Humidity effects on the gas/particle partitioning of the different types of semivolatile organic compounds (SOCs) in the organic layer of wood soot, diesel soot, and secondary aerosols were studied in outdoor environmental chambers. Experimentalpartitioning coefficients, K{sub}p, of different SOCs were measured using outdoor Teflon film chambers and compared to theoretical K{sub}p values calculated with the aid of activity coefficients and vapor pressures. A thermodynamic model based on groupcontribution methods was used to estimate SOC activity coefficients in the liquid organic layer of different atmospheric particles. The equilibrated water content in the organic phase of chemically different particles was estimated from the activitycoefficient of water in the particle's organic liquid and the ambient relative humidity (RH). It was found that predicted SOC activity coefficients ((i{sup left}γ{sup}∞){sub}om) for diesel soot particles were not a strong function of RH. There was,however, dramatic change in ; (i{sup left}γ{sup}∞){sub}om on wood soot particles for hydrophobic compounds such as alkanes and polycyclic aromatic hydrocarbons with changing RH. For polar SOCs, such as n-alkanoic acids and substituted phenols on woodsoot particles, (i{sup left}γ{sup}∞){sub}om did not change with increasing RH. Similar behavior to wood soot particles was observed for (i{sup left}γ{sup}∞){sub}om on secondary aerosols from the reaction ofα-pinene with O{sub}3. It was concluded that humidity effect on partitioning was most significant for hydrophobic compounds in polar aerosols.
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