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 (-72% NE NOx 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 NOx largely controls the shape of surface O-3 seasonal cycles. In the absence of regional NOx controls, climate warming contributes to a higher surface O-3 summertime peak over the NE. A doubling of the global CH4 abundance by 2100 partially offsets summertime surface O-3 decreases attained via NOx 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.