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

Abstract

Regional air pollution controls imposed by the US Clean Air Act of 1970 reduced emissions of precursors to ozone (O₃), 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 O₃ stratospheric-adjusted radiative forcing (SARF) using an O₃ radiative kernel. We estimate an avoided global annual tropospheric O₃ 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 O₃ 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 O₃ SARF (>70%), the shortwave contributes >60% over the Arctic.

Plain Language Summary

Ozone (O₃) is a harmful surface air pollutant and a greenhouse gas in the troposphere. Observations suggest that human activities have increased the concentration of tropospheric O₃ since the preindustrial era, exerting a positive radiative forcing (RF) and warming influence on the planet. Historical air pollution controls that reduced regional emissions of O₃ precursors likely avoided additional positive tropospheric O₃ RF. Here, we estimate the global and local avoided tropospheric O₃ RF from US air pollution controls between 1970 and 2013 using simulations from a global climate model with atmospheric chemistry. We find that US air pollution controls may have avoided an additional 3%–6% of the total global tropospheric O₃ RF relative to the preindustrial era. Due to the strong seasonality of O₃, US air pollution controls primarily avoided additional tropospheric O₃ RF in the summertime, particularly over the eastern US and North Atlantic Ocean, where O₃ is efficiently transported to the upper troposphere. The strong seasonal and spatial concentration of tropospheric O₃ RF from regional emissions controls may induce climate responses that differ from those of well-mixed greenhouse gases like carbon dioxide and methane.