Assessment of summertime O3 formation and the O3-NOX-VOC sensitivity in Zhengzhou, China using an observation-based model.

Zhengzhou, the provincial capital of Henan province in Central China and a major hub of the country's transportation network, has been suffering from severe summertime ozone (O3) pollution. Simultaneous field measurements of O3 and its precursors, including NOx, CO, HONO, and 106 volatile organic compounds (VOCs), were conducted at an urban site (the municipal environmental monitoring station, MEM) in Zhengzhou in July 2019. The Community Multiscale Air Quality (CMAQ) model, which incorporates the Master Chemical Mechanism (MCMv3.3.1), was modified to work as a 0-D observation-based photochemical box model to assess the sources and sinks of HOx radicals and O3, and the OH reactivity (KOH) and ozone formation potential (OFP) of major VOC groups. In addition, the O3-NOx-VOC sensitivity was evaluated using the relative incremental reactivity (RIR) and O3 formation isopleth techniques. The OH radicals were mainly generated from the propagation reaction of HO2 + NO (91-95%). The daily average mixing ratios of OH and HO2 radicals were significantly higher during high O3 days, reaching as high as 4.8 × 106 and 7.7 × 108 molecules cm-3, respectively. Photochemical O3 formation was mostly due to the conversion of NO to NO2 by HO2 radicals (52-54%), while the NO2 + OH reaction was the main contributor to O3 destruction (70- 76%). Alkenes and aromatics were the main anthropogenic VOC contributors to KOH and OFP. Contributions of biogenic VOCs became much more important on high O3 days, correlating with the increase in temperature and solar radiation. RIR analysis showed that the O3 formation was under the VOC-limited on low O3 days but was in the transition regime during the O3 pollution buildup and persisting days. These results are generally consistent with those based on the O3 formation isopleth. This paper provides important corroborative scientific evidence urgently needed by local governments to formulate O3 pollution control strategies.