An estimate of the potential significance of heterogeneous loss to aerosols as an additional sink for hydroperoxy radicals in the troposphere

A simple analysis has been performed to estimate the potential significance for tropospheric chemistry of heterogeneous loss of hydroperoxy (HO2) radicals to aerosols. By comparing the magnitude of the heterogeneous loss as a function of mass accommodation coefficient and particle number density to the gas-phase chemical loss of HO2 radicals for urban and remote marine scenarios, it is shown that this process may be Bn important loss mechanism for HO2 radicals in the troposphere. Under typical urban conditions, the magnitude of heterogeneous loss of HO2 may be a significant fraction of the gas-phase chemical loss. This loss process may also be important for remote tropospheric chemistry in that for typical remote marine conditions where NOx and O-3 concentrations are low, heterogeneous loss may be nearly the same order of magnitude as the gas-phase chemical loss. Two major uncertainties are inherent in the current analysis. One is the value of the mass accommodation coefficient for HO2 uptake to aerosols. Measurements (Hanson et al., 1992; Mozurkewich et al., 1987) indicate that this parameter is most likely > 0.05 and may, in fact, approach unity; however, more laboratory investigations are necessary, especially ones specifically designed to investigate this loss process under typical tropospheric conditions. The second uncertainty is the fate of HO2 radicals upon being accommodated into an aerosol particle. The current analysis is based on the simplifying assumption that HO2 is destroyed irreversibly immediately upon entering the aerosol phase. Potential mechanisms for this aerosol-phase destruction have been identified but not definitively tested. A more detailed analysis, including quantitative estimates of these aerosol-phase HO2 reactions, is necessary to further evaluate the feasibility of this process for tropospheric conditions. Additionally, analysis of coincident measurements of gas-phase species concentrations and aerosol physical and chemical properties are necessary to further examine the significance of this loss process in tropospheric photochemistry. (C) 1997 Elsevier Science Ltd.