particulate matter (PM2.5) mass determined by filter samplersare compared for summer and winter at 35 locationsin and around California's San Joaquin Valley from December2, 1999 to February 3, 2001. The relationship isdescribed using particle mass scattering efficiency (sp)derived from linear regression of Bsp on PM2.5 that can beapplied to estimated PM2.5 from nephelometer datawithin the 24-hr filter sampling periods and between theevery-6th-day sampling frequency. An average of sp 4.9 m2/g was found for all of the sites and seasons; however,sp averaged by site type and season provided betterPM2.5 estimates. On average, the sp was lower in summerthan winter, consistent with lower relative humidities,lower fractions of hygroscopic ammonium nitrate, andhigher contributions from fugitive dust. Winter averagesp were similar at non-source-dominated sites, rangingfrom 4.8 m2/g to 5.9 m2/g. The sp was 2.3 m2/g at theroadside, 3.7 m2/g at a dairy farm, and 4.1 m2/g in theKern County oilfields. Comparison of Bsp from nephelometerswith and without a PM2.5 inlet at the Fresno Supersiteshowed that coarse particles contributed minoramounts to light scattering. This was confirmed by poorercorrelations between Bsp and coarse particulate mattermeasured during a fall sampling period.