Measurement report: Variations and environmental impacts of atmospheric N ₂ O ₅ concentrations in urban Beijing during the 2022 Winter Olympics

Abstract. The chemistry of nitrate radical (NO3) and dinitrogen pentoxide (N2O5) plays a pivotal role in tropospheric nighttime chemistry. Given their close linkage to precursor variations, emission reduction during the 2022 Beijing Winter Olympics likely affected NO3 and N2O5 behavior. In this study, we measured N2O5, NO2, O3, etc. during and after the Olympics, and compared pollutant levels as well as the contributions of reaction pathways to the loss of NO3 and N2O5. Throughout the entire observation period, NO3 production rate averaged 0.5 ± 0.4 ppbv h−1, and the N2O5 mixing ratio could reach up to 875 pptv within 1 min, indicating their active production. The relatively long nighttime N2O5 lifetime (τN2O5), with an average of 11.9 ± 11.8 min, suggested a slow N2O5 loss rate during winter season. Despite low NO mixing ratio (below 3 ppbv), it dominated NO3 loss (79.0 %). VOCs oxidation contributed 0.2 %, primarily driven by styrene. During the Olympics, emission reductions led to decreased NO and VOCs, which in turn reduced their reaction with NO3. The heterogeneous uptake of N2O5, another key NO3 loss pathway – accounted for 20.8 % of NO3 loss during the Olympics, but this contribution decreased to 10.6 % after the Olympics. This uptake is crucial for nighttime NO3 removal and would be essential for winter nitrate formation in urban Beijing. Our results highlight that under emission control scenarios, the relative importance of heterogeneous processes in nocturnal NO3 cycling increases, providing new insights into how emission reduction measures shape nighttime oxidation processes in polluted urban environments.