Heterogeneous uptake of N₂O₅ in sand dust and urban aerosols observed during the dry season in Beijing

The uptake of dinitrogen pentoxide (N₂O₅) on aerosols affects the nocturnal removal of NO_x and particulate nitrate formation in the atmosphere. This study investigates N₂O₅ uptake processes using field observations from an urban site in Beijing during April-May 2017, a period characterized by dry weather conditions. For the first time, a very large N₂O₅ uptake rate (k(N₂O₅) up to ~0.01 s^-1) was observed during a sand storm event, and the uptake coefficient ( (N₂O₅)) was estimated to be 0.044. The γ(N₂O₅) in urban air masses was also determined and exhibited moderate correlation (r = 0.68) with aerosol volume to surface ratio (V_a/S_a), but little relation to aerosol water, nitrate, and chloride, a finding that contrasts with previous results. Several commonly used parameterizations of γ(N₂O₅) underestimated the field-derived γ(N₂O₅). A new parameterization is suggested for dry conditions, which considers the effect of V_a/S_a, temperature, and relative humidity.