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&#8211;May 2017, a period characterized by dry weather conditions. For the first time, a very large N₂O₅ uptake rate (<i>k</i>(N₂O₅) up to ~0.01 s^&#8722;1) was observed during a sand storm event, and the uptake coefficient (&#947;(N₂O₅)) was estimated to be 0.044. The &#947;(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 &#947;(N₂O₅) underestimated the field-derived &#947;(N₂O₅). A new parameterization is suggested for dry conditions, which considers the effect of V_a/S_a, temperature, and relative humidity.