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Nonpolar organic compounds (NPOCs) in ambient particulate matter (PM) commonly include n-alkanes, branched alkanes, hopanes and steranes, and polycyclic aromatic hydrocarbons (PAHs). The recent development of thermal desorption-gas chromatography/mass spectrometry (TD-GC/MS) has greatly reduced time and labor in their quantification by eliminating the laborious solvent extraction and sample concentration steps in the traditional approach that relies on solvent extraction. The simplicity of the TD-GCMS methods has afforded us concentration data of NPOCs in more than 90 aerosol samples in two aerosol field studies and 20 vehicular emissions-dominated source samples in Hong Kong over the past few years. In this work, we examine the interspecies relationships between select NPOCs and their concentration ratios to elemental carbon (EC) among the ambient samples and among the source samples. Our analysis indicates that hopanes were mainly from vehicular emissions and they were significantly oxidized in ambient PM. The hopane/EC ratio in ambient samples was on average less than half of the ratio in vehicular emissions-dominated source samples. This highlights the necessity in considering oxidation loss in applying organic tracer data in source apportionment studies. Select PAH/EC ratio-ratio plots reveal that PAHs had diverse sources and vehicular emissions were unlikely a dominant source for PAHs in Hong Kong. Biomass burning and other regional sources likely dominated ambient PAHs in Hong Kong. [ABSTRACT FROM AUTHOR]

Organic materials make up a significant fraction of ambient particulate mass. It is important to quantify their contributions to the total aerosol mass for the identification of aerosol sources and subsequently formulating effective control measures. The organic carbon (OC) mass can be determined by an aerosol carbon analyzer; however, there is no direct method for the determination of the mass of organic compounds, which also contain N, H, and O atoms in addition to C. The often-adopted approach is to estimate the organic mass (OM) from OC multiplying by a factor. However, this OC-to-OM multiplier was rarely measured for a lack of appropriate methods for OM. We report here a top-down approach to determine OM by coupling thermal gravimetric and chemical analyses. OM is taken to be the mass difference of a filter before and after heating at 550°C in air for 4h minus mass losses due to elemental carbon (EC), volatile inorganic compounds (e.g., NH4NO3), and loss of aerosol-associated water that arise from the heating treatment. The losses of EC and inorganic compounds are determined through chemical analysis of the filter before and after the heating treatment. We analyzed 37 ambient aerosol samples collected in Hong Kong during the winter of 2003, spring of 2004, and summer of 2005. A value of 2.1±0.3 was found to be the appropriate factor to convert OC to OM in these Hong Kong aerosol samples. If the dominant air mass is classified into two categories, then an OM-to-OC ratio of 2.2 was applicable to aerosols dominated by continent-originated air mass, and 1.9 was applicable to aerosols dominated by marine air mass. [Copyright &y& Elsevier]