HR: 0800h
AN: A51E-0830    [Abstracts]
TI: Surfactant Control of HCl and HBr Uptake Into Supercooled Sulfuric Acid
AU: * Park, S
EM: scpark1@wisc.edu
AF: University of Wisconsin-Madison, Department of Chemistry, 1101 University Ave., Madison, WI 53706-1322 United States
AU: Samuel, G
EM: glass@chem.wisc.edu
AF: University of Wisconsin-Madison, Department of Chemistry, 1101 University Ave., Madison, WI 53706-1322 United States
AU: Lawrence, J
EM: splan@chem.wisc.edu
AF: University of Wisconsin-Madison, Department of Chemistry, 1101 University Ave., Madison, WI 53706-1322 United States
AU: Nathanson, G
EM: nathanso@chem.wisc.edu
AF: University of Wisconsin-Madison, Department of Chemistry, 1101 University Ave., Madison, WI 53706-1322 United States
AB: Surfactant molecules on sulfuric acid droplets potentially alter the rates of heterogeneous reactions in the upper troposphere and lower stratosphere by blocking gas molecules from entering the acid. We perform molecular beam experiments with deuterated sulfuric acid solutions (56-68 wt % D$_{2}$SO$_{4}$/D$_{2}$O at 213 K) with varying concentrations of surfactants including 1-butanol and 1-hexanol, which segregate to the surface to form a nearly complete monolayer. We direct a beam of a protic gas HX (X = Cl or Br) at a continuously renewed film of supercooled D$_{2}$SO$_{4}$/D$_{2}$O in vacuum and measure the fraction of thermalized HX which undergo HX \rightarrow$ DX exchange. We have further shown that this HX \rightarrow$ DX exchange fraction is approximately equal to the probability of HX entering the acid. Our results appear to contradict the notion that surfactants generally impede gas transport. The presence of surface butanol does not alter the rate of D$_{2}$O evaporation from the liquid surface, whereas surface hexanol slightly does. Our most striking result is that the surface butanol molecules actually increase the HX \rightarrow$ DX exchange fraction, implying that HX dissociates more readily at the interface when butanol is present. This enhancement may be caused by the dilution of the acid near the surface by segregated butanol molecules, which provide additional OH groups for protonation by HX. For hexanol, we observe an enhancement in the HX \rightarrow$ DX exchange fraction at higher acidity and a small reduction at lower acidity.
DE: 0305 Aerosols and particles (0345, 4801)
DE: 0320 Cloud physics and chemistry
DE: 0341 Middle atmosphere--constituent transport and chemistry (3334)
DE: 0365 Troposphere--composition and chemistry
DE: 0368 Troposphere--constituent transport and chemistry
SC: Atmospheric Sciences [A]
MN: 2004 AGU Fall Meeting