1National Exposure Research Laboratory, US Environmental Protection Agency, Research Triangle Park, 27711 North Carolina, USA
2Alion Science and Technology, P.O. Box 12313, Research Triangle Park, 27709 North Carolina, USA
Abstract. Laboratory smog chamber experiments have been carried out to investigate secondary organic aerosol (SOA) formation from the photooxidation of naphthalene and its methyl analogs, 1- and 2-methylnaphthalene (1-MN and 2-MN, respectively). Laboratory smog chamber irradiations were conducted in a flow mode to ensure adequate collection of the aerosol at reasonably low reactant concentrations and in the presence and absence of nitrogen oxides. Phthalic acid and methyl analogs were identified following BSTFA derivatization of the aerosol extract. These compounds were examined to determine whether they could serve as reasonable molecular tracers to estimate the contributions of these precursors to ambient PM2.5. Measurements were also made to determine aerosol parameters from secondary organic aerosol from naphthalene, 1-MN, and 2-MN.
A mass fraction approach was used to establish factors which could be applied to phthalic acid concentrations in ambient aerosols, assuming a negligible contribution from primary sources. In addition, the hydrolysis of phthalic anhydride was tested and found to represent a moderate filter artifact in side-by-side filter measurements with and without in-line denuders. This study also provided the opportunity to examine numeric differences using authentic standards for phthalic acid compared to surrogate standards. While the mass fraction based on a surrogate compounds was somewhat lower, the differences are largely unimportant. For naphthalene, mass fractions of 0.023 and 0.019 were determined in the presence and absence of nitrogen oxides, respectively, based on the phthalic acid standards.
The mass fractions determined from the laboratory data were then applied to ambient samples where phthalic acid was found and expressed "as naphthalene" since phthalic acid was found to be produced in the particle phase from other PAHs tested. The mass fraction values were applied to samples taken during the 2005 SOAR Study in Riverside, CA and 2010 CalNex Study in Pasadena. In both studies an undetermined isomer of methylphthalic acid was detected in addition to phthalic acid. Laboratory experiment retention times and mass spectra suggest that the major precursor for this compound is 2-MN. For the CalNex Study, SOC values for the gas-phase PAHs (as naphthalene) were found to range from below the detection limit to 20 ng C m−3 which together with the laboratory mass fraction data suggests an upper limit of 1 μg m−3 for SOA due to PAHs. Temporal data over the course of the one-month CalNex study suggest that primary sources of phthalic acid were probably negligible during this study period. However, the values must still be considered upper limits given a potential gas-phase hydrolysis reaction or uptake of phthalic anhydride (subsequently hydrolyzed) onto the collection medium.