Temporal and spatial variations of PM2.5 and PM10 aerosol in the Southern California air quality study

Abstract

During the summer and fall of 1987, the Southern California Air Quality Study (SCAQS) was conducted at more than 40 locations to acquire a database with meteorological, air quality, and visibility measurements, and to assess the causes of elevated ozone and suspended particulate matter concentrations throughout metropolitan Los Angeles, CA (the South Coast Air Basin—SoCAB). PM2.5 and PM10 (particles with aerodynamic diameters equal to or less than 2.5 and 10 μm, respectively) mass, elements, water-soluble chloride, nitrate, sulfate, ammonium ions, organic and elemental carbon as well as gaseous ammonia, nitric acid, and sulfur dioxide were measured on eleven episode days during the summer and six episode days during the fall. This paper examines the statistical, temporal, and spatial distributions of the SCAQS aerosol measurements. PM2.5 constituted one-half to two-thirds of PM10 at all sampling sites. PM10 mass concentrations were highest during the fall and were dominated by PM2.5. Nitrate, sulfate, ammonium, and organic and elemental carbon were the most abundant species in the PM2.5 fraction. The coarse particle (PM10 minus PM2.5) fraction was enriched with soil-related elements (e.g. aluminum, silicon, calcium, iron) at the inland sites and with marine-related elements (e.g. sodium, chloride) at the coastal sites. Average concentrations for most chemical compounds were higher during the fall than during the summer, except for sulfate which was more abundant during the summer. PM2.5 nitrate and ammonium concentrations were negatively biased for daytime samples with respect to nighttime samples, consistent with diurnal changes in temperature and the effect of these changes on the equilibrium between particulate ammonium nitrate and gaseous ammonia and nitric acid. Highest sulfur dioxide concentrations were found at the Long Beach site, the location closest to SoCAB oil refineries and ship traffic; and highest ammonia concentrations were found at the Rubidoux site, a location downwind of dairies and agricultural operations.