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

The nitrate radical (NO3) 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 NO3 in inland regions of China, while fewer studies target marine regions. Here, we present a field measurement of the NO3 reservoir, dinitrogen pentoxide (N2O5), 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 similar to 84 %, and the airmass from eastern coastal areas (CAM) with similar to 16 %. During the IAM period, the NO3 production rate reached 1.6 +/- 0.9 ppbv h(-1) due to the transportation of the polluted urban plume with high NOx and O-3. The average nocturnal N2O5 and the calculated NO3 mixing ratios were 119.5 +/- 128.6 and 9.9 +/- 12.5 pptv, respectively, and the steady-state lifetime of NO3 was 0.5 +/- 0.7 min on average, indicating intensive nighttime chemistry and rapid NO3 loss at this site. By examining the reaction of NO3 with volatile organic compounds (VOCs) and N2O5 heterogeneous hydrolysis, we revealed that these two reaction pathways were not responsible for the NO3 loss (< 20 %) since the NO3 reactivity (k(NO3)) towards VOCs was small ( 5.2 x 10(-3)s(-1)) and the aerosol loading was low. Instead, NO was proposed to significantly contribute to nocturnal NO3 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 NO3 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 NOx was much lower. The NO3 production was still very fast, with a rate of 1.2 ppbv h(-1). With the absence of N2O5 measurement in this period, the NO3 reactivity towards VOCs and N2O5 uptake were calculated to assess NO3 loss processes. We showed that the average k(NO3) from VOCs (56.5 %, 2.6 +/- 0.9 x 10(-3) s(-1)) was higher than that from N2O5 uptake (43.5 %, 2.0 +/- 1.5 x 10(-3) s(-1)) during the CAM period, indicating a longer NO3 / N2O5 lifetime than that during IAM period. This study improves the understanding of the nocturnal NO3 budget and environmental impacts with the interaction of anthropogenic and natural activities in marine regions.