O3 and OH Sensitivity to heterogeneous sinks of HOx and CH3O2 on aerosol particles

Abstract

A new modification of matrix-isolation EPR has been developed for the measurement of uptake coefficients of polyatomic radicals. The technique is combined either with a cylinder reactor having a movable source of radicals or a coaxial reactor having a movable rod and a fixed source of radicals. The walls of the cylinder reactor or the rod surface are covered with the aerosol material. A simple way to treat the experimental data on the basis of the kinetic resistance additivity formula is proved. The results of HO₂ and CH₃O₂ uptake coefficient measurements are given and the possible implications for atmospheric chemistry are discussed. The measured value of the HO₂ diffusion coefficient was closer to D_H2O2 than to D_O2. The HO₂ uptake coefficient on the concentrated 80 and 96% w/w H₂SO₄ was >0.2 at 243 K. For NaCl, KCl, NH₄NO₃ and (NH₄)₂SO₄, the γ_HO2 values were in the range (1–2)× 10^–2 and γ_CH3O2= 0.3 × 10^–2 at 295 K for NaCl. No changes in γ_HO2 were found at [HO₂]= 4 × 10⁹–3 × 10¹¹ cm^–3. The results are discussed from the viewpoint of stratospheric ozone in volcanic clouds and tropospheric hydroxyl abundance in urban regions.