Measurement of organic mass to organic carbon ratio in ambient aerosol samples using a gravimetric technique in combination with chemical analysis

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 4 h 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.