Hyper-local to regional exposure contrast of source-resolved PM components across the contiguous United States: implications for health assessment.

Background: Improved understanding of sources and processes that drive exposure contrast of fine particulate matter (PM) is essential for designing and interpreting epidemiological study outcomes.

Objective: We investigate the contribution of various sources and processes to PM exposure contrasts at different spatial scales across the continental United States.

Methods: We consider three cases: exposure contrast within a metro area, nationwide exposure contrast with high spatial resolution, and nationwide exposure contrast with low spatial resolution. Using national empirical model estimates of source- and chemically specific PM concentration predictions, we quantified the contribution of various sources and processes to PM exposure contrasts in these three cases.

Results: At the metro level (i.e., metropolitan statistical area; MSA), exposure contrasts of PM vary between -1.8 to 1.4 µg m relative to the MSA-mean with about 50% of within-MSA exposure contrast of PM caused by cooking and mobile source primary PM. For the national exposure contrast at low-resolution (i.e., using MSA-average mean concentrations), exposure contrasts (relative to the national mean: -3.9 to 3.2 µg m) are larger than within an MSA with ~80% of the variation due to secondary PM. National exposure contrast at high resolution (census block) has the largest absolute range (relative to the national mean: -4.7 to 3.7 µg m) due to both regional and intra-urban contributions; on average, 65% of the national exposure contrast is due to secondary PM with the remaining from the primary PM (cooking and mobile source 26%, other 9%).

Impact: Our study provides a comprehensive analysis of the sources and processes that contribute to exposure contrasts of PM across different geographic areas in the US. For the first time on a national scale, we used high spatial resolution source-specific exposure estimates to identify the primary contributors to PM exposure contrasts. The study also highlights the advantages of different study designs for investigating the health impacts of specific PM components. The findings provide novel insights that can inform public health policies aimed at reducing PM exposure and advance the understanding of the epidemiological study outcomes.