Estimating the Avoided Tropospheric Ozone Radiative Forcing From US Air Pollution Controls

Regional air pollution controls imposed by the US Clean Air Act of 1970 reduced emissions of precursors to ozone (O 3 ), a radiatively active gas that primarily acts as a greenhouse gas in the troposphere. From a pair of CESM2(WACCM6) initial‐condition ensembles that simulate a “world avoided” by US air pollution controls from 1970 to 2013, we estimate the avoided tropospheric O 3 stratospheric‐adjusted radiative forcing (SARF) using an O 3 radiative kernel. We estimate an avoided global annual tropospheric O 3 SARF of +15 mW m −2 (+10–24, ensemble range) at the end of the period (2004–2013 average), approximately 4% (3%–6%) of the preindustrial‐to‐present‐day tropospheric O 3 SARF. The avoided SARF varies spatially and seasonally, peaking in the summer over the eastern US and North Atlantic Ocean at +169 mW m −2 (+155–182). While the longwave component dominates the near‐source summertime avoided O 3 SARF (>70%), the shortwave contributes >60% over the Arctic.