Laboratory studies on the heterogeneous chemistry of clay minerals in the Earth's atmosphere

Atmospheric mineral aerosol is a potentially important reactive surface that may provide a heterogeneous sink for gas phase species such as nitric acid and oxygenated organic compounds in the Earth's troposphere. Smectite clays, such as montmorillonite, are particularly interesting reactive surfaces because they are commonly found in the atmosphere and have a unique ability to swell. The swelling properties of montmorillonite allow for substantial adsorbed water under humid conditions, possibly promoting further reactivity. The heterogeneous uptake of water, nitric acid and a series of small organic acids on Na-montmorillonite clay under upper tropospheric temperatures and humidities was studied in a high vacuum chamber equipped with a quadrupole mass spectrometer (MS) and a transmission Fourier transform infrared (FT-IR) spectrometer used to detect the gas and condensed phases, respectively.; Water adsorption on montmorillonite clay was measured using FT-IR as a function of relative humidity (RH) with respect to liquid water at temperatures from 212 to 232 K. The specific surface area and adsorbed water content of the swollen clay were determined and are consistent with previous results from gravimetric methods at room temperature. Thus, water adsorption appears to be independent of temperature down to upper tropospheric temperatures. However, the amount of adsorbed water and swollen surface area was found to increase significantly as the RH was raised. Na-montmorillonite was found to contain 10% water by mass at 50% RH and the observed growth curve is comparable to that of ammonium sulfate, a well characterized hygroscopic species. Thus, swelling clays entrained in the Earth's atmosphere may be important cloud condensation nuclei and may indirectly affect the Earth's climate.; The heterogeneous uptake of the C1 to C4 organic acids on Na-montmorillonite clay was studied at 212 K as a function of RH, from 0% to 45% RH, organic acid pressure and clay mass. While the initial uptake was found to be independent of organic acid pressure, linear dependence of the initial uptake on clay mass was observed. However, the organic acid content was found to increase significantly with increasing acid pressure. A significant enhancement in the organic acid content, water content and the initial uptake coefficients was observed as the RH was raised. Additionally, the presence of organic acids was found to slightly enhance the water content of the clay above 45% RH. Results suggest that that heterogeneous uptake of organic acids on swelling clay minerals is an important heterogeneous removal mechanism for carboxylic acids in the atmosphere at all RH values studied, and probably more so at higher humidities.; The heterogeneous uptake of nitric acid on Na-montmorillonite clay at low temperatures as a function of RH, nitric acid pressure and clay mass has also been studied. Below 16% RH, uptake of nitric acid on Na-montmorillonite was below the detection limit. However, similar to the organic acids, the nitric acid content, water content and initial uptake coefficient all increase with increasing RH. Additionally, these values were all found to be independent of temperature from 210 to 232 K and independent of pressure from 1x10 -5 to 3x10-4 Torr nitric acid. However, a linear dependence on mass was observed for small sample masses. The adsorbed water content of Na-montmorillonite clay was also significantly enhanced compared to experiments performed when only water was present. Results suggest that heterogeneous uptake of nitric acid on swelling clay minerals is an important heterogeneous removal mechanism in the troposphere under humid conditions when clay is expected to be swollen.; Higher in the atmosphere, it is currently questionable as to whether mineral particles are effective nuclei for polar stratospheric clouds (PSCs), which play a crucial role in polar ozone loss. The heterogeneous formation of nitric acid trihydrate (NAT) on Na-mont...