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 typ es of semivolatile organic compounds (SDCs) in the organic layer of wo od soot, diesel soot, and secondary aerosols were studied in outdoor e nvironmental chambers. Experimental partitioning coefficients, K-p, of different SOCs were measured using outdoor Teflon film chambers and c ompared 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 i n the liquid organic layer of different atmospheric particles. The equ ilibrated water content in the organic phase of chemically different p articles was estimated from the activity coefficient of water in the p article's organic liquid and the ambient relative humidity (RH). It wa s found that predicted SOC activity coefficients ((i) gamma(om)(infini ty)) for diesel soot particles were not a strong function of RH. There was, however, a dramatic change in (i) gamma(om)(infinity) on wood so ot particles for hydrophobic compounds such as alkanes and polycyclic aromatic hydrocarbons with changing RH. For polar SOCs, such as n-alka noic acids and substituted phenols on wood soot particles, (i) gamma(o m)(infinity) did not change with increasing RH. Similar behavior to wo od soot particles was observed for i gamma(om)(infinity) on secondary aerosols from the reaction of alpha-pinene with O-3. It was concluded that humidity effect on partitioning was most significant for hydropho bic compounds in polar aerosols.