Human exposure to ambient ozone (O
3) has been linked to a variety of adverse health effects. The ozone level at a location is contributed by local production, regional transport, and background ozone. This study combines detailed emission inventory, air quality modeling, and census data to investigate the source–receptor relationships between nitrogen oxides (NO
x) emissions and population exposure to ambient O
3 in 48 states over the continental United States. By removing NO
x emissions from each state one at a time, we calculate the change in O
3 exposures by examining the difference between the base and the sensitivity simulations. Based on the 49 simulations, we construct state-level and census region-level source–receptor matrices describing the relationships among these states/regions. We find that, for 43 receptor states, cumulative NO
x emissions from upwind states contribute more to O
3 exposures than the state's own emissions. In-state emissions are responsible for less than 15% of O
3 exposures in 90% of U.S. states. A state's NO
x emissions can influence 2 to 40 downwind states by at least a 0.1 ppbv change in population-averaged O
3 exposure. The results suggest that the U.S. generally needs a regional strategy to effectively reduce O
3 exposures. But the current regional emission control program in the U.S. is a cap-and-trade program that assumes the marginal damage of every ton of NO
x is equal. In this study, the average O
3 exposures caused by one ton of NO
x emissions ranges from ? 2.0 to 2.3 ppm-people-hours depending on the state. The actual damage caused by one ton of NO
x emissions varies considerably over space.
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