Summertime state-level source-receptor relationships between nitrogen oxides emissions and surface ozone concentrations over the continental United States

Interstate transport of ozone (O₃) and its precursors can contribute substantially to state-level surface O₃ concentrations, making it difficult for some states to meet the National Ambient Air Quality Standards (NAAQS) for O₃ by limiting only their own emissions. We analyze the effect of interstate transport on surface O₃ in each continental U.S. state in July 1996 using the community multiscale air quality (CMAQ) model. By examining the difference between a baseline simulation and perturbation simulations in which each state's nitrogen oxides (NO_x) emissions are removed, we establish for the first time a summertime source-receptor matrix for all 48 continental states. We find that for 16 (20) states at least one neighboring state's NO_x emissions are responsible for a larger increase in monthly mean peak 8 h (all-hour) O₃ concentrations than the state's own emissions. For over 80% of the contiguous states, interstate transport is more important than local emissions for summertime peak O₃ concentrations. Our source-receptor matrices indicate that the geographic range of the clean air interstate rule (CAIR) was sufficient to address interstate transport of O₃ in most of the states included in the program. However, the exclusion of Texas, which has particularly large NO_x emissions, from the CAIR O₃ program left emission sources uncontrolled that contribute more than 1 ppbv to the July mean of peak 8 h O₃ concentrations in over a dozen states.