Optical properties of the San Joaquin Valley aerosol collected during the 1995 integrated monitoring study

Optical, filter chemistry, and cascade impactor data collected during the w inter intensive of the IMS95 Study in the San Joaquin Valley (SJV) of Calif ornia were analyzed to determine the light-extinction efficiency of aerosol species. Regression of light scattering by particles (b(sp)) measured by a heated nephelometer without a size selective inlet against PM2.5 front fil ter mass gave a scattering efficiency of 3.67 +/- 0.05 m(2)/g with an R-2 ( fraction of variance explained) of 0.94. Division of the aerosol into two c omponents and applying two different corrections to the filter data for nit rate and organic carbon on the backup filter gave scattering efficiencies o f 3.7 +/- 0.3 or 3.1 +/- 0.2 m(2)/g for the salts composed of sulfate: nitr ate, and ammonium and 2.9 +/- 0.2 or 3.1 +/- 0.2 m2/g for all other species with R-2 of 0.985 and 0.986. The ambient b(sp) measured by an open nephelo meter was a simple function of PM2.5 mass and relative humidity (RH), givin g R-2 of 0.90 and 0.88 for two different RH sensors. Variations in PM2.5 si ze distribution and composition did not have an important effect on ambient b(sp). The RH data from each sensor were repeatable enough to show the exi stence of a simple dependence of aerosol water uptake on RH, but RH sensor calibration uncertainties prevented determining this dependence. Inversion of MOUDI cascade impactor data gave sulfate and nitrate mass median diamete rs (MMD) between 0.4 and 0.8 mu m. Mie scattering calculations based on MOU DI data provided humidity-dependent extinction efficiencies for the princip al aerosol chemical species. These efficiencies combined with particle filt er data showed that ammonium nitrate was the dominant contributor to winter time light extinction. Source apportionment showed that light extinction wa s dominated by emissions sources contributing to the formation of secondary species, especially nitrate. These wintertime data are not expected to app ly to summertime in the SJV. (C) 1999 Published by Elsevier Science Ltd. Al l rights reserved.