Community-level spatial heterogeneity of chemical constituent levels of fine particulates and implications for epidemiological research

Abstract

Studies of the health impacts of airborne particulates' chemical constituents typically assume spatial homogeneity and estimate exposure from ambient monitors. However, factors such as local sources may cause spatially heterogeneous pollution levels. This work examines the degree to which constituent levels vary within communities and whether exposure misclassification is introduced by spatial homogeneity assumptions. Analysis considered PM(2.5) elemental carbon (EC), organic carbon matter, ammonium, sulfate, nitrate, silicon, and sodium ion (Na(+)) for the United States, 1999-2007. Pearson correlations and coefficients of divergence were calculated and compared to distances among monitors. Linear modeling related correlations to distance between monitors, long-term constituent levels, and population density. Spatial heterogeneity was present for all constituents, yet lower for ammonium, sulfate, and nitrate. Lower correlations were associated with higher distance between monitors, especially for nitrate and sulfate, and with lower long-term levels, especially for sulfate and Na(+). Analysis of colocated monitors revealed measurement error for all constituents, especially EC and Na(+). Exposure misclassification may be introduced into epidemiological studies of PM(2.5) constituents due to spatial variability, and is affected by constituent type and level. When assessing health effects of PM constituents, new methods are needed for estimating exposure and accounting for exposure error induced by spatial variability.

Figure 1

Correlation of monitor pairs versus distance for PM_2.5 chemical constituents. Note: Each circle represents a monitor pair. The size of the circle represents the number of observation days in each monitor pair; larger circles have larger sample sizes. EC, elemental carbon; OCM, organic carbon matter.

Figure 2

Percent change in correlation of constituent levels at monitor pairs as a function of distance between monitor pairs, long-term constituent levels, and population density. Note: The circles represent central estimates and the horizontal lines the 95% intervals. Interquartile range (IQR) values are 1.1, 2.8, 1.4, 3.5, 0.5, 0.1, and 0.1 µg/m³ for nitrate, sulfate, ammonium, organic carbon matter, elemental carbon, sodium ion, and silicon, respectively.