and temporal variations of near-surface O3 and PM2.5 are consistent well with the measurement at the ambient monitoring stations. The simulated daily mass concentrations of aerosol constituents, including sulfate, nitrate, ammonium, elemental and organic carbon, are also in good agreement with the filter measurements. High aerosol concentrations in Xi'an and surrounding areas significantly decrease the photolysis frequencies and can reduce near-surface O3 concentrations by more than 50 μg m-3 (around 25 ppb) on average. Sensitivity studies show that the O3 production regime in Xi'an and surrounding areas is complicated, varying from NOx to VOC-sensitive chemistry. The industry emissions contribute the most to the O3 concentrations compared to the natural and other anthropogenic sources, but still do not play a determined role in the O3 formation. The complicated O3 production regime and high aerosol levels constitute a dilemma for O3 control strategies in Xi'an and surrounding areas. In the condition with high O3 and PM2.5 concentrations, decreasing various anthropogenic emissions cannot efficiently mitigate the O3 pollution, and a 50 % reduction of all the anthropogenic emissions only decreases near-surface O3 concentrations by less than 14 % during daytime. Further studies need to be performed for O3 control strategies considering manifest changes of the emission inventory and uncertainties of meteorological field simulations.