Measurement report: Atmospheric nitrate radical chemistry in the South China Sea influenced by the urban outflow of the Pearl River Delta

Wang, Jie; Wang, Haichao; Tham, Yee Jun; Ming, Lili; Zheng, Zelong; Fang, Guizhen; Sun, Cuizhi; Ling, Zhenhao; Zhao, Jun; Fan, Shaojia

The nitrate radical (NOinline-formula3) is a critical nocturnal atmospheric oxidant in the troposphere, which widely affects the fate of air pollutants and regulates air quality. Many previous works have reported the chemistry of NOinline-formula3 in inland regions of China, while fewer studies target marine regions. Here, we present a field measurement of the NOinline-formula3 reservoir, dinitrogen pentoxide (Ninline-formula2Oinline-formula5), and related species at a typical marine site (Da Wan Shan Island) located in the South China Sea in the winter of 2021. Two patterns of air masses were captured during the campaign, including the dominant airmass from inland China (IAM) with a percentage of inline-formula∼ 84 %, and the airmass from eastern coastal areas (CAM) with inline-formula∼ 16 %. During the IAM period, the NOinline-formula3 production rate reached 1.6 inline-formula± 0.9 ppbv hinline-formula−1 due to the transportation of the polluted urban plume with high NOinline-formulax and Oinline-formula3. The average nocturnal Ninline-formula2Oinline-formula5 and the calculated NOinline-formula3 mixing ratios were 119.5 inline-formula± 128.6 and 9.9 inline-formula± 12.5 pptv, respectively, and the steady-state lifetime of NOinline-formula3 was 0.5 inline-formula± 0.7 min on average, indicating intensive nighttime chemistry and rapid NOinline-formula3 loss at this site. By examining the reaction of NOinline-formula3 with volatile organic compounds (VOCs) and Ninline-formula2Oinline-formula5 heterogeneous hydrolysis, we revealed that these two reaction pathways were not responsible for the NOinline-formula3 loss (inline-formula< 20 %) since the NOinline-formula3 reactivity (inline-formulak(NOinline-formula3)) towards VOCs was small (inline-formula M29inlinescrollmathml normal 5.2 × normal 10 - normal 3 51pt14ptsvg-formulamathimg94ffc44f9641ee4e070f864f704e2fb7 acp-24-977-2024-ie00001.svg51pt14ptacp-24-977-2024-ie00001.png  sinline-formula−1) and the aerosol loading was low. Instead, NO was proposed to significantly contribute to nocturnal NOinline-formula3 loss at this site, despite the nocturnal NO concentration always below the parts per billion by volume level and near the instrument detection limit. This might be from the local soil emission or something else. We infer that the nocturnal chemical NOinline-formula3 reactions would be largely enhanced once without NO emission in the open ocean after the air mass passes through this site, thus highlighting the strong influences of the urban outflow to the downwind marine areas in terms of nighttime chemistry. During the CAM period, nocturnal ozone was higher, while NOinline-formulax was much lower. The NOinline-formula3 production was still very fast, with a rate of 1.2 ppbv hinline-formula−1. With the absence of Ninline-formula2Oinline-formula5 measurement in this period, the NOinline-formula3 reactivity towards VOCs and Ninline-formula2Oinline-formula5 uptake were calculated to assess NOinline-formula3 loss processes. We showed that the average inline-formulak(NOinline-formula3) from VOCs (56.5 %, 2.6 inline-formula± 0.9 inline-formula× 10inline-formula−3 sinline-formula−1) was higher than that from Ninline-formula2Oinline-formula5 uptake (43.5 %, 2.0 inline-formula± 1.5 inline-formula× 10inline-formula−3 sinline-formula−1) during the CAM period, indicating a longer NOinline-formula3inline-formula M55inlinescrollmathml / 8pt14ptsvg-formulamathimgab921b68c9e3b26bd3fdb564f7f7e866 acp-24-977-2024-ie00002.svg8pt14ptacp-24-977-2024-ie00002.png  Ninline-formula2Oinline-formula5 lifetime than that during IAM period. This study improves the understanding of the nocturnal NOinline-formula3 budget and environmental impacts with the interaction of anthropogenic and natural activities in marine regions.

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Wang, Jie / Wang, Haichao / Tham, Yee Jun / et al: Measurement report: Atmospheric nitrate radical chemistry in the South China Sea influenced by the urban outflow of the Pearl River Delta. 2024. Copernicus Publications.

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