A Thermodynamic Approach for Modeling Partitioning of Semivolatile Organic Compounds on Atmospheric Particulate Matter:  Humidity Effects

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. Experimental partitioning coefficients, K_p, of different SOCs were measured using outdoor Teflon film chambers and compared to theoretical K_p values calculated with the aid of activity coefficients and vapor pressures. A thermodynamic model based on group contribution 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 chemi cally different particles was estimated from the activity coefficient of water in the particle's organic liquid and the ambient relative humidity (RH). It was found that predicted SOC activity coefficients (ⁱ ) for diesel soot particles were not a strong function of RH. There was, however, a dramatic change in ⁱ 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 wood soot particles, ⁱ did not change with increasing RH. Similar behavior to wood soot particles was observed for ⁱ on secondary aerosols from the reac tion of α-pinene with O₃. It was concluded that humidity effect on partitioning was most significant for hydrophobic compounds in polar aerosols.