Nighttime N2O5 chemistry in an urban site of Beijing in winter based on the measurements by cavity ring-down spectroscopy

Dinitrogen pentoxide (N2O5) plays an important role in nighttime chemistry. A field campaign was conducted to measure N2O5 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 N2O5 concentrations were above the detection limit due to high NO emissions near the ground. The average N2O5 concentration in these 4 days was 23.4 +/- 25.9 pptv. The calculated NO3 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 N2O5 lifetime was 2.5 +/- 2.5 min, indicating the N2O5 chemistry in urban Beijing during winter time was active. The different loss pathways of N2O5 were discussed. The results show that the loss of N2O5 in winter urban area mainly attributed to the direct heterogeneous uptake reaction of N2O5, and indirect loss of NO3 reaction with NO, which contributed 28-89.6% and 9.8-65% to the overall N2O5 loss, respectively. Indirect loss reaction of NO3 with VOC was of lesser importance. Biogenic VOCs were the dominant NO3 reaction partners, and alkenes along with aromatics contributed less to NO3 reactivity.

Automated summary

The study found that in the winter urban area of Beijing, the loss of N2O5 is mainly attributed to direct heterogeneous uptake and indirect loss reactions, with the indirect loss reaction of NO3 with NO contributing significantly, while the reaction of NO3 with VOC contributed less to N2O5 loss.