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

Abstract

Photochemical smog characterized by high concentrations of ozone (O₃) is a serious air pollution issue in the North China Plain (NCP) region, especially in summer and autumn. For this study, measurements of O₃, nitrogen oxides (NO_x), 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 O₃ formation and find an optimal way to control O₃ pollution. Here, the radical chemistry and O₃ 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 RO_x (OH + HO₂ + RO₂) is 3.9 ppbv h⁻¹. HONO photolysis is the largest primary RO_x 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 O₃ 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 RO_x production by 55% and O₃ photochemical production by 42%, highlighting the importance of accurate HONO measurements when quantifying the RO_x budget and O₃ photochemical production. Considering heterogeneous reactions of trace gases and radicals on aerosols, aerosol uptake of HO₂ contributes 11% to RO_x sink, and the daytime average O₃ photochemical production decreases by 14%. The O₃-NO_x-VOCs sensitivity shows that the O₃ production at Xianghe during the investigation period is mainly controlled by VOCs.

摘 要

夏、秋季节以高浓度臭氧(O₃)为主的光化学烟雾是华北地区的主要空气污染问题。本研究以2018年7月华北地区香河郊区站的O₃,亚硝酸(HONO),氮氧化物(NO_x)、挥发性有机化合物(VOCs)、一氧化碳(CO)的观测浓度,以及气象和光解系数观测资料为基础,以观测资料约束NCAR-MM化学箱模式,研究RO_x自由基(OH、HO₂和RO₂)收支和O₃光化学生成。研究结果表明:香河站白天时段(0600¬¬¬–1800 LST),新生RO_x自由基的平均值为3.9ppbv h^-1。其中,HONO光解的贡献最大,占到新生RO_x自由基的41%。RO_x自由基终止反应中, NO₂和OH生成HNO₃的反应占到了RO_x消耗的41%,RO₂和NO₂反应生成PAN从而造成RO_x的净消耗反应占到了26%。香河站白天O₃光化学生成的日平均最大值为32.9ppb h^-1,O₃光化学消耗最大可以达到4.5ppb h^-1。模式敏感性试验表明:不使用实测HONO浓度值做模拟约束时,白天时段新生RO_x降低了55%,O₃化学生成降低了42%,说明HONO的观测很重要,直接影响RO_x自由基化学和O₃生成。模拟时考虑气体和自由基在气溶胶表面的非均相反应过程,则HO₂在气溶胶表面的非均相反应占到RO_x自由基消耗的11%,O₃化学生成降低了14%,由此表明非均相过程不能忽略。O₃-NO_x-VOCs的EKMA曲线表明,观测期间香河站受VOC控制,可以减排VOC降低臭氧生成。