GC34A-06 Quantifying the tropospheric ozone radiative forcing from US air pollution controls

Regional air quality controls imposed by the US Clean Air Act (CAA) of 1970 reduced the concentrations of short-lived climate forcers (SLCFs) in the troposphere, particularly of ozone (O₃) and aerosols, which have opposing radiative effects on Earth’s energy budget. Accurately quantifying the radiative forcing from changes in regional air pollutant emissions is challenging because of the highly variable abundances of SLCFs that depend on a combination of emissions (both natural and anthropogenic), transport, and chemistry. These factors are also influenced by natural (internal) climate variability, which is large on regional scales. In previous work, we used a pair of initial-condition ensembles generated by the fully-coupled chemistry-climate model, CESM2–WACCM6, to simulate a “world avoided” by US air pollution controls (USfrozenEF) relative to historical emissions (HIST) from 1970–2013 (Elkins et al., 2025). We identified the emissions-driven signal in tropospheric column O₃ and aerosols relative to internal variability for the decade 2004–2013. Here, we expand upon this work to quantify the tropospheric O₃ radiative forcing from avoided US air pollutant emissions, assessing the magnitude and spatial pattern of the ensemble mean radiative forcing from tropospheric O₃ and the influence of internal variability. We apply an O₃ radiative kernel (developed via idealized O₃ perturbation simulations using a radiative transfer model) to derive the net stratospheric-adjusted radiative forcing (SARF) from the transient O₃ fields simulated by each ensemble member. We find an ensemble mean net SARF of +0.016 W m^-2 from O₃ in USfrozenEF relative to HIST for 2004–2013, ranging from +0.011–0.024 W m^-2 across individual USfrozenEF ensemble members due to the influence of internal variability. This result implies that US emissions controls may have lowered the total preindustrial to present day O₃ radiative forcing (as estimated by CMIP6 models) by ~3–6%, a small but non-negligible contribution from regional policy. The ensemble mean signal in net SARF from O₃ largely lies north of 20°N, with over 65% attributable to the upper troposphere (<380 hPa), highlighting the substantial role of transport to the upper troposphere where O₃ acts as a potent greenhouse gas.