HR: 0800h
AN: A51E-0827    [Abstracts]
TI: Uptake of C$_{1}$ $-$ C$_{5}$ Monocarboxylic Acids on Ammonium Nitrate as a Function of Temperature and Relative Humidity.
AU: * Shilling, J E
EM: john.shilling@colorado.edu
AF: CIRES and The Department of Chemistry and Biochemistry, University of Colorado Campus Box 216, CIRES Room 318, Boulder, CO 80309 United States
AU: Tolbert, M A
EM: tolbert@colorado.edu
AF: CIRES and The Department of Chemistry and Biochemistry, University of Colorado Campus Box 216, CIRES Room 318, Boulder, CO 80309 United States
AB: The uptake of C$_{1}$ $-$ C$_{5}$ monocarboxylic acids on ammonium nitrate (AN) has been investigated as a function of temperature and relative humidity using a Knudsen cell flow reactor coupled with FTIR-Reflection Absorption Spectroscopy (FTIR-RAS). Initial uptake coefficients ($\gamma$'s) and coverages ($\Theta$'s) were determined over the temperature range 200 to 240 K. The uptake of the C$_{1}$ $-$ C$_{4}$ acids was quite efficient but temperature dependent, with larger coverages and uptake coefficients being observed at lower temperatures. The values of $\gamma$ and $\Theta$ were also dependent on the carbon chain length, with the shorter chain acids displaying larger coverages but smaller uptake coefficients at a given temperature. Valeric acid (C$_{5}$) did not adsorb onto ammonium nitrate under any of the conditions studied. Infrared spectra revealed that the acids ionized on the surface, despite the fact that the ammonium nitrate films were effloresced. The initial uptake coefficients were analyzed using a precursor-mediated adsorption model to determine $\Delta$H$_{ads}$ and $\Delta$S$_{ads}$ values for each species. Adding small amounts of water vapor (4% RH) to the chamber resulted in unlimited uptake and dramatically increased values of $\gamma$ and $\Theta$. Higher relative humidity resulted in further increases of both $\gamma$ and $\Theta$. The IR spectra revealed that the organic acids ionized on the surface. Furthermore, the IR spectra revealed that a liquid layer is formed when many of the acids adsorbed onto the film at RH $>$ 10%. These liquid water features were not observed at a similar relative humidity in the absence of the acids. These studies suggest that uptake of organic acids on AN dramatically increases the water uptake properties of the inorganic salt.
DE: 0305 Aerosols and particles (0345, 4801)
DE: 0320 Cloud physics and chemistry
DE: 0345 Pollution--urban and regional (0305)
DE: 0365 Troposphere--composition and chemistry
SC: Atmospheric Sciences [A]
MN: 2004 AGU Fall Meeting