A high ozone event over Beijing after the May 2017 Belt and Road Forum

Abstract The Beijing area experienced daily ozone increases after the Belt and Road Forum in May 2017, with the maximum hourly O3 peak exceeding 400 μg/m3. The high O3 event was analyzed by using observed data and model simulations. Results indicated that the local photochemistry was the major driver boosting O3 formation for northern suburban sites and regional transport in the late afternoon made another peak in O3 diurnal variation for urban sites. A prominent VOC-limited sensitivity regime was dominant in the Beijing area, even over suburban areas far away from the Beijing urban center. The rapid increase in VOCs emissions from both biogenic and anthropogenic sources under high air temperatures provided the needed VOCs for O3 formation, resulting in a dramatic O3 increase after the summit. It was also found that PM2.5 experienced steady daily increases after the summit, leading to a gradual decrease in the UV radiation measured at the ground; however, O3 showed a dramatically increasing trend, in contrast to the expectation that the high-level PM2.5 would restrain the O3 formation by the theoretical deduction in the O3 and aerosol interaction. The results suggest that the abrupt change in emissions during the study period overwhelmed the effects of the O3 and aerosol interaction. Such temporary changes in emissions should be considered in mitigation strategies of O3 control.