A study of gas-particle partitioning of PAH according to adsorptive models and season

Vapour-phase and particulate air samples were collected in a rural area of England in a cold period (20/11/2003-22/12/2003) and a warm period (5/5/2004-26/5/2004) to investigate the seasonal effect on gas-particle partitioning of PAHs. Three different sorptive models, namely Dachs-Eisenreich (ab/adsorption), K_oa (absorption into organic layer) and K _sa (adsorption on the soot surface) were applied and, as expected, the inclusion of an adsorptive mechanism provided better predictions of K _p values for all PAHs. The ratios of measured-K_p/Dachs- Eisenreich-K_p are 1.03 ± 0.12 and 1.06 ± 0.15 in the cold period and warm period respectively for a group of medium molecular weight compounds. The model results indicate a far greater importance for both adsorptive and absorption processes rather than the absorption mechanism alone in determining the gas-particle partitioning of PAH. There was little difference between log K_p values for the medium and high molecular weight PAH between the warm period and cold period campaigns despite a difference in mean temperature of 10°C. This can be explained by a difference in aerosol properties between the two campaigns and it is postulated to be attributable in minor part to a greater fraction of elemental carbon present in the warm period aerosol, which derives from a greater proportion of continental air masses than in the cold period, but predominantly to the lesser relative humidity in the warm period leading to lower hygroscopic growth of internally-mixed aerosol and consequently less blockage of adsorption sites by liquid water.