| Abstract
| - The uptake of formic (C1), propanoic (C3), butanoic (C4), and pentanoic (C5) acids onto ammonium nitrate(AN) has been investigated as a function of temperature and relative humidity using a Knudsen cell flowreactor coupled with FTIR−reflection absorption spectroscopy (FTIR−RAS). The uptake of acetone andmethanol onto AN was also briefly studied. Initial uptake coefficients (γ) were determined over the temperaturerange 200−240 K. Formic, propanoic, and butanoic acids exhibited efficient but temperature-dependent uptakeon AN, with larger uptake coefficients observed at lower temperatures. Pentanoic acid was not taken up byAN under any of the conditions studied. Uptake of acetone and methanol onto AN was observed, but ininsignificant amounts under atmospherically relevant conditions. Infrared spectra revealed that propanoic andbutanoic acids ionized on the surface, despite the fact that the AN films were effloresced. Formic acid reactedwith the AN film to produce ammonium formate and ionized nitric acid. Adding small amounts of watervapor (4% RH) to the chamber resulted in dramatically increased γ values for all of the acids. Furthermore,the IR spectra showed the formation of a liquid layer when propanoic and butanoic acids adsorbed on thesurface at RH = 20% and greater. Liquid water features were not observed at a similar relative humidity inthe absence of the acids. These results show that small organic acids can be efficiently scavenged by AN andlead to enhanced water uptake under upper tropospheric conditions.
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