Measurements of HO2 uptake to aqueous aerosol: Mass accommodation coefficients and net reactive loss

Abstract

We present room temperature measurements of HO2 uptake to aqueous submicron aerosol obtained with an entrained aerosol flow tube coupled to a chemical ionization mass spectrometer. Two aqueous aerosol types were examined, H2SO4 at 35–40% relative humidity (RH) and (NH4)2SO4 at 40–45% RH. By doping the aerosol with Cu(II) (∼0.1 M in aerosol) to create an efficient aerosol sink, we determine lower limits to the mass accommodation coefficient, αHO2, to be 0.8 ± 0.3 for H2SO4 and 0.5 ± 0.1 for (NH4)2SO4 particles. In the absence of Cu(II), net reactive loss of HO2 on H2SO4 aerosol was slow, and we measured a reaction probability, γHO2, less than 0.01. In contrast, loss of HO2 to (NH4)2SO4 aerosol, buffered to pH = 5.1 but without Cu(II), was efficient in our experiment. For this system, aerosol‐induced loss of gas‐phase HO2 was observed to follow second‐order kinetics, and we infer from our measurements a second‐order aqueous phase reaction rate coefficient of 1 ± 0.25 × 107M−1 s−1 in good agreement with an estimate of 2 ± 1 × 107M−1 s−1 based on literature values of HO2 aqueous‐phase chemical parameters. These results imply that heterogeneous loss of HO2 to submicron aqueous sulfate aerosol will be strongly temperature dependent with negligible contribution to odd hydrogen radical (HOx) removal rates at temperatures warmer than ∼270 K, but potentially large contributions to HOx loss (e.g., >50% of total HOx loss) in colder regions of the troposphere depending on the available aqueous aerosol volume.