Elsevier

Atmospheric Environment

Volume 45, Issue 24, August 2011, Pages 3924-3936
Atmospheric Environment

A source apportionment of U.S. fine particulate matter air pollution

https://doi.org/10.1016/j.atmosenv.2011.04.070Get rights and content

Abstract

Using daily fine particulate matter (PM2.5) composition data from the 2000–2005 U.S. EPA Chemical Speciation Network (CSN) for over 200 sites, we applied multivariate methods to identify and quantify the major fine particulate matter (PM2.5) source components in the U.S. Novel aspects of this work were: (1) the application of factor analysis (FA) to multi-city daily data, drawing upon both spatial and temporal variations of chemical species; and, (2) the exclusion of secondary components (sulfates, nitrates and organic carbon) from the source identification FA to more clearly discern and apportion the PM2.5 mass to primary emission source categories. For the quantification of source-related mass, we considered two approaches based upon the FA results: 1) using single key tracers for sources identified by FA in a mass regression; and, 2) applying Absolute Principal Component Analysis (APCA). In each case, we followed a two-stage mass regression approach, in which secondary components were first apportioned among the identified sources, and then mass was apportioned to the sources and to other secondary mass not explained by the individual sources. The major U.S. PM2.5 source categories identified via FA (and their key tracers) were: Metals Industry (Pb, Zn); Crustal/Soil Particles (Ca, Si); Motor Vehicle Traffic (EC, NO2); Steel Industry (Fe, Mn); Coal Combustion (As, Se); Oil Combustion (V, Ni); Salt Particles (Na, Cl) and Biomass Burning (K). Nationwide spatial plots of the source-related PM2.5 impacts were confirmatory of the factor interpretations: ubiquitous sources, such as Traffic and Soil, were found to be spread across the nation, more unique sources (such as Steel and Metals Processing) being highest in select industrialized cities, Biomass Burning was highest in the U.S. Northwest, while Residual Oil combustion was highest in cities in the Northeastern U.S. and in cities with major seaports. The sum of these source contributions and the secondary PM2.5 components agreed well with the U.S. PM2.5 observed during the study period (mean = 14.3 μg m−3; R2 = 0.94). Apportionment regression analyses using single-element tracers for each source category gave results consistent with the APCA estimates. Comparisons of nearby sites indicated that the PM2.5 mass and the secondary aerosols were most homogenous spatially, while traffic PM2.5 and its tracer, EC, were among the most spatially representative of the source-related components. Comparison of apportionment results to a previous analysis of the 1979–1982 IP Network revealed similar and correlated major U.S. source category factors, albeit at lower levels than in the earlier period, suggesting a consistency in the U.S. spatial patterns of these source-related exposures over time, as well. These results indicate that applying source-apportionment methods to the nationwide CSN can be an informative avenue for identifying and quantifying source components for the subsequent estimation of source-specific health effects, potentially contributing to more efficient regulation of PM2.5.

Highlights

► First nationwide source apportionment of U.S. PM2.5 Chemical Speciation Network data. ► A focus on primary emission tracers gave clearer source category interpretations. ► Spatial distributions of estimated impacts consistent with source interpretations. ► Co-pollutant correlations (e.g. SO2 and Hg with Coal) support source interpretations. ► Correlations with 1979–1982 IP Network results suggest spatial consistency over time.

Section snippets

Background

Long-term exposure to fine particulate matter (PM2.5) air pollution has been associated with increased risk of human mortality (e.g., Ozkaynak and Thurston, 1987, Dockery et al., 1993, Pope et al., 2002). However, the composition of PM2.5 mass can vary significantly with its origins, and it is likely that particles from different sources can have differing toxicities (NRC, 2004, WHO, 2007). The U.S. EPA decision to add a PM2.5 fine particle National Ambient Air Quality Standard (NAAQS) in 1997

Data

The U.S. EPA Air Quality System provides routine air monitoring measurements for PM2.5 mass, PM2.5 anions (sulfate, nitrate) and cations (ammonium, sodium, and potassium), trace elements (Na through Pb on the periodic table), total carbon [including organic carbon (OC) and elemental carbon (EC)], and gaseous pollutant data (CO, NO2, SO2, O3). These data have been compiled by the Health Effects Institute (HEI) and for this work PM2.5 chemical speciation data, EC, OC, Nitrates (NO3−), SO4, NH4

Results

The analysis dataset ultimately included 46,478 daily observations for 212 CSN monitoring sites distributed throughout the U.S. (Fig. 1). For the sites considered in this work, there was an average of 220 observations per site (or almost two years of data per site at the common network data collection rate of every-third-day sampling). Table 1 provides a nationwide summary of PM2.5 and its constituents considered in the factor model. Averages by season and by the five U.S. regions considered in

Discussion

This is the first study to conduct a nationwide factor analysis and source apportionment of the newly available daily CSN data. Additionally, this work takes a unique approach to the identification and quantification of PM sources: secondary aerosol constituents were not included in the source component identification factorization step. By only including tracers of primary emissions in the factor analysis (e.g., Se, As, Ni, V, EC), while excluding tracers of secondary formation (i.e., S, OC,

Acknowledgements

Research supported by the Health Effects Institute (HEI) National Particle Component Toxicity (NPACT) Initiative and NYU’s National Institute of Environmental Health Sciences (NIEHS) Center Grant (5P30ES000260).

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