GAS SOLID AND GAS/LIQUID PARTITIONING OF ORGANIC-COMPOUNDS - CRITICAL-EVALUATION OF THE INTERPRETATION OF EQUILIBRIUM-CONSTANTS/

Gas/solid and gas/liquid partition processes are pivotal for the trans port and residence time of organic pollutants in the atmosphere. Commo nly, experimentally determined partition constants (K) between air and condensed phases (i.e., aerosols, rain, fog,snow, Soils, plants) of a series of compounds are evaluated as a function of their (subcooled) liquid saturation vapor pressure p(L)degrees. Frequently, a linear fre e energy relationship of the form in K = m In p(L)degrees + constant h as been found for groups of structurally related cbmpounds. In many ca ses, in the literature, deviations of the slope m from -1 have been co nsidered to be in contradiction to theory and have thus been taken as indicative of nonequilibrium conditions or experimental artifacts. In this paper, it is shown by:theoretical considerations as well as by ex perimental data from the literature that m may deviate significantly f rom -1 for equilibrium adsorption and absorption and that such deviati ons do not-necessarily indicate nonequilibrium effects. In fact; if tr ue equilibrium partitioning data are available,the slope m is a charac teristic parameter for the specific sorption process and can thus be h elpful for obtaining information about unknown sorbents (e.g., from fi eld data for gas/particle partitioning). Some examples demonstrate tha t earlier interpretations of experimental sorption constants may have to be revised. A review of slopes m for field data of gas/particle par titioning from the literature leads to the conclusion that aerosol par ticles sampled in different events at the same location may differ con siderably in their chemical I properties. Combining the information of the slopes m and the absolute sorption constants K,we further conclud e that; absorption rather than adsorption must have been the dominatin g sorption mechanism in many cases. Finally, it is shown that a linear free energy relationship directly relating the sorption coefficients for two sorbents (In K-1 vs In K-2) may provide more information with respect to the question of the chemical similarity of the investigated sorbents than a.comparison of m values from plots of In K-1 and, In K -2 vs In p(L)degrees..