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

Abstract

[1] We present a study of the potential importance of known reaction pathways for HO 2 loss in atmospheric aerosols. As a baseline case, we calculate the reaction probability for HO 2 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 HO 2 loss by this process. An algebraic parameterization of the reaction probability, g HO2 , due to self-reaction is valid for aerosol pH < 6 and the existence of a homogeneous gas-phase HO x source greater than 1 Â 10 5 molec cm À3 s À1 . In this formulation g HO2 depends strongly on particle phase, size, pH and temperature; the latter causing g HO2 > 0.1 in the upper troposphere and g 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 g HO2 . Relative to simulations where g 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, HO 2 , and H 2 O 2 of À1, À2, and +2%, respectively. These minor effects of heterogeneous loss are significantly different from a simulation assuming g HO2 = 0.2 on all particles, as is currently recommended, with implications for predictions of regional HO x levels, ozone production rates and their sensitivity to NO x .Citation: Thornton, J. A., L. Jaeglé, and V. F. McNeill (2008), Assessing known pathways for HO 2 loss in aqueous atmospheric aerosols: Regional and global impacts on tropospheric oxidants,

Impacts of bromine and iodine chemistry on tropospheric OH and HO&lt;sub&gt;2&lt;/sub&gt;: comparing observations with box and global model perspectives

“…A range of aerosol uptake coefficients for HO 2 have been reported in the literature, with recent measurements indicating values of γ HO 2 between 0.003 and 0.02 on aqueous aerosols (George et al, 2013), while others have reported values of γ HO 2 ∼ 0.1 (Taketani et al, 2008) and increased uptake coefficients in the presence of Cu and Fe ions (Thornton et al, 2008;Mao et al, 2013). In this work we use a value of γ HO 2 = 0.1 in order to maintain consistency with previous modelling studies at the site and to account for potential impacts of ions of copper and iron in aerosol particles influenced by mineral dust Müller et al, 2010;Fomba et al, 2014;Matthews et al, 2014;Lakey et al, 2015).…”

“…Treatment of seasalt aerosol is described by Jaegle et al (2011). The uptake coefficient for N 2 O 5 uses the parameterisation by Evans and Jacob (2005), while that for HO 2 uses the parameterisation of Thornton et al (2008). A full description of the organic aerosol chemistry in the model is given by Heald et al (2011).…”

“…Anthropogenic emissions of CO, NO x and SO 2 are described by the EDGAR 3.2 monthly global inventory (Olivier et al, 2005). Emissions of volatile organic compounds (VOCs) are described by the RETRO monthly global inventory (van het Bolscher, 2008) for anthropogenic sources, with ethane emissions described by Xiao et al (2008), and the MEGAN v2.1 inventory (Guenther et al, 2006;Barkley et al, 2011) for biogenic sources.…”

Heterogeneous reaction of HO&lt;sub&gt;2&lt;/sub&gt; with airborne TiO&lt;sub&gt;2&lt;/sub&gt; particles and its implication for climate change mitigation strategies

“…At RH relevant to the lower stratosphere (<40%) the measurements showed that γ (HO 2 ) is in the range 0.020-0.028 at 295 K. An inverse temperature dependence of γ (HO 2 ) onto dry seasalt aerosols has previously been observed (Remorov et al, 2002); although there have been no systematic experimental studies of the temperature dependence of γ (HO 2 ), parameterisations have developed (Thornton et al, 2008;Macintyre and Evans, 2011). At stratospherically relevant temperatures (T = 200-220 K), γ (HO 2 ) is likely to be considerably larger than observed at 295 K; however it is not possible to cool the aerosol flow tube/SMPS system to verify this experimentally.…”

Measurements of uptake coefficients for heterogeneous loss of HO2 onto submicron inorganic salt aerosols

“…We plan to use the KM-SUB model to further investigate the HO 2 concentration dependence on the HO 2 uptake to aerosols to attempt to elucidate the reaction mechanisms involved. Tables 1 and 2 The HO 2 uptake coefficient obtained at much higher concentrations can be extrapolated to lower HO 2 concentrations using the currently understood aqueous chemistry of HO 2 , as outlined by Thornton et al 48 For conditions that resemble this work for (NH 4 ) 2 SO 4 aerosols, namely 293 K, HO2 = 0.4, [HO 2 ]~10 9 molecule cm -3 and assuming an aerosol pH=5, an extrapolated value  HO2~0 .002 was calculated using Equation (7) in Thornton et al 48 It should be noted that this calculation is extremely sensitive to aerosol pH due to the pH-dependent HO 2 solubility. For example, the calculated uptake coefficient for NaCl aerosols at pH~7 becomes  HO2~0 .12.…”