Absorption of semi-volatile compounds in oiled impaction substrates: Measurement of pyrene absorption

Oiled impaction substrates have been used to prevent particle bounce during the collection of size-segregated aerosol samples, which have been analyze d for trace-level airborne organic compounds, including polycyclic aromatic hydrocarbons (PAHs). The use of the oiled impaction substrates, however, m ay introduce another sampling artifact-the absorption of semivolatile speci es from the gas phase which could artificially increase the amount of PAHs attributed to the aerosol. In this article, laboratory measurements of the absorption of a particular PAH, pyrene, from the gas phase to impaction sub strates of polytetrafluoroethylene membranes impregnated with dibutyl phtha late are reported. Overall mass transfer coefficients are determined from t he data. These results are used to calculate the absorption of gas phase PA Hs during sampling of size-segregated atmospheric particles. Criteria are d eveloped to determine if the absorption artifact is negligible. The first c riterion requires that the analyte be negligibly soluble in the oil; this c riterion is met by none of the impaction oils reported in the literature. T he second criterion is that species do not have time to reach an equilibriu m distribution between the gas phase and impaction oil; this criterion is m et for nonvolatile species, those with vapor pressures equal to or less tha n that of benzo[a]pyrene (3.5 x 10(-6) Pa). We recommend that oiled impacti on substrates be used only if the absorption artifact is expected to be neg ligible on the basis of these criteria.