Adsorption of HOx on aerosol surfaces: Implications for the atmosphere of Mars

The potential impact of heterogeneous chemistry on the abundance and distribution of HO x in the atmosphere of Mars has been assessed by combining observational data of dust and ice aerosol distributions with an updated photochemical model. Critical parameters include the altitude distributions of aerosols, and the surface loss coefficients (γ) of HO 2 on dust and ice in the lower atmosphere, and H on ice above 40 km. We find that adsorption of HO 2 on dust (γHO 2 ≥ 0.01), or ice near 30 km (γHO 2 ≥ 0.1), can deplete OH abundances in the lower atmosphere by 10% or more. Such depletions approach those obtained by lowering the water vapor abundance by an order of magnitude below the global average observed by Viking (≈ 25%). Since the oxidation of CO is catalyzed by HO x in the lower atmosphere via the reaction CO + OH → CO 2 + H, loss of OH due to adsorption of HO 2 on dust or ice at low altitudes could have a significant effect on the ratio CO : CO 2 . The adsorption of H on ice at 50 km (γ H ≥ 0.01) can result in even larger OH depletions. However, this effect is localized to altitudes > 40 km, where CO oxidation is relatively unimportant. Laboratory data suggest that γHO 2 ≈ 0.01 is a reasonable estimate for adsorption on dust. Larger values are plausible, but are not stongly supported by experimental evidence. The reactivity of HO 2 on ice is unknown, while γH on ice appears to be < 0.001. There is a need for measurements of HO x adsorption on surfaces representative of Martian aerosols at temperatures < 220 K.