Assessing known pathways for HO2 loss in aqueous atmospheric aerosols:: Regional and global impacts on tropospheric oxidants

We present a study of the potential importance of known reaction pathways for HO2 loss in atmospheric aerosols. As a baseline case, we calculate the reaction probability for HO2 loss by its self-reaction in aqueous particles. Detailed calculations assessed the effects of aerosol pH, temperature, particle size, and aqueous phase diffusion limitations on the rate of HO2 loss by this process. An algebraic parameterization of the reaction probability, gamma(HO2), due to self-reaction is valid for aerosol pH < 6 and the existence of a homogeneous gas-phase HOx source greater than 1 x 10(5) molec cm(-3) s(-1). In this formulation gamma(HO2) depends strongly on particle phase, size, pH and temperature; the latter causing gamma(HO2) > 0.1 in the upper troposphere and gamma(HO2) < 0.01 in the extra-polar lower troposphere. We contrast the self-reaction pathway with catalytic oxidation by dissolved Cu ions. Using IMPROVE network data we assess the atmospheric importance and uncertainties associated with the Cu pathway. Simulations of tropospheric chemistry were performed using the GEOS-Chem global chemical transport model with different parameterizations of gamma(HO2). Relative to simulations where gamma(HO2) = 0 for all aerosol types, assuming that only the aqueous-phase self- reaction proceeds on pollution and sea salt particles causes global annual mean differences in surface OH, HO2, and H2O2 of -1, -2, and +2%, respectively. These minor effects of heterogeneous loss are significantly different from a simulation assuming gamma(HO2) = 0.2 on all particles, as is currently recommended, with implications for predictions of regional HOx levels, ozone production rates and their sensitivity to NOx.