Heterogeneous Uptake of Gaseous N₂O₅by (NH₄)₂SO₄, NH₄HSO₄, and H₂SO₄Aerosols

Abstract

The heterogeneous uptake of gaseous N 2 O 5 by ammonium sulfate [(NH 4 ) 2 SO 4 ], ammonium bisulfate [NH 4 HSO 4 ], and sulfuric acid [H 2 SO 4 ] 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 SO 3 and H 2 O in a borosilicate vessel. Addition of a dry or wet N 2 flow controlled the relative humidity (RH) of these aerosol flows. Using a chemical ionization mass spectrometer (CIMS) for N 2 O 5 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 N 2 O 5 were determined as a function of RH. The results are expressed as follows: γ [(NH 4 ) 2 SO 4 ] = 2.79 × 10 -4 + 1.30 × 10 -4 × (RH) − 3.43 × 10 -6 × (RH) 2 + 7.52 × 10 -8 × (RH) 3, γ [NH 4 HSO 4 ] = 2.07 × 10 -3 − 1.48 × 10 -4 × (RH) + 8.26 × 10 -6 × (RH) 2, and γ [H 2 SO 4 ] = 0.052 − 2.79 × 10 -4 × (RH). We suggest that the water content and phase in the ammonium-containing aerosols control the reactivity of N 2 O 5 while liquid-phase ionic reactions primarily dominate the uptake in sulfuric acid aerosols.