The kinetics of the uptake of HNO3 on ice, solid H2SO4–H2O and solid ternary solutions of H2SO4–HNO3–H2O in the temperature range 180–211 K

Abstract

The uptake of nitric acid on ice, solid sulfuric acid solutions and solid ternary solutions was investigated in a Teflon-coated Knudsen flow reactor at 180–211 K using mass spectrometric detection. The uptake coefficient, γ, decreases from 0.30 at 180 K to 0.06 at 211 K. The Arrhenius representation shows two distinct regimes. The first, at low temperatures (180–190 K), shows a constant uptake coefficient, whereas the second, at high temperatures (>195 K), corresponds to an activation energy E_a = − 7 ± 1 kcal mol⁻¹. At a fixed temperature the uptake coefficient is independent of the concentration of HNO₃, thus confirming a rate law first order in [HNO₃]. The uptake coefficient of HNO₃ on binary H₂SO₄–H₂O solid solutions decreases linearly from 0.20 at 10 wt.% H₂SO₄ to 0.05 at 98 wt.% H₂SO₄ at 180 K. At 200 K γ decreases from 0.10 to 0.03 over the same concentration range. The uptake coefficient for the heterogeneous interaction of HNO₃ with solid ternary solutions of H₂SO₄–HNO₃–H₂O is 0.10 ± 0.03 in the temperature range 185–195 K under conditions where the composition of the interface was held constant using an external flow of H₂O. HNO₃ uptake on ice fitted a simple Langmuir–Hinshelwood model.