The effect of viscosity and diffusion on the HO₂ uptake by sucrose and secondary organic aerosol particles

We report the first measurements of HO₂ uptake coefficients, γ, for secondary organic aerosol particles (SOA) and for the well-studied model compound sucrose which was doped with copper. Above 65% relative humidity (RH), γ for copper doped sucrose aerosol particles equalled the surface mass accommodation coefficient α= 0.22 ± 0.06 but decreased to γ = 0.012 ± 0.007 upon decreasing the RH to 17 %. The trend of γ with RH can be explained by an increase in aerosol viscosity, as demonstrated using the kinetic multi-layer model of aerosol surface and bulk chemistry (KM-SUB). SOA from two different precursors, α-pinene and 1,3,5- trimethylbenzene (TMB), was investigated, yielding small uptake coefficients of γ < 0.001 and γ = 0.004 ± 0.002, respectively. It is postulated that the larger values measured for TMB derived SOA compared to α-pinene derived SOA are either due to differing viscosity, a different liquid water content of the aerosol particles or a HO₂ + RO₂ reaction occurring within the aerosol particles.