| Abstract
| - Airborne clay mineral particles have long atmospheric lifetimes due to their relatively small size. To assesstheir impact on trace atmospheric gases, we investigated heterogeneous reactions on prototype clay minerals.Diffuse reflectance infrared spectroscopy identified surface-adsorbed products formed from the uptake ofgaseous nitric acid and nitrogen dioxide on kaolinite and pyrophyllite. For kaolinite, a 1:1 phyllosilicate,HNO3 molecularly adsorbed onto the octahedral aluminum hydroxide and tetrahedral silicon oxide surfaces.Also detected on the aluminum hydroxide surface were irreversibly adsorbed monodentate, bidentate, bridged,and water-coordinated nitrate species as well as surface-adsorbed water. Similar adsorbed products formedduring the uptake of NO2 on kaolinite at relative humidity (RH) of 0%, and the reaction was second orderwith respect to reactive surface sites and 1.5 ± 0.1 for NO2. Reactive uptake coefficients, calculated usingBrunauer, Emmett, and Teller surface areas, increased from (8.0 ± 0.2) × 10-8 to (2.3 ± 0.4) × 10-7 forNO2 concentrations ranging from 0.56 × 1013 to 8.8 × 1013 molecules cm-3. UV−visible spectroscopy detectedgaseous HONO as a product for the reaction of NO2 on wet kaolinite. The uptake of HNO3 on pyrophyllite,a 2:1 phyllosilicate, resulted in stronger signal for nitric acid molecularly adsorbed on the silicon oxide surfacecompared to kaolinite. Monodentate, bridged, and water-coordinated nitrate species bound to aluminum sitesalso formed during this reaction indicating that reactive sites on edge facets are important for this system.The uptake of NO2 on pyrophyllite, γBET = (7 ± 1) × 10-9, was significantly lower than kaolinite becauseNO2 did not react with the dominant tetrahedral silicon oxide surface. These results highlight general trendsregarding the reactivity of tetrahedral silicon oxide and octahedral aluminum hydroxide clay surfaces andindicate that the heterogeneous chemistry of clay aerosols varies with mineralogy and cannot be predicted byelemental analysis.
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