Spatial and Temporal Variation in PM2.5 Chemical Composition in the United States for Health Effects Studies

BACKGROUND Although numerous studies have demonstrated links between particulate matter (PM) and adverse health effects, the chemical components of the PM mixture that cause injury are unknown. OBJECTIVES This work characterizes spatial and temporal variability of PM(2.5) (PM with aerodynamic diameter < 2.5 microm) components in the United States; our objective is to identify components for assessment in epidemiologic studies. METHODS We constructed a database of 52 PM(2.5) component concentrations for 187 U.S. counties for 2000-2005. First, we describe the challenges inherent to analysis of a national PM(2.5) chemical composition database. Second, we identify components that contribute substantially to and/or co-vary with PM(2.5) total mass. Third, we characterize the seasonal and regional variability of targeted components. RESULTS Strong seasonal and geographic variations in PM(2.5) chemical composition are identified. Only seven of the 52 components contributed >/= 1% to total mass for yearly or seasonal averages [ammonium (NH(4) (+)), elemental carbon (EC), organic carbon matter (OCM), nitrate (NO(3) (-)), silicon, sodium (Na(+)), and sulfate (SO(4) (2-))]. Strongest correlations with PM(2.5) total mass were with NH(4) (+) (yearly), OCM (especially winter), NO(3) (-) (winter), and SO(4) (2-) (yearly, spring, autumn, and summer), with particularly strong correlations for NH(4) (+) and SO(4) (2-) in summer. Components that co-varied with PM(2.5) total mass, based on daily detrended data, were NH(4) (+), SO(4) (2-) (,) OCM, NO(3) (2-), bromine, and EC. CONCLUSIONS The subset of identified PM(2.5) components should be investigated further to determine whether their daily variation is associated with daily variation of health indicators, and whether their seasonal and regional patterns can explain the seasonal and regional heterogeneity in PM(10) (PM with aerodynamic diameter < 10 microm) and PM(2.5) health risks.