Heterogeneous Uptake of HO2 Radicals onto Atmospheric Aerosols

The hydroxyl (OH) and hydroperoxyl (HO₂) radicals, together known as HO_x, play a vital role in atmospheric chemistry by controlling the oxidative capacity of the troposphere. The atmospheric lifetime and concentrations of many trace reactive species, such as volatile organic compounds (VOCs), are determined by HO_x radical levels. Therefore, the ability to accurately predict atmospheric HO_x concentrations from a detailed knowledge of their sources and sinks is a very useful diagnostic tool to assess our current understanding of atmospheric chemistry. Several recent field studies have observed significantly lower concentrations of HO₂ radicals than predicted using box models, where HO₂ loss onto aerosols was suggested as a possible missing sink [1, 2]. However, the mechanism on HO₂ uptake onto aerosols and its impact on ambient HO_x levels are currently not well understood. To improve our understanding of this process, we have conducted laboratory experiments to measure HO₂ uptake coefficients onto submicron aerosol particles. The FAGE (Fluorescence Assay by Gas Expansion) technique, a highly sensitive laser induced fluorescence based detection method, was used to monitor HO₂ uptake kinetics onto aerosol particles in an aerosol flow tube. The application of the FAGE technique allowed for kinetic experiments to be performed under low HO₂ concentrations, i.e. [HO₂] < 10⁹ molecules cm^-3. HO₂ radicals were produced by the photolysis of water vapour in the presence of O₂ and aerosol particles were produced either by atomizing dilute salt solutions or by homogeneous nucleation. HO₂ uptake coefficients (γ) have been measured for single-component solid and aqueous inorganic salt and organic aerosol particles with a wide range of hygroscopicities. HO₂ uptake coefficients on solid particles were below the detection limit (γ < 0.001), whereas on aqueous aerosols uptake coefficients were somewhat larger (γ = 0.001 - 0.008). HO₂ uptake coefficients were highest on aerosols containing metal ions, such as Cu and Fe. Humidity and aerosol pH did not significantly impact the reactive HO₂ uptake. Preliminary experiments have also been conducted to study the temperature dependence of the uptake coefficients. These results suggest that particle phase and transition metal concentration are the most important factors to consider when modeling the impact of heterogeneous uptake onto aerosols as a HO_x sink. References [1] R. Sommariva, A.-L.Haggerstone, L. J. Carpenter, N. Carslaw, D. J. Creasey, D. E. Heard, J. D. Lee, A. C. Lewis, M. J. Pilling, J. Zador, Atmos. Chem. Phys. 4 (2004) 839. [2] Y. Kanaya, R. Cao, S. Kato, Y. Miyakawa, Y. Kajii, H. Tanimoto, Y. Yokouchi, M. Mochida, K. Kawamura, H. J. Akimoto, Geophys. Res. 112 (2007) D11308.