Concentration and composition of atmospheric aerosols from the 1995 SEAVS experiment and a review of the closure between chemical and gravimetric measurements

We summarize the results from the various measurements and the inter-sample r comparisons from Southeastern Aerosol and Visibility Study (SEAVS), a stu dy with one of its objectives to test for closure among chemical, gravimetr ic and optical measurements of atmospheric aerosol particles. Sulfate and o rganics are the dominant components of the SEAVS fine particles (nominally, particles with aerodynamic diameter less than or equal to 2.5 mu m) but be tween 28 and 42% (range over various samplers) of the gravimetrically measu red total fine particle concentration is unidentified by the chemical measu rements. Estimates of water associated with inorganic components and measur ement imprecision do not totally explain the observed difference between gr avimetric and chemical measurements. We examine the theoretical and empiric al basis for assumptions commonly made in the published literature to extra polate total fine particle concentration on the basis of chemical measureme nts of ions, carbon and elements. We then explore the more general question of closure using the SEAVS data a s well as data from other, similar studies reported in the literature. In s o combining the SEAVS measurements with Other similar studies, we find a st rong association between organic carbon and the unidentified component, tha t is, the fraction of the total fine particle concentration not identified by chemical measurements. We offer several tenable hypotheses for the relat ionship between the organic and unidentified components that deserve to be tested in future work. Specifically, we hypothesize that (1) errors in the sampling and analysis of organic carbon; (2) estimates of organic mass from measurements of organic carbon; and/or (3) water absorption by organics ma y all contribute to the observed relationship.