Laboratory studies of the heterogeneous chemistry and phase transitions of ice and ammonium nitrate surfaces representative of tropospheric aerosol

The uptake of trace gases on ammonium nitrate and ice, the heterogeneous depositional nucleation of ice, and the vapor pressure of the cubic phase of ice (Ic) have been studied using a Knudsen cell flow reactor coupled with FTIR-Reflection Absorption Spectroscopy (FTIR-RAS). The coupling of these techniques allowed heterogeneous reaction rates to be determined from mass spectroscopy (MS) data while simultaneously monitoring the condensed phase reaction products with the IR.; The uptake of methanol, acetone, and a homologous series of monocarboxylic acids (C1-C5) onto ammonium nitrate films as a function of temperature and relative humidity has been investigated. The uptake of formic, acetic, propanoic, and butanoic acid was efficient under all conditions. IR spectra revealed that acetic, propanoic, and butanoic acids ionized on the crystalline surface. Adsorption of these acids in the presence of water vapor dramatically enhanced the water content of the AN film. Formic acid reacted with the AN film to produce ammonium formate and nitric acid. Uptake of pentanoic acid, acetone and methanol was insignificant under atmospherically relevant conditions.; The critical saturation ratio (Sice) required for heterogeneous depositional nucleation of ice on micrometer-sized (NH4)2 SO4, maleic acid (C4H4O4), and mixed (NH4)2SO4/maleic acid particles and on thin NH4NO3 films was measured. Sice was found to be similar for all substrates, ranging from Sice = 1.42 +/- 0.04 at 190 K to Sice = 1.04 +/- 0.05 at 240 K. Deliquescence of the particles was not observed in any experiment. The observed saturation ratios for heterogeneous nucleation were all substantially lower than the saturation ratios required for homogeneous nucleation.; The first measurements of the vapor pressure of Ic over the temperature range 180--190 K are reported. On average, the vapor pressure of the cubic phase is 10.5 +/- 2.5 % higher than that of the hexagonal phase. From the vapor pressure data, it was determined that DeltaHsub is 155 +/- 30 J/mole larger for Ih than for Ic. These data will help to explain measurements of atmospheric water vapor and can be used in models to predict the effect of Ic on cloud properties.