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

Measurements of the reactive uptake coefficient for N2O5 hydrolysis, gamma(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 gamma(m) on solid malonic acid was determined to be less than 0.001 (RH = 10 - 50%), and on solid azelaic acid, gamma(m) was 0.0005 +/- 0.0003. Aqueous malonic acid aerosol yielded gamma(m) = 0.0020 +/- 0.0005 at 10% RH and increased with RH to similar to 0.03 at RH = 50 - 70%. We report the first evidence of an inverse dependence of the gamma(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 similar to 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.