Determination of OM/OC ratios and specific attenuation coefficients (SAC) in ambient fine PM at a rural site in southern Ontario: implications for emission sources, particle aging, and radiative forcing

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Abstract
Ambient particulate matter (PM) samples were collected on quartz filters at a rural site in southern Ontario during intensive studies in 2005 and 2007. The concentrations of organic carbon (OC), pyrolysis organic carbon (POC), and elemental carbon (EC) were determined by thermal analysis. These results were compared to the organic aerosol mass concentration (OM) measured by an Aerodyne Aerosol Mass Spectrometer (AMS) and to the particle absorption coefficient (b_asp) obtained from a Radiance Research Particle Soot Absorption Photometer (PSAP). The total organic mass to organic carbon ratios (OM/OC) and specific attenuation coefficients (SAC) were also derived. According to the results, the POC mass is proportional to the approximated oxygen mass in the aerosols and OM/OC ratios can be estimated directly from thermal measurements. The study also suggests that the air masses from the south, with relatively low OC/EC ratios, high EC, sulphate contents and OM/OC ratios, were originated from urban and industrial emissions and subsequently experienced photo-oxidations in the atmosphere, implying that the oxygenated organics could come from both primary and secondary sources. Whereas the air masses from the north, with relatively high OC/EC ratios, low EC, sulphate contents and OM/OC ratios, were dominant by the background clean air with relatively larger contributions from biogenic emissions. The mean SAC derived from the 2005 and 2007 studies are 4.9 m2 g−1 and 3.8 m2 g−1, respectively. When POC mass approaching zero (i.e. the impact of atmospheric aging is minimized), the SAC for primary emitted soot is estimated to be 5.8 m2 g−1 and 6.3 m2 g−1 for the northern and southern air masses, respectively, supported by the corresponding values when particulate sulphate concentration approaches zero. A decreasing trend in the SAC value with atmospheric aging of the aerosol was observed at the site, suggesting that during the study, the light absorption enhancement due to the presence of coating on particles was likely to be offset by the decrease in light absorption caused by increasing soot particle diameter and collapsing of soot particle structure. This result may imply that model simulations of atmospheric warming by BC could be 50% too high.
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