Efficient N 2 O 5 uptake and NO 3 oxidation in the outflow of urban Beijing

Abstract. Nocturnal reactive nitrogen compounds play an important role in regional air pollution. Here we present the measurements of dinitrogen pentoxide ( N 2 O 5 ) associated with nitryl chloride ( ClNO 2 ) and particulate nitrate ( p NO 3 - ) at a suburban site of Beijing in the summer of 2016. High levels of N 2 O 5 and ClNO 2 were observed in the outflow of the urban Beijing air masses, with 1 min average maxima of 937 and 2900 pptv , respectively. The N 2 O 5 uptake coefficients, γ , and ClNO 2 yield, f , were experimentally determined from the observed parameters. The N 2 O 5 uptake coefficient ranged from 0.012 to 0.055, with an average of 0.034 ± 0.018, which is in the upper range of previous field studies reported in North America and Europe but is a moderate value in the North China Plain (NCP), which reflects efficient N 2 O 5 heterogeneous processes in Beijing. The ClNO 2 yield exhibited high variability, with a range of 0.50 to unity and an average of 0.73 ± 0.25. The concentration of the nitrate radical ( NO 3 ) was calculated assuming that the thermal equilibrium between NO 3 and N 2 O 5 was maintained. In NO x -rich air masses, the oxidation of nocturnal biogenic volatile organic compounds (BVOCs) was dominated by NO 3 rather than O 3 . The production rate of organic nitrate (ON) via NO 3 + BVOCs was significant, with an average of 0.10 ± 0.07 ppbv h −1 . We highlight the importance of NO 3 oxidation of VOCs in the formation of ON and subsequent secondary organic aerosols in summer in Beijing.