The interaction of methanol, acetone, and acetaldehyde with ice and nitric acid-doped ice: Implications for cirrus clouds

The uptake of methanol, acetone, and acetaldehyde by ice and nitric acid/ice surfaces representative of cirrus clouds has been measured using a Knudsen cell flow reactor equipped with FTIR spectroscopic observation of the condensed phase. Sticking coefficients and total equilibrium coverage for the organics on ice were obtained over the temperature range of 120-200 K. Both the sticking coefficients and coverage were found to decrease with increasing temperature for all three species. The coverage data was fit using a Langmuir adsorption model, and the results were extrapolated to typical cirrus cloud temperatures. By the use of estimates for the gaseous concentrations of the organic species, calculated coverages on ice at 210 K are extrapolated to be on the order of 10¹⁰ molecules/cm² (10^-4 ML) for methanol and acetone and <5 × 10⁹ molecules/cm² (<10^-5 ML) for acetaldehyde. Such small coverages constitute an insignificant portion of each gas-phase species being lost to cirrus clouds. Additionally, the surface coverages of ≤10^-4 monolayers (ML) are likely to be too small to impact heterogeneous chemistry. Uptake of methanol, acetone, and acetaldehyde was also studied on several forms of nitric acid-coated ice at temperatures from 190 to 200 K. Studies on nitric acid trihydrate (NAT) and ice with monolayer coverage of HNO₃ Showed no measurable uptake in the examined temperature range for all three species. While no uptake was observed on a supercooled HNO₃/H₂O solution for acetone or acetaldehyde at T = 200 K, this solution did show enhanced uptake of methanol at 200 K relative to pure ice.