Measurements of the HO2 uptake coefficient onto aqueous salt and organic aerosols and interpretation using the kinetic multi-layer model of aerosol surface and bulk chemistry (KM-SUB)

Abstract:

HO₂ is closely coupled with OH which is responsible for the majority of the oxidation in the troposphere. Therefore, it is important to be able to accurately predict OH and HO₂ concentrations. However, many studies have reported a large discrepancy between HO₂ radical concentrations measured during field campaigns and predicted by constrained box models using detailed chemical mechanisms (1,2). However, there have been very few laboratory studies (3,4) on HO₂ uptake by aerosols and the rates and mechanism is still uncertain.

The HO₂ uptake coefficients were measured for deliquesced ammonium nitrate and sodium chloride aerosols and copper doped sucrose aerosols. The measurements were performed using an aerosol flow tube coupled to a Fluorescence Assay by Gas Expansion (FAGE) detector. By either placing the HO₂ injector in set positions and varying the aerosol concentration or by moving it along the flow tube at given aerosol concentrations, uptake coefficients could be measured. The aerosols were generated using an atomiser and the total aerosol surface area was measured using a SMPS.

Larger uptake coefficients were measured at shorter times and lower HO₂ concentrations for aqueous salt aerosols. The time dependence was able to be modelled by the KM-SUB model (5) as the HO₂ concentration decreases along the flow tube and the HO₂ uptake mechanism is known to be a second order reaction. Measurements have shown that at higher HO₂ concentrations there was also more H₂O₂ exiting the injector which could convert back to HO₂ if trace amounts of metals are present within the aerosol via Fenton reactions. Preliminary results have shown that the inclusion of a Fenton-like reaction within the KM-SUB model has the potential to explain the apparent HO₂ concentration dependence. Finally, the KM-SUB model has been used to demonstrate that the increase in uptake coefficient observed when increasing the relative humidity for copper doped sucrose aerosols could be explained by an increase in the diffusion coefficient of HO₂ in the aerosol.

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(2) Whalley, L.K. et al. Atmos. Chem. Phys. 2010, 10, 1555-1576.

(3) Taketani, F. et al. J. Phys. Chem. 2008, 112, 2370-2377.

(4) Thornton, J. et al. J. Geophys. Atm. 2005, 110, D08309.

(5) Shiraiwa, M. et al. Atmos. Chem. Phys. 2010, 10, 3673–3691.