Halogen chemistry reduces tropospheric O₃ radiative forcing

Abstract. Tropospheric ozone (O₃) is a global warming gas, but the lack of a firm observational record since the preindustrial period means that estimates of its radiative forcing (RF_TO3) rely on model calculations. Recent observational evidence shows that halogens are pervasive in the troposphere and need to be represented in chemistry-transport models for an accurate simulation of present-day O₃. Using the GEOS-Chem model we show that tropospheric halogen chemistry is likely more active in the present day than in the preindustrial. This is due to increased oceanic iodine emissions driven by increased surface O₃, higher anthropogenic emissions of bromo-carbons, and an increased flux of bromine from the stratosphere. We calculate preindustrial to present-day increases in the tropospheric O₃ burden of 113 Tg without halogens but only 90 Tg with, leading to a reduction in RF_TO3 from 0.43 to 0.35 Wm⁻². We attribute  ∼ 50 % of this reduction to increased bromine flux from the stratosphere,  ∼ 35 % to the ocean–atmosphere iodine feedback, and  ∼ 15 % to increased tropospheric sources of anthropogenic halogens. This reduction of tropospheric O₃ radiative forcing due to halogens (0.087 Wm⁻²) is greater than that from the radiative forcing of stratospheric O₃ (∼ 0.05 Wm⁻²). Estimates of RF_TO3 that fail to consider halogen chemistry are likely overestimates (∼ 25 %).