Measurements of HO2 uptake to aqueous aerosol:: Mass accommodation coefficients and net reactive loss -: art. no. D08309

[1] 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)(2)SO4 at 40 - 45% RH. By doping the aerosol with Cu(II) ( similar to 0.1 M in aerosol) to create an efficient aerosol sink, we determine lower limits to the mass accommodation coefficient, alpha(HO2), to be 0.8 +/- 0.3 for H2SO4 and 0.5 +/- 0.1 for (NH4)(2)SO4 particles. In the absence of Cu( II), net reactive loss of HO2 on H2SO4 aerosol was slow, and we measured a reaction probability, gamma(HO2), less than 0.01. In contrast, loss of HO2 to (NH4)(2)SO4 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 x 10(7) M-1 s(-1) in good agreement with an estimate of 2 +/- 1 x 10(7) M-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 similar to 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.