Laboratory studies on the uptake of aromatic hydrocarbons by ice crystals during vapor depositional crystal growth

Abstract

Uptake of aromatic hydrocarbons (AH) by ice crystals during vapor deposit growth was investigated in a walk-in cold chamber at temperatures of 242, 25 1, and 260 K, respectively. Ice crystals were grown from ambient air in the presence of gaseous AH namely: benzene (C6H6), toluene (methylbenzene, C7H8, the C7H8 isomers ethylbenzene, o-, m-, p-xylene (dimethylbenzenes), the C9H12 isomers n-propylbenzene, 4-ethyltoluene, 1,3,5-trimethylbenzene (1,3,5-TMB), 1,2,4-trimethylbenzene (1,2,4-TMB), 1,2,3trimethylberizene (1,2,3-TMB), and the C10H14 compound tert.-butylbenzene. Gas-phase concentrations calculated at 295 K were 10.3-20.8 mu g m(-3). Uptake of AH was detected by analyzing vapor deposited ice with a very sensitive method composed of solidphase micro-extraction (SPME), followed by gas chromatography/mass spectrometry (GC/MS). 3 Ice crystal size was lower than 1 cm. At water vapor extents of 5.8, 6.0 and 8.1 g m-, ice crystal shape changed with decreasing temperatures from a column at a temperature of 260 K, to a plate at 251 K, and to a dendrite at 242 K. Experimentally observed ice growth rates were between 3.3 and 13.3 x 10(-3) g s(-1) m(-2) and decreased at lower temperatures and lower value of water vapor concentration. Predicted growth rates were mostly slightly higher. Benzene, toluene, ethylbenzene, and xylenes (BTEX) were not detected in ice above their detection limits (DLs) of 25 mu g g(ice)(-1) (toluene, ethylbenzene, xylenes) and 125 mu g g(ice)(-1) (benzene) over the entire temperature range. Median concentrations of n-propylbenzene, 4-ethyltoluene, 1,3,5-TMB, tert.-butylbenzene, 1,2,4-TMB, and 1,2,3-TMB were between 4 and 176 pg g(ice)(-1) at gas concentrations of 10.3-10.7 mu g m(-3) calculated at 295 K. Uptake coefficients (K) defined as the product of concentration of AH in ice and density of ice related to the product of their concentration in the gas phase and ice mass varied between 0.40 and 10.23. K increased with decreasing temperatures. Values of Gibbs energy (Delta G) were between -4.5 and 2.4 kJ mol(-1) and decreased as temperatures were lowered. From the uptake experiments, the uptake enthalpy (Delta H) could be determined between - 70.6 and - 33.9 kJ mol(-1). The uptake entropy (Delta S) was between - 281.3 and - 126.8 J mol(-1) K-1. Values of Delta H and Delta S were rather similar for 4-ethlytoluene, 1,3,5-TMB and tert-butylbenzene, whereas 1,2,3-TMB showed much hi her values. (c) 2007 Elsevier Ltd. All rights reserved.