The chemistry of nitrogen oxides (NO_x) affects both air quality and climate through its role in the production of ozone and secondary aerosols. We find that recent changes in emissions have caused a significant shift in the chemical loss of NO_x away from direct production of HNO₃ and towards production of organic nitrates. This shift is leading to a flatter distribution of NO_x across the United States and helping transform air pollution from a local issue into a broader regional concern.