Articles | Volume 16, issue 20
Atmos. Chem. Phys., 16, 13035–13047, 2016
Atmos. Chem. Phys., 16, 13035–13047, 2016

Research article 21 Oct 2016

Research article | 21 Oct 2016

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

Pascale S. J. Lakey1,2, Thomas Berkemeier2, Manuel Krapf3, Josef Dommen3, Sarah S. Steimer3, Lisa K. Whalley1,4, Trevor Ingham1,4, Maria T. Baeza-Romero5, Ulrich Pöschl2, Manabu Shiraiwa2,6, Markus Ammann3, and Dwayne E. Heard1,4 Pascale S. J. Lakey et al.
  • 1School of Chemistry, University of Leeds, Woodhouse Lane, Leeds, LS2 9JT, UK
  • 2Multiphase Chemistry Department, Max Planck Institute for Chemistry, Hahn-Meitner-Weg 1, 55128 Mainz, Germany
  • 3Paul Scherrer Institute, Villigen, Switzerland
  • 4National Centre for Atmospheric Chemistry, University of Leeds, Woodhouse Lane, Leeds, LS2 9JT, UK
  • 5Escuela de Ingeniería Industrial de Toledo, Universidad de Castilla la Mancha, Avenida Carlos III s/n Real Fábrica de Armas, 45071 Toledo, Spain
  • 6Department of Chemistry, University of California, Irvine, CA 92617, USA

Abstract. We report the first measurements of HO2 uptake coefficients, γ, 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), γ for copper(II)-doped sucrose aerosol particles equalled the surface mass accommodation coefficient α  =  0.22 ± 0.06, but it 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 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, α-pinene and 1,3,5-trimethylbenzene (TMB), was investigated, yielding low 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 an HO2 + RO2 reaction occurring within the aerosol particles.

Short summary
Chemical oxidation in the atmosphere removes pollutants and greenhouse gases but generates undesirable products such as secondary organic aerosol. Radicals are key intermediates in oxidation, but how they interact with aerosols is still not well understood. Here we use a laser to measure the loss of radicals onto oxidised aerosols generated in a smog chamber. The loss of radicals was controlled by the thickness or viscosity of the aerosols, confirmed by using sugar aerosols of known thickness.
Final-revised paper