Uptake of HCl(g) and HBr(g) on ethylene glycol surfaces as a function of relative humidity and temperature

Organic compounds are a significant component of tropospheric aerosols. In the present experiments, ethylene glycol was selected as a surrogate hydrophilic organic compound for a study of surface properties of organic liquid-aqueous solutions. The mass accommodation coefficient of gas-phase HCl and HBr on ethylene glycol-water surfaces has been measured as a function of water mole fraction (0-1) and temperature. The gas uptake studies were performed under liquid-vapor equilibrium conditions using a droplet train flow reactor. The mass accommodation coefficient (alpha) for HCl on pure ethylene glycol increases from about 0.40 +/- 0.06 at 303 K to 0.79 +/- 0.12 at 258 K. Such negative temperature dependence for alpha has been observed in studies of gas uptake by aqueous surfaces. The HBr mass accommodation coefficient on ethylene glycol is near unity independent of temperature in the range studied. The mass accommodation coefficient on the ethylene glycol-water solution is well represented by alpha = alpha(H2O)X(s)(H2O) + alpha(EG)(1 - X(s)(H2O)), where X(s)(H2O) is the water fractional surface coverage obtained via the surface tension of the solution and the subscripted alpha's are the mass accommodation coefficients on pure water and pure ethylene glycol. The D-H isotope exchange probability of DCl on the gas-liquid interface of ethylene glycol was also studied and was measured to be 1, implying that the thermal accommodation coefficient is also 1.