ROx Budgets and O3 Formation during Summertime at Xianghe Suburban Site in the North China Plain

Photochemical smog characterized by high concentrations of ozone (O-3) is a serious air pollution issue in the North China Plain (NCP) region, especially in summer and autumn. For this study, measurements of O-3, nitrogen oxides (NOx), volatile organic compounds (VOCs), carbon monoxide (CO), nitrous acid (HONO), and a number of key physical parameters were taken at a suburban site, Xianghe, in the NCP region during the summer of 2018 in order to better understand the photochemical processes leading to O-3 formation and find an optimal way to control O-3 pollution. Here, the radical chemistry and O-3 photochemical budget based on measurement data from 1-23 July using a chemical box model is investigated. The daytime (0600-1800 LST) average production rate of the primary radicals referred to as ROx (OH + HO2 + RO2) is 3.9 ppbv h(-1). HONO photolysis is the largest primary ROx source (41%). Reaction of NO2 + OH is the largest contributor to radical termination (41%), followed by reactions of RO2 + NO2 (26%). The average diurnal maximum O-3 production and loss rates are 32.9 ppbv h(-1) and 4.3 ppbv h(-1), respectively. Sensitivity tests without the HONO constraint lead to decreases in daytime average primary ROx production by 55% and O-3 photochemical production by 42%, highlighting the importance of accurate HONO measurements when quantifying the ROx budget and O-3 photochemical production. Considering heterogeneous reactions of trace gases and radicals on aerosols, aerosol uptake of HO2 contributes 11% to ROx sink, and the daytime average O-3 photochemical production decreases by 14%. The O-3-NOx-VOCs sensitivity shows that the O-3 production at Xianghe during the investigation period is mainly controlled by VOCs.