PNNL

Abstract

Since the 1970s, air pollutant emissions controls in the United States (US) have lowered concentrations of ozone (O3) and aerosols, which have opposing radiative effects on surface temperature. Using a pair of initial-condition ensembles generated by a fully-coupled chemistry-climate model, we simulate the “world avoided” by US air pollution controls. Increases in free tropospheric O3, robust to natural internal variability, extend across the Northern Hemisphere while the aerosol signal remains localized near sources. Remote O3 increases partially reflect production from transported oxidized nitrogen that releases precursor nitrogen oxides downwind. Wintertime Northwest Atlantic cloud droplet number concentration is particularly sensitive to US aerosol. An ensemble mean US regional cooling signal (-0.4°C) implies that aerosol-driven cooling prevails over any O3-induced warming, but we show that internal variability will confound its detection in observations. Larger signal-to-noise ratios for composition versus climate variables highlight the greater detectability of emissions-driven changes in tropospheric composition versus their associated climate impacts.