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)

Monday, 15 December 2014
Pascale Sylvie Jeanne Matthews1, Thomas Berkemeier2, Ingrid J George3, Lisa K Whalley1, Daniel R Moon1, Markus Ammann4, Maria T Baeza-Romero5, Ulrich Poeschl6, Manabu Shiraiwa2 and Dwayne E Heard7, (1)University of Leeds, School of Chemistry, Leeds, LS2, United Kingdom, (2)Max Planck Institute for Chemistry, Mainz, Germany, (3)Environmental Protection Agency Research Triangle Park, Durham, NC, United States, (4)Paul Scherrer Institute, Villingen, Switzerland, (5)Universidad de Castilla la Mancha, Toledo, Spain, (6)Max Planck Inst. f. Chemistry, Mainz, Germany, (7)University of Leeds, School of Chemistry, Leeds, United Kingdom
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.

(1) Sommariva, R. et al. Atmos. Chem. Phys.2006, 6, 1135-1153.

(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.