Measurement report: Fast photochemical production of peroxyacetyl nitrate (PAN) over the rural North China Plain during haze events in autumn

Yulu Qiu,Zhiqiang Ma,Tingting Han,Ping Tian,Huaigang Zhou,Hong Liao,Weiwei Pu,Mengyu Huang,Jia Zhu,Yingxiao Tang,Ke Li,Jiujiang Sheng

doi:10.5194/acp-21-17995-2021

Abstract

Abstract. Photochemical pollution over the North China Plain (NCP) is attracting much concern. We usually view peroxyacetyl nitrate (PAN) as the second most important photochemical pollutant featuring high mixing ratios during warm seasons. Our observations at a background site in the NCP identified high PAN concentrations, even during haze events in autumn. The substantial increasing ratios of PAN, by 244 % and 178 %, over the morning hours (08:00–12:00 local time) on 20 and 25 October 2020 were 10.6 and 7.7 times larger than those on clean days. Polluted days are characterized by higher temperature, higher humidity, and anomalous southerly winds compared with clean days. Enhanced local photochemistry has been identified as being the dominant factor that controls the PAN increase in the morning at the rural site, as the time when prevailing wind turns to a southerly wind is too late to promote direct transport of PAN from the polluted urban region. By removing the effect of direct transport of PAN, we provide a quantitative assessment of net PAN chemical production rate of 0.45 ppb h−1 for the mornings of polluted days, also demonstrating the strong local photochemistry. Using observations and calculated photolysis rates, we find that acetaldehyde oxidation by hydroxyl radical (OH) is the primary pathway of peroxyacetyl radical formation at the rural site. Acetaldehyde concentrations and production rates of HOx (HOx= OH + HO2) on polluted days are 2.8 and 2 times as large as those on clean days, leading to a remarkable increase in PAN in the morning. Formaldehyde (HCHO) photolysis dominates the daytime HOx production, thus contributing to fast photochemistry of PAN. Our observational results suggest the cause of a rapid increase in PAN during haze events in autumn at a rural site of the NCP and provide evidence of important role of HCHO photolysis in secondary pollutants at lower nitrogen oxide emissions. This highlights the urgency of carrying out strict volatile organic compound controls over the NCP during the cold season and not just in summer.

Highlights

Since the late 1960s, peroxyacetyl nitrate (PAN) has been identified as being a key photochemical pollutant in the atmosphere, having adverse effects on human health and vegetation (Heuss and Glasson, 1968; Taylor, 1969)
Our results suggest the cause of the rapid PAN increase over the rural North China Plain (NCP), and provide a new insight into assessing the impacts of high reactive volatile organic compounds (VOCs) from urban plumes on the atmospheric oxidation capacity and secondary pollutant formation over the rural NCP
Compared with recent studies in China (Table S2), the observed PAN concentration at the SDZ site is generally lower than 1.5–1.89 ppb over the urban NCP (Zhang et al, 2017; Liu et al, 2018) and 2.05 ppb in urban southwestern China (Sun et al, 2020), comparable to 0.93–1.04 ppb in the suburban NCP region

Summary

Introduction

Since the late 1960s, peroxyacetyl nitrate (PAN) has been identified as being a key photochemical pollutant in the atmosphere, having adverse effects on human health and vegetation (Heuss and Glasson, 1968; Taylor, 1969). Y. Qiu et al.: Fast photochemical production of PAN over the rural North China Plain ondary pollutant formed through reactions between peroxyacetyl radical (CH3C(O)O2; PA) and nitrogen dioxide (NO2; Xue et al, 2014).

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Conclusion