Intra-urban spatial variability of PM2.5-bound carbonaceous components

Abstract

The Denver Aerosol Sources and Health (DASH) study was designed to evaluate associations between PM2.5 species and sources and adverse human health effects. The DASH study generated a five-year (2003–2007) time series of daily speciated PM2.5 concentration measurements from a single, special-purpose monitoring site in Denver, CO. To evaluate the ability of this site to adequately represent the short term temporal variability of PM2.5 concentrations in the five county Denver metropolitan area, a one year supplemental set of PM2.5 samples was collected every sixth day at the original DASH monitoring site and concurrently at three additional sites. Two of the four sites, including the original DASH site, were located in residential areas at least 1.9 km from interstate highways. The other two sites were located within 0.3 km of interstate highways. Concentrations of elemental carbon (EC), organic carbon (OC), and 58 organic molecular markers were measured at each site. To assess spatial variability, site pairs were compared using the Pearson correlation coefficient (r) and coefficient of divergence (COD), a statistic that provides information on the degree of uniformity between monitoring sites. Biweekly co-located samples collected from July 2004 to September 2005 were also analyzed and used to estimate the uncertainty associated with sampling and analytical measurement for each species. In general, the two near-highway sites exhibited higher concentrations of EC, OC, polycyclic aromatic hydrocarbons (PAHs), and steranes than did the more residential sites. Lower spatial heterogeneity based on r and COD was inferred for all carbonaceous species after considering their divergence and lack of perfect correlations in co-located samples. Ratio–ratio plots combined with available gasoline- and diesel-powered motor vehicle emissions profiles for the region suggested a greater impact to high molecular weight (HMW) PAHs from diesel-powered vehicles at the near-highway sites and a more uniformly distributed impact to ambient hopanes from gasoline-powered motor vehicles at all four sites.

Intra-urban spatial variability and uncertainty assessment of PM2.5 sources based on carbonaceous species

“…However, gasoline versus diesel vehicle contributions to these heavy PAHs could not be separated in this work, likely due to their similar emission factors (Phuleria et al, 2007). In a separate paper, Xie et al (in press) presented a ratio-ratio plot comparing indeno[1,2,3-cd]pyrene and benzo[ghi]perylene normalized by EC, which also suggested that gasoline and diesel were both significant sources.…”

“…This association is supported by observation that EC concentrations show a greater proportional reduction on the weekend than that in overall traffic counts (Dutton et al, 2010a). In ratio-ratio plots, the ambient sterane data at the four sites were mostly explained by a continuum of gasoline vehicle source profiles, and partially ascribed to light duty diesel vehicles (Xie et al, in press). As a result, this sterane/EC factor was categorized as lubricating oil combustion, which is also obtained by Dutton et al, (2010b).…”

“…A detailed description of the four sampling sites was given in Xie et al (in press); here the sites are summarized in Table 1. Details of the sampling equipment, protocols and chemical analysis were described previously (Dutton et al, 2009a, b; Vedal et al, 2009).…”

“…using alkanoic acids resulted in very low factor matching rates (<20%) due to their high variability in factor profiles). The samples collected on November 25, 2008 at all four sites were excluded from the source apportionment analysis as concentrations of all groups of species were unusually high (1.6 – 11 times greater than the annual averages) (Xie et al, in press). These extreme measurements were caused by heavy traffic combined with a stagnant atmosphere and cold weather.…”

“…In order to examine the spatial variability of source contributions to ambient PM 2.5 , samples were collected at four sites in Denver every sixth day for one year. The carbonaceous composition of all the PM 2.5 samples was characterized and reported in a separate paper (Xie et al, in press). In this work, PMF2 was applied to the pooled dataset, combining measurements of all sites, to identify and quantify PM 2.5 factor/source profiles and factor/source contributions.…”