Heterogeneous uptake of the C₁ to C₄ organic acids on a swelling clay mineral

Abstract

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. Sub-monolayer coverage of organic acid on montmorillonite was observed under dry conditions and relevant organic acid pressures. However, the organic acid content increased significantly with increasing humidity. Additionally, 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. Results from this study show that the initial uptake efficiency for n-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 uptake of formic, acetic and propionic acids increase only slightly at higher humidities. Additionally, the presence of organic acids was found to 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 heterogeneous sink for these species and may modify the cloud forming potential of the clay particles.