The Humidity Dependence of N2O5 Uptake to Citric Acid Aerosol Particles

Dinitrogen pentoxide is a significant reactive intermediate in the night time chemistry of nitrogen oxides. Depending on atmospheric conditions it can act either as a NO₃ radical reservoir or as a major NO_x sink by heterogeneous hydrolysis on aerosol surfaces. As such, it can influence tropospheric ozone production and therefore the oxidative capacity of the atmosphere. The heterogeneous loss of N₂O₅ to aerosol particles has remained uncertain, and reconciling lab and field data has demonstrated some gaps in our understanding of the detailed mechanism. We used the short-lived radioactive tracer ¹³N to study N₂O₅ uptake kinetics on aerosol particles in an aerosol flow reactor at ambient pressure, temperature and relative humidity. Citric acid, representing strongly oxidized polyfunctional organic compounds in atmospheric aerosols, has been chosen as a proxy due to its well established physical properties. Aerosol uptake measurements were performed with citric acid aerosols in a humidity range of 15-75 % RH, within which the uptake coefficient varies between about 0.001 and about 0.02. Taking into account the well established hygroscopic properties of citric acid, we interpret uptake in terms of disproportionation of N₂O₅ into nitrate ion and nitronium ion and reaction of the latter with liquid water.