Articles | Volume 16, issue 7
Research article
07 Apr 2016
Research article |  | 07 Apr 2016

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

Tian Feng, Naifang Bei, Ru-Jin Huang, Junji Cao, Qiang Zhang, Weijian Zhou, Xuexi Tie, Suixin Liu, Ting Zhang, Xiaoli Su, Wenfang Lei, Luisa T. Molina, and Guohui Li

Abstract. In this study, the ozone (O3) formation in China's northwest city of Xi'an and surrounding areas is investigated using the Weather Research and Forecasting atmospheric chemistry (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. The model generally performs well compared to measurements in simulating the surface temperature, relative humidity, and wind speed and direction, near-surface O3 and PM2.5 mass concentrations, and aerosol constituents. High aerosol concentrations in Xi'an and surrounding areas significantly decrease the photolysis frequencies and can reduce 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 (volatile organic compound)-sensitive chemistry. The industrial emissions contribute the most to the O3 concentrations compared to biogenic and other anthropogenic sources, but neither individual anthropogenic emission nor biogenic emission plays a dominant role in the O3 formation. Under high O3 and PM2.5 concentrations, a 50 % reduction in all the anthropogenic emissions only decreases near-surface O3 concentrations by about 14 % during daytime. The complicated O3 production regime and high aerosol levels pose a challenge for O3 control strategies in Xi'an and surrounding areas. Further investigation regarding O3 control strategies will need to be performed, taking into consideration the rapid changes in anthropogenic emissions that are not reflected in the current emission inventories and the uncertainties in the meteorological field simulations.

Short summary
The occurrence of high O3 levels with high PM2.5 concentrations constitutes a dilemma for the design of O3 control strategies in Xi’an and surrounding areas. If the O3 mitigation approach decreases aerosols in the atmosphere directly or indirectly, the enhanced photolysis caused by aerosol reduction would compensate for the O3 loss. If only the PM2.5 control strategy is implemented, the O3 pollution will decrease.
Final-revised paper