Comparison of particle light scattering and fine particulate matter mass in central California

Abstract

Particle light scattering (B-sp) from nephelometers and fine particulate matter (PM2.5) mass determined by filter samplers are compared for summer and winter at 35 locations in and around California's San Joaquin Valley from December 2, 1999 to February 3, 2001. The relationship is described using particle mass scattering efficiency (sigma(sp)) derived from linear regression of B-sp on PM2.5 that can be applied to estimated PM2.5 from nephelometer data within the 24-hr filter sampling periods and between the every-6th-day sampling frequency. An average of sigma(sp) = 4.9 m(2)/g was found for all of the sites and seasons; however, sigma(sp) averaged by site type and season provided better PM2.5 estimates. On average, the sigma(sp) was lower in summer than winter, consistent with lower relative humidities, lower fractions of hygroscopic ammonium nitrate, and higher contributions from fugitive dust. Winter average sigma(sp) were similar at non-source-dominated sites, ranging from 4.8 m(2)/g to 5.9 m(2)/g. The sigma(sp) was 2.3 m(2)/g at the roadside, 3.7 m(2)/g at a dairy farm, and 4.1 m(2)/g in the Kern County oilfields. Comparison of B-sp from nephelometers with and without a PM2.5 inlet at the Fresno Supersite showed that coarse particles contributed minor amounts to light scattering. This was confirmed by poorer correlations between B-sp and coarse particulate matter measured during a fall sampling period.