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 winter intensive of the IMS95 Study in the San Joaquin Valley (SJV) of California were analyzed to determine the light-extinction efficiency of aerosol species. Regression of light scattering by particles (bsp) measured by a heated nephelometer without a size selective inlet against PM2.5 front filter mass gave a scattering efficiency of 3.67±0.05 m2/g with an R2 (fraction of variance explained) of 0.94. Division of the aerosol into two components and applying two different corrections to the filter data for nitrate and organic carbon on the backup filter gave scattering efficiencies of 3.7±0.3 or 4.1±0.2 m2/g for the salts composed of sulfate, nitrate, and ammonium and 2.9±0.2 or 3.1±0.2 m2/g for all other species with R2 of 0.985 and 0.986. The ambient bsp measured by an open nephelometer was a simple function of PM2.5 mass and relative humidity (RH), giving R2 of 0.90 and 0.88 for two different RH sensors. Variations in PM2.5 size distribution and composition did not have an important effect on ambient bsp. The RH data from each sensor were repeatable enough to show the existence 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 diameters (MMD) between 0.4 and 0.8 μm. Mie scattering calculations based on MOUDI data provided humidity-dependent extinction efficiencies for the principal aerosol chemical species. These efficiencies combined with particle filter data showed that ammonium nitrate was the dominant contributor to wintertime light extinction. Source apportionment showed that light extinction was dominated by emissions sources contributing to the formation of secondary species, especially nitrate. These wintertime data are not expected to apply to summertime in the SJV.