Abstract
The heterogeneous uptake of gaseous N2O5 by ammonium sulfate [(NH4)2SO4], ammonium bisulfate [NH4HSO4], and sulfuric acid [H2SO4] aerosols as a function of relative humidity has been investigated at room temperature and atmospheric pressure. Ammonium-containing aerosols were generated by a constant-output atomizer and conditioned by passing through a diffusion dryer. Sulfuric acid aerosols were produced by the homogeneous reaction of SO3 and H2O in a borosilicate vessel. Addition of a dry or wet N2 flow controlled the relative humidity (RH) of these aerosol flows. Using a chemical ionization mass spectrometer (CIMS) for N2O5 concentration monitoring and a scanning mobility particle spectrometer (SMPS) for aerosol characterization, reaction probabilities (γ) in the range of 0.001 to 0.1 for the uptake of N2O5 were determined as a function of RH. The results are expressed as follows: γ[(NH4)2SO4] = 2.79 × 10-4 + 1.30 × 10-4 × (RH) − 3.43 × 10-6 × (RH)2 + 7.52 × 10-8 × (RH)3, γ[NH4HSO4] = 2.07 × 10-3 − 1.48 × 10-4 × (RH) + 8.26 × 10-6 × (RH)2, and γ[H2SO4] = 0.052 − 2.79 × 10-4 × (RH). We suggest that the water content and phase in the ammonium-containing aerosols control the reactivity of N2O5 while liquid-phase ionic reactions primarily dominate the uptake in sulfuric acid aerosols.
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