Secondary Particulate Matter in the United States: Insights from the Particulate Matter Supersites Program and Related Studies

Abstract

Secondary aerosols comprise a major fraction of fine particulate matter (PM 2.5 ) in all parts of the country, during all seasons, and times of day. The most abundant secondary species include sulfate, nitrate, ammonium, and secondary organic aerosols (SOAs). The relative abundance of each species varies in space and time as a function of meteorology, source emissions strength and type, thermodynamics, and atmospheric processing. Transport of secondary aerosols from upwind locations can contribute significantly at downwind receptor sites, especially regionally in the eastern United States, and across a given urbanized area, such as in Los Angeles. Processes governing the formation of the inorganic secondary species (sulfate, nitrate, and ammonium) are fairly well understood, although the occurrence of nucleation bursts initiated with the formation of ultrafine sulfuric acid particles observed regionally on clean days in the eastern United States was unexpected. Because of the complex nature of organic material in air, much is still to be learned about the sources, formation, and even spatial and temporal distributions of SOAs. For example, a considerable fraction of ambient organic PM is oxidized organic species, many of which still need to be identified, quantified, and their sources and formation mechanisms determined.Furthermore, significant uncertainty (approaching 50% or more) is associated with estimating the SOA fraction of organic material in air with current methods. This review summarizes the findings of the Supersites Program and related studies addressing secondary particulate matter (PM), including spatial and temporal variations of secondary PM and its precursor species, data and methods for determining the primary and secondary fractions of PM mass, and findings on the anthropogenic and natural fractions of secondary PM.

Air Quality Measurements from the Southern Particulate Matter Supersite in Taiwan

“…Table 4 are SO 4 2-(24-29%), OC (21-22%) and NO 3 -(10-16%). This is similar to levels observed in the eastern U.S. (Fine et al, 2008), but differ from those in the western U.S. where only NO 3 -and OC dominate . The annual PM 10 averages (Table 4) According to Tsai et al (2006), when relatively humidity is > 85%, the BAM reading will increase as humidity increases.…”

“…Notably, EC is a primary pollutant and is generated by incomplete combustion of local sources (Fine et al, 2008), while OC can be from primary and secondary sources. (Fig.…”

“…The U.S. EPA constructed Particulate Matter Supersites in (U.S. EPA, 1999Solomon and Hopke, 2008) with objectives to: (1) evaluate advanced measurement methods, (2) further understand atmospheric processes, and (3) establish relationships between PM and adverse health effects (Watson et al, 2000). Over 400 studies have focused on U.S. Supersites (e.g., Solomon and Hopke, 2008;Chow et al, 2008;Solomon and Sioutas, 2008;Watson et al, 2008). Several special journal issues (Brock et al, 2004;Middlebrook et al, 2004;Ondov et al, 2004;Chow et al, 2004;Stanier and Solomon, 2006;Wittig and Solomon, 2006;Geller and Solomon, 2006;Chow and Solomon, 2006;Solomon and Hopke, 2008) increase may be due to the increase in the number of vehicles, 6-10% per year (Lin et al, 2002).…”

Source Apportionment: Findings from the U.S. Supersites Program

“…The remaining mass could be highly polar humic-like substance (HULIS), which are not fully characterized and quantified. 105 EC is a useful marker for combustion emission, but the differences between EC produced from various combustion sources are uncertain. Wood smoke and diesel exhaust are reported to have different spectral absorption efficiencies and Raman spectra.…”

“…101 A tracer solution CMB was tested by Gray et al 104 using EC as the marker for primary source types. 105 Assuming that primary OC and EC concentrations are correlated at a specific location because they have common sources, SOA is estimated as:…”