HR: 0800h
AN: A51E-0825    [Abstracts]
TI: Measurements of HO$_{2}$ Uptake to Sulfuric Acid and Ammonium Sulfate Aerosol: Mass Accommodation Coefficients and Net Reactive Loss
AU: * Thornton, J A
EM: thornton@atmos.washington.edu
AF: Department of Atmospheric Sciences, University of Washington, Seattle, 408 ATG Building UW Mailbox 351640, Seattle, WA 98195 United States
AU: Abbatt, J P
EM: jabbatt@chem.utoronto.ca
AF: Department of Chemistry, University of Toronto, 80 St. George St., Toronto, ON M5S 3H6 Canada
AB: We present room temperature measurements of HO$_{2}$ uptake to aqueous submicron H$_{2}$SO$_{4}$ and (NH$_{4}$)$_{2}$SO$_{4}$ aerosol particles at 35% and 40% relative humidity, respectively, obtained with an entrained aerosol flow tube coupled to a chemical ionization mass spectrometer. By doping the aerosol with Cu(II) ($\sim$ 0.1 M in aerosol) to create an efficient aerosol sink, we determine lower limits to the mass accommodation coefficient to be 0.8 $\pm$ 0.3 for H$_{2}$SO$_{4}$ and 0.5 $\pm$ 0.1 for (NH$_{4}$)$_{2}$SO$_{4}$ particles. In the absence of Cu(II), net reactive loss of HO$_{2}$ on H$_{2}$SO$_{4}$ aerosol was slow, and we measured a reaction probability less than 0.01. In contrast, loss of HO$_{2}$ to (NH$_{4}$)$_{2}$SO$_{4}$ aerosol, buffered to pH = 5.1 but without Cu(II), was efficient in our experiment. For this system, aerosol-induced loss of gas-phase HO$_{2}$ was observed to follow second-order kinetics and we infer from our measurements a second-order aqueous phase reaction rate coefficient of 1 $\pm$ 0.25 x 10$^{7}$ M$^{-1}$ s$^{-1}$ in good agreement with an estimate of 2 $\pm$ 1 x 10$^{7}$ M$^{-1}$ s$^{-1}$ based on literature values of HO$_{2}$ aqueous-phase chemical parameters. These results imply that heterogeneous loss of HO$_{2}$ to submicron aqueous sulfate aerosol will be strongly temperature dependent with negligible contribution to odd hydrogen radical (HO$_{x}$) removal rates at temperatures warmer than $\sim$ 270 K, but potentially large contributions to HO$_{x}$ loss (e.g., $>$ 50% of total HO$_{x}$ loss) in colder regions of the troposphere depending on the available aqueous aerosol volume. We discuss these results and the importance of heterogeneous chemistry on tropospheric HO$_{x}$ budgets.
DE: 0305 Aerosols and particles (0345, 4801)
DE: 0317 Chemical kinetic and photochemical properties
DE: 0320 Cloud physics and chemistry
DE: 0365 Troposphere--composition and chemistry
DE: 0368 Troposphere--constituent transport and chemistry
SC: Atmospheric Sciences [A]
MN: 2004 AGU Fall Meeting