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

The uptake of nitric acid on ice, solid sulfuric acid solutions and solid t ernary solutions was investigated in a Teflon-coated Knudsen flow reactor a t 180-211 K using mass spectrometric detection. The uptake coefficient, gam ma, decreases from 0.30 at 180 K to 0.06 at 211 K. The Arrhenius representa tion shows two distinct regimes. The first, at low temperatures (180-190 K) , shows a constant uptake coefficient, whereas the second, at high temperat ures (> 195 K), corresponds to an activation energy E-a = - 7 +/- 1 kcal mo l(-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 l inearly from 0.20 at 10 wt.% H2SO4 to 0.05 at 98 wt.% H2SO4 at 180 K. At 20 0 K gamma decreases from 0.10 to 0.03 over the same concentration range. Th e uptake coefficient for the heterogeneous interaction of HNO3 with solid t ernary solutions of H2SO4-HNO3-H2O is 0.10 +/- 0.03 in the temperature rang e 185-195 K under conditions where the composition of the interface was hel d constant using an external flow of H2O. HNO3 uptake on ice fitted a simpl e Langmuir-Hinshelwood model.