The ratio of organic-mass-to-organic-carbon, typically taken to be between 1.4 and 1.7, has an uncertainty higher than 50, but this value is used in every measurement to date of the organic fraction of atmospheric particles. A recently developed technique with errors reduced to between 9 and 33 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. The molecular masses associated with each functional group are not unique and do not account for highly branched organic compositions. For the organic mixtures described by the less than 20 of atmospheric organic mass that has been speciated by GCMS, the theoretical discrepancy in the composite organic-mass-toorganic-carbon ratio is less than 5. The measured ratios for submicron particle samples are skewed: over 90 of the measurements collected lie between 1.2 and 1.6, with mean values just below 1.4. This variability highlights the importance of measured organic-mass-to-organic-carbon ratios to reduce the uncertainty associated with atmospheric organic aerosol.
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