High N₂O₅ Concentrations Observed in Urban Beijing: Implications of a Large Nitrate Formation Pathway

The heterogeneous hydrolysis of dinitrogen pentoxide (N₂O₅) is important to understanding the formation of particulate nitrate (pNO₃^-). Measurements of N₂O₅ in the surface layer taken at an urban site in Beijing are presented here. N₂O₅ was observed with large day-to-day variability. High N₂O₅ concentrations were determined during pollution episodes with the co-presence of large aerosol loads. The maximum value was 1.3 ppbv (5 s average), associated with an air mass characterized by a high level of O₃. N₂O₅ uptake coefficients were estimated to be in the range of 0.025-0.072 using the steady-state lifetime method. As a consequence, the nocturnal pNO₃^- formation potential by N₂O₅ heterogeneous uptake was calculated to be 24-85 μg m^-3 per night and, on average, 57 μg m^-3 during days with pollution. This was comparable to or even higher than that formed by the partitioning of HNO₃. The results highlight that N₂O₅ heterogeneous hydrolysis is vital in pNO₃^- formation in Beijing.