Uptake of Organic Acids on Ice Surfaces: Evidence for Surface Modification and Hydrate Formation

Abstract

Abstract The uptake of gaseous HC(O)OH, CH3 C(O)OH and CF3C(O)OH on ice films at temperatures corresponding to the upper troposphere (UT) has been investigated using a coated-wall flow tube with mass spectrometric measurement of gas concentration. Uptake was largely reversible and followed Langmuir-type kinetic behavior, i.e. surface coverage increased with trace gas concentration approaching a maximum surface coverage at N max ~2 to 3×1014 molecules cm−3, corresponding to ~25% monolayer (ML). The partition constants, KLinC , were obtained from the experimental data by analysis using the simple Langmuir model and also using a simple one-dimensional numerical model to simulate individual uptake profiles and retrieve partition constants for specific conditions of temperature and concentration, over the temperature range 208–238 K. The analysis showed that Langmuir constants decreased at high surface coverages, possibly due to adsorbate-adsorbate interaction or modification of the ice surface. At low coverage, the following expressions described the temperature dependence of the partition coefficients (KLinC) for HC(O)OH (KLinC = (1.5±3.5 1.0)×10−8exp((5143±268)/T) cm), for CH3 C(O)OH (KLinC = (0.55±5.4 0.5)×10−8exp((5703±536)/T) cm), and CF3C(O)OH (KLinC = (512±1903 404)×10−8exp((309±331)/T) cm). For CF3C(O)OH there was an irreversible component of uptake, which was attributed to hydrate formation on the surface.