Uptake of gaseous aromatic hydrocarbons by non-growing ice crystals

Laboratory studies were performed in a walk-in cold chamber to investigate the uptake of aromatic hydrocarbons by non-growing ice crystals at −20°C. Dendritic ice crystals were grown by vapor deposition and exposed to organic gases (benzene, toluene, ethylbenzene, m/p-xylene, o-xylene, n-propylbenzene, 4-ethyltoluene, 1,3,5-trimethylbenzene, tert-butylbenzene, 1,2,4-trimethylbenzene, and 1,2,3-trimethylbenzene) at gas-phase concentrations between 2.8 and 33.1μgm−3. During all exposure experiments, the gas/air stream was maintained at ice saturation to avoid ice crystal growth or evaporation. An analytical method comprising of solid-phase-micro-extraction followed by gas chromatography/mass spectrometry (SPME/GC–MS) was applied, which allows detection of organic compounds in melted ice at 0.025nggice−1. The SPME/GC–MS method was an appropriate tool to determine the uptake of organic compounds by ice crystals at the applied gas-phase concentrations. However, it was not possible to detect any of the test substances in ice samples after exposure. No adsorption could be detected by increasing gas-phase concentrations. Neither increasing exposure time nor lowering flow rate of the carrier gas caused detectable adsorption effects of aromatic compounds on ice. Our results indicate that adsorption of aromatic hydrocarbons is either insignificant or highly reversible at −20°C. These findings are consistent with reversible adsorption processes reported already for many oxygenated organic compounds like alcohols, acids, and aldehydes. Although the specific surface area of dendritic ice crystals is large, the results of our study demonstrate that gas uptake by ice surfaces is negligible for the removal of aromatic hydrocarbons in the atmosphere. This is an indication that the occurrence of aromatic hydrocarbons in precipitation cannot be explained by surface adsorption. There must be another accumulation process leading to concentrations of aromatic hydrocarbons found in field studies which is still unknown.