Understanding the formation of high-ozone episodes at Raoyang, a rural site in the north China plain

Abstract

Abstract An observational field campaign was carried out from 20 June to July 31, 2016 at Raoyang, a rural site in the North China Plain (NCP). In-situ measurements of O3 and related gases were obtained and analyzed in this work. Heavy photochemical pollution episodes lasting for a few days were observed in the campaign. The episodes could be divided into two categories, with one being influenced by plume from northeast (NE) via Beijing-Tianjin cities and the other from southeast (SE) via Shandong province. The SE episodes were more frequent with higher O3 concentrations and lagged daily peaks of O3 despite of lower concentrations of precursors. Such episodes could last for several days or even a week. In contrast, the NE episode was associated with relatively lower O3 levels and earlier daily O3 peaks though the levels of precursors were higher. The WRF-Chem model was used to study the differences between typical SE and NE episodes in regional distributions of O3 concentrations, chemical regimes of O3 formation, ozone production efficiency (OPE), photochemical age of plume, and evolution of O3 concentration within the boundary layer. Our results suggest that the O3 production regimes were different for the Raoyang site, with the NE episode being VOCs-sensitive and the SE episode in the VOC-sensitive to NOx-sensitive transition regime. In both episodes, a major part of the NCP was under the VOCs-sensitive regime. Plumes in the SE episodes were more aged than those in the NE episode. The OPE values were higher in more aged SE plumes, resulting in higher O3 concentrations despite of lower precursor concentrations. Our results also indicate that downward mixing of O3 in the nocturnal residual layer played an important role in rapid buildup of surface O3 in the next morning in both NE and SE episodes. The formation of photochemical smog seems to be particularly effective in the SE episode since O3 is rapidly produced during daytime, stored aloft at night, and mixed downward to the ground-level, and the process can circulate for several consecutive days. Our study highlights the complicated role of chemical reaction, transport and boundary layer processes in the formation of severe photochemical smog in the region.