Organic-Mass-to-Organic-Carbon Aerosol Ratio Measurements by FTIR

The ratio of organic-mass-to-organic-carbon has an uncertainty higher than 50%, but is used in every measurement to date of the organic fraction of atmospheric particles. A recently developed technique with limited systematic errors between 10% and 30% (not including size-dependent variations in organic compositions) provides measurements of this ratio that show its large variability for samples measured in northeastern Asia and the Caribbean. The technique uses functional groups measured by FTIR spectroscopy to estimate composite organic carbon from the number of carbon bonds present and organic mass from the molecular mass of each functional group associated with the measured bond type for submicron filter samples. The molecular mass associated with each functional group is not unique and does not account for highly branched organic compositions, but for the organic mixtures described by the 10% of atmospheric organic mass that has been speciated by GCMS, the theoretical discrepancy in the composite organic-mass-to-organic-carbon ratio is less than 5%. The measured values are skewed and bimodal: over 90% of the measurements collected lie between 1.1 and 1.4, with the remaining values between 2.3 and 2.4, although the mean for each sampling project was 1.4. This variability highlights the importance of an improved approach; measured values of organic-mass-to-organic-carbon will significantly reduce the uncertainty associated with ambient aerosol organic measurements, as well as source and global inventories.