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 NOx 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 (γ(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 (Va/Sa), 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 Va/Sa, temperature, and relative humidity.