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, gamma, for secondary organic aerosol (SOA) particles and for the well-studied model compound sucrose which we doped with copper(II). Above 65% relative humidity (RH), gamma for copper(II)-doped sucrose aerosol particles equalled the surface mass accommodation coefficient alpha = 0.22 +/- 0.06, but it decreased to gamma = 0.012 +/- 0.007 upon decreasing the RH to 17 %. The trend of gamma with RH can be explained by an increase in aerosol viscosity and the contribution of a surface reaction, as demonstrated using the kinetic multilayer model of aerosol surface and bulk chemistry (KM-SUB). At high RH the total uptake was driven by reaction in the near-surface bulk and limited by mass accommodation, whilst at low RH it was limited by surface reaction. SOA from two different precursors, alpha-pinene and 1,3,5-trimethylbenzene (TMB), was investigated, yielding low uptake coefficients of gamma < 0.001 and gamma = 0.004 +/- 0.002, respectively. It is postulated that the larger values measured for TMB-derived SOA compared to alpha-pinene-derived SOA are either due to differing viscosity, a different liquid water content of the aerosol particles, or an HO2 + RO2 reaction occurring within the aerosol particles.