Seasonal and spatial variability of the size‐resolved chemical composition of particulate matter (PM₁₀) in the Los Angeles Basin

Abstract

[1] For a period of almost 3 years, sampling of size-fractionated ambient particulate matter with diameter below 10 mm (PM 10 ) was performed at urban source sites (Downey and University of Southern California) and inland receptor sites (Claremont and Riverside) in the Los Angeles Basin as part of the Southern California Particle Center and Supersite. Results for size-resolved PM 10 mass, inorganic ions (sulfate and nitrate), metals, elemental carbon, and organic carbon were obtained. Three collocated micro-orifice uniform deposit impactors (MOUDIs) were deployed to collect 24-hour samples roughly once a week. Ultrafine particle concentrations (particle diameter d p < 0.1 mm) were found to be the highest at the source sites resulting from fresh vehicular emissions. Mass concentrations in the accumulation mode (0.1 < d p < 2.5 mm) were lower in winter than in summer, especially at the receptor sites. PM concentrations in the coarse mode (2.5 < d p < 10 mm) were lower in winter and were composed mostly of nitrate and crustal elements (iron, calcium, potassium, silicon, and aluminum). Consistent relative levels of these elements indicate a common source of soil and/or road dust. In the accumulation mode, nitrate and organic carbon were predominant with higher nitrate levels found at the receptor sites. The ultrafine mode PM consisted of mostly organic carbon, with higher wintertime levels at the source sites due to increased organic vapor condensation from vehicles at lower temperatures. Conversely, higher ultrafine organic carbon levels at the receptor areas are due to secondary organic aerosol formation by photochemical reactions as well as increased advection of polluted air masses from upwind.

Field Validation of the New Miniature Versatile Aerosol Concentration Enrichment System (mVACES)

“…Further, as performance validation of the mVACES is the principal focus of this work, discussion of the physical and chemical characteristics of the observed aerosol is subsequently limited. Our observations, however, are consistent with extensive measurements previously reported at the same site (Sardar et al 2005;Miguel et al 2004). …”

“…Field validation measurements were conducted over a fourmonth period, from September to December 2005 at an urban sampling site-the Particle Instrumentation Unit of the Southern California Particle Center and Supersite (Sardar et al 2005)-near the University Park campus of the University of Southern California in Los Angeles. The sampling site is within 100-150 m of a major freeway and adjacent to a multi-story parking structure, industrial and construction sites.…”

“…Therefore, it is impor- tant to also characterize the mVACES' concentration enhancement for carbonaceous species as well, particularly as they are important contributors to aerosol mass at this location (Sardar et al 2005). Elevated black carbon concentrations are associated with proximity to the freeway.…”

Application of a Diffusion Charger for the Measurement of Particle Surface Concentration in Different Environments

“…This site is about 100 m downwind of a major (mainly gasoline vehicle) freeway and just south of downtown Los Angeles. The ambient air at this site represents a typical urban mix of mobile, industrial, and construction sources (Sardar et al 2005). Here, an NSAM, a W-CPC, and an SMPS operated continuously.…”

Characterization of Subgrid-Scale Variability in Particulate Matter with Respect to Satellite Aerosol Observations

“…Observed seasonal differences were likely due to the added contribution of wind-blown dust, a large portion of PM 10 , which is transported from desert areas by southwesterly summer winds. This, coupled with meteorological conditions, tends to lead to higher overall PM 2.5-10 concentrations in the warm season [44]. In the cool season the subgrid variability can be attributed more directly to localized traffic sources.…”