N2O5 hydrolysis on sub-micron organic aerosols: the effect of relative humidity, particle phase, and particle size

Abstract

Measurements of the reactive uptake coefficient for N₂O₅ hydrolysis, γ_m, on sub-micron organic aerosols were performed in an entrained aerosol flow tube as a function of relative humidity (RH), aerosol phase, N₂O₅ partial pressure, and mean aerosol size. Aerosol phase and relative humidity were determined simultaneously, and chemical ionization mass spectrometry was used to detect the decay rate of N₂O₅ in the presence of malonic acid or azelaic acid aerosol. The γ_m on solid malonic acid was determined to be less than 0.001 (RH = 10–50%), and on solid azelaic acid, γ_m was 0.0005 ± 0.0003. Aqueous malonic acid aerosol yielded γ_m = 0.0020 ± 0.0005 at 10% RH and increased with RH to ∼0.03 at RH = 50–70%. We report the first evidence of an inverse dependence of the γ_m on the initial partial pressure of N₂O₅ in the flow reactor, and a dependence on particle size for aerosol with surface area-weighted radii less than ∼100 nm at 50% RH. We find that the super-saturated malonic acid aerosol results are consistent with N₂O₅ hydrolysis being both aerosol volume-limited where, for RH < 50%, water is the limiting reagent, and also with a surface-specific process.