Changing emissions can alter the surface O₃ seasonal cycle, as detected from northeastern U.S. (NE) observations during recent decades. Under continued regional precursor emission controls (>80% decreases in NE NOx by 2100), the NE surface O₃ seasonal cycle reverses (to a winter maximum) in 21st century transient chemistry-climate simulations. Over polluted regions, regional NOx largely controls the shape of surface O₃ seasonal cycles. In the absence of regional NOx controls, climate warming contributes to a higher surface O₃ summertime peak over the NE. A doubling of the global CH₄ abundance by 2100 partially offsets summertime surface O₃ decreases attained via NOx reductions and contributes to raising surface O₃ during December-March when the O₃ lifetime is longer. The similarity between surface O₃ seasonal cycles over the NE and the Intermountain West by 2100 indicates a NE transition to a region representative of baseline surface O₃ conditions.
