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 (PM_2.5) 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 (σ_sp) derived from linear regression of B_sp on PM_2.5 that can be applied to estimated PM_2.5 from nephelometer data within the 24-hr filter sampling periods and between the every-6th-day sampling frequency. An average of σ_sp =4.9 m²/g was found for all of the sites and seasons; however, σ_sp averaged by site type and season provided better PM_2.5 estimates. On average, the σ_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 σ_sp were similar at non-source-dominated sites, ranging from 4.8 m²/g to 5.9 m²/g. The σ_sp was 2.3 m²/g at the roadside, 3.7 m²/g at a dairy farm, and 4.1 m²/g in the Kern County oilfields. Comparison of B_sp from nephelometers with and without a PM_2.5 inlet at the Fresno Super-site 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.