Variation and trend of nitrate radical reactivity towards volatile organic compounds in Beijing, China

Nitrate radical (NO3) is an important nocturnal atmospheric oxidant in the troposphere that significantly affects the lifetime of pollutants emitted by anthropogenic and biogenic activities, especially volatile organic compounds (VOCs). Here, we used 1 year of VOC observation data obtained in urban Beijing in 2019 to look into the level, composition, and seasonal variation in NO3 reactivity (kNO(3)). We show that hourly kNO(3) towards measured VOC varied widely from < 10(-4) to 0.083 s(-1) with a campaign-average value ( +/- standard deviation) of 0.0032 +/- 0.0042 s(-1). There was large seasonal difference in NO3 reactivity towards VOC with averaged values (+/- standard deviation) of 0:0024 +/- 0:0026 s(-1) (spring), 0:0067 +/- 0:0066 s(-1) (summer), 0:0042 +/- 0:0037 s(-1) (autumn), and 0:0027 +/- 0:0028 s(-1) (winter). Alkenes such as isoprene and styrene accounted for the majority. Isoprene was the dominant species in spring, summer, and autumn, accounting for 40.0 %, 77.2 %, and 43.2 %, respectively. Styrene only played a leading role in winter, with a percentage of 39.8 %. A sensitivity study shows monoterpenes, the species we did not measure, may account for a large fraction of kNO(3). Based on the correlation between the calculated kNO(3) and VOC concentrations in 2019, we established localized parameterization schemes for predicting the reactivity by only using a part of VOC species. The historically published VOC data were collected using the parameterization method to reconstruct the long-term kNO(3) in Beijing. The lower kNO(3) during 2014-2021 compared with that during 2005-2013 may be attributed to anthropogenic VOC emission reduction. Finally, we revealed that NO3 dominated the nocturnal VOC oxidation, with 83% of the annual average in Beijing in 2019, which varied seasonally and was strongly regulated by the level of kNO(3), nitrogen oxide, and ozone. Our results improve the understanding of nocturnal atmospheric oxidation in urban regions and contribute to our knowledge of nocturnal VOC oxidation and secondary organic pollution.