Twenty‐first century reversal of the surface ozone seasonal cycle over the northeastern United States

Changing emissions can alter the surface O 3 seasonal cycle, as detected from northeastern U.S. (NE) observations during recent decades. Under continued regional precursor emission controls (>80% decreases in NE NO x by 2100), the NE surface O 3 seasonal cycle reverses (to a winter maximum) in 21st century transient chemistry‐climate simulations. Over polluted regions, regional NO x largely controls the shape of surface O 3 seasonal cycles. In the absence of regional NO x controls, climate warming contributes to a higher surface O 3 summertime peak over the NE. A doubling of the global CH 4 abundance by 2100 partially offsets summertime surface O 3 decreases attained via NO x reductions and contributes to raising surface O 3 during December–March when the O 3 lifetime is longer. The similarity between surface O 3 seasonal cycles over the NE and the Intermountain West by 2100 indicates a NE transition to a region representative of baseline surface O 3 conditions.