Variation and Trend of Nitrate radical reactivity towards volatile organic compounds in Beijing, China

Abstract

Abstract. Nitrate radical (NO3) is an important nocturnal atmospheric oxidant in the troposphere, which significantly affects the lifetime of pollutants emitted by anthropogenic and biological activities, especially volatile organic compounds (VOC). Here, we used one-year VOC observation data obtained in urban Beijing in 2019 to look insight to the level, compositions and seasonal variation of NO3 reactivity (kNO3). We show the hourly kNO3 towards measured VOC highly varied from <10-4 to 0.083 s-1 with campaign-averaged value (± standard deviation) of 0.0032 ± 0.0042 s-1. There was large seasonal difference in NO3 reactivity towards VOC with the average of 0.0024 ± 0.0026 s-1, 0.0067 ± 0.0066 s-1, 0.0042 ± 0.0037 s-1, 0.0027 ± 0.0028 s-1 from spring to 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 the percentage of 39.8 %. Sensitivity study shows monoterpenes, the species we did not measure, may account a large fraction of kNO3. Based on the correlation between the calculated kNO3 and VOC concentrations in 2019, we established localized parameterization schemes for predicting the reactivity by only using a part of VOC species. The historical published VOC data was collected to reconstruct the long-term NO3 reactivity in Beijing by the parameterization method. The downward trend of kNO3 during 2011–2020 may be responded to the reduction of anthropogenic VOC emission. At last, we revealed that NO3 dominated the nocturnal VOC oxidation with 83 % on the annual average in Beijing in 2019, which varied seasonally and was strongly regulated by the level of kNO3, nitrogen oxide and ozone. Our results improve the understanding of nocturnal atmospheric oxidation in urban regions, and gain the knowledge of nocturnal VOC oxidation and secondary organic pollution.