Nighttime N 2 O 5 chemistry in an urban site of Beijing in winter based on the measurements by cavity ring-down spectroscopy

Dinitrogen pentoxide (N 2 O 5 ) plays an important role in nighttime chemistry. A field campaign was conducted to measure N 2 O 5 by cavity ring-down spectroscopy method from November 13 to December 3, 2016, in an urban site in Beijing. During the campaign, there were only 4 days that N 2 O 5 concentrations were above the detection limit due to high NO emissions near the ground. The average N 2 O 5 concentration in these 4 days was 23.4 ± 25.9 pptv. The calculated NO 3 production rate varied between 0.03 and 6.26 ppbv/h, with a mean value of 0.94 ± 0.83 ppbv/h, and the average N 2 O 5 lifetime was 2.5 ± 2.5 min, indicating the N 2 O 5 chemistry in urban Beijing during winter time was active. The different loss pathways of N 2 O 5 were discussed. The results show that the loss of N 2 O 5 in winter urban area mainly attributed to the direct heterogeneous uptake reaction of N 2 O 5 , and indirect loss of NO 3 reaction with NO, which contributed 28–89.6% and 9.8–65% to the overall N 2 O 5 loss, respectively. Indirect loss reaction of NO 3 with VOC was of lesser importance. Biogenic VOCs were the dominant NO 3 reaction partners, and alkenes along with aromatics contributed less to NO 3 reactivity.