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Abstract:
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.
