Evaluation of PM_(2.5) and PM_(10) Mass Closure Formulae

摘要

Some of the oxygen (O), hydrogen (H), and nitrogen (N) components of PM_(2.5) and PM_(10) samples are not quantified by the normally applied elemental, ionic, and carbon fraction analysis methods. These are compensated for by multipliers that assume certain chemical forms. Thirteen different weighting formulae have been applied, with the IMPROVE formula being the one most commonly used. These formulae are applied to different data sets, including those from IMPROVE acquired over 20 years in central California to determine the extent to which mass closure can be improved. The 1.8 OC multiplier is optimum for most samples at IMPROVE sites, except when there is evidence of fire and a larger multiplier is more appropriate. The 1.4 OC multiplier is more appropriate for urban areas, except when there is evidence of residential wood combustion or secondary organic aerosol. The method by which the organic vapor adsorption artifact adds uncertainty to the weighting is described. Soil is a small fraction of PM_(2.5) and mass closure is not sensitive to the weighting scheme, but different soil compositions in central California call for different weighting factors. The IMPROVE sea salt assumption is sufficient at all but coastal sites where sea salt sometime reacts with nitric acid. Characteristic ratios of measured chemical components provide indications of the source material and more appropriate weighting factors that improve mass closure.