Heterogeneous uptake of the C1 to C4 organic acids on a swelling clay mineral C. D. Hatch, R. V. Gough, and M. A. Tolbert University of Colorado, Department of Chemistry and Biochemistry and the Cooperative Institute for Research in Environmental Sciences, CIRES Room 318, Boulder, Colorado 80309, USA
Abstract. Mineral aerosol is of interest due to its physiochemical impacts on the
Earth's atmosphere. However, adsorbed organics could influence the chemical
and physical properties of atmospheric mineral particles and alter their
impact on the biosphere and climate. In this work, the heterogeneous uptake
of a series of small organic acids on the swelling clay, Na-montmorillonite,
was studied at 212 K as a function of relative humidity (RH), organic acid
pressure and clay mass. A high vacuum chamber equipped with a quadrupole
mass spectrometer and a transmission Fourier transform infrared spectrometer
was used to detect the gas and condensed phases, respectively. Our results
show that while the initial uptake efficiency was found to be independent of
organic acid pressure, it increased linearly with increasing clay mass.
Thus, the small masses studied allow access to the entire surface area of
the clay sample with minimal effects due to surface saturation.
Additionally, results from this study show that the initial uptake
efficiency for butanoic (butyric) acid on the clay increases by an order of
magnitude as the RH is raised from 0% to 45% RH at 212 K while the
initial uptake efficiency of formic, acetic and propanoic (propionic) acids
increases only slightly at higher humidities. However, the initial uptake
efficiency decreases significantly in a short amount of time due to surface
saturation effects. Thus, although the initial uptake efficiencies are
appropriate for initial times, the fact that the uptake efficiency will
decrease over time as the surface saturates should be considered in
atmospheric models. Surface saturation results in sub-monolayer coverage of
organic acid on montmorillonite under dry conditions and relevant organic
acid pressures that increases with increasing humidity for all organic acids
studied. Additionally, the presence of large organic acids may slightly
enhance the water content of the clay above 45% RH. Our results indicate
that heterogeneous uptake of organic acids on swelling clay minerals
provides an important irreversible heterogeneous sink for these species.
Citation: Hatch, C. D., Gough, R. V., and Tolbert, M. A.: Heterogeneous uptake of the C1 to C4 organic acids on a swelling clay mineral, Atmos. Chem. Phys., 7, 4445-4458, doi:10.5194/acp-7-4445-2007, 2007.