Decadal tropospheric ozone radiative forcing estimations with offline radiative modelling and IAGOS aircraft observations

Abstract. We use an offline radiative transfer model driven by IAGOS aircraft observations, to estimate the tropospheric ozone radiative forcing (RF) at decadal time scale (two time intervals between 1994–2004 and 2011–2016 or 2019), over 11 selected Northern Hemispheric regions. On average, we found a systematic positive trend in the tropospheric ozone column (TOC) for both time intervals, even if trends are reduced in 2019 (∆TOC +2.5±1.7 DU, +9.3±7.7 %) with respect to 2011–2016 (∆TOC +3.6±2.0 DU, +14.9±11.5 %). The reduced TOC average trend in 2019 with respect to 2011–2016, originates mostly from decreases of the lower tropospheric ozone column (LTOC) trends and limited variations for upper tropospheric ozone column (UTOC) trends, in the tropics. These average reductions in TOC trends are not accompanied with reductions of the tropospheric ozone RF, between 2011–2016 (4.2±2.4 mW m^-2 per year) and 2019 (3.8±3.6 mW m^-2 per year). This disconnection depends by the smaller RF sensitivity to LTOC than UTOC changes. Correspondingly, the total tropospheric ozone RF sensitivity varies between 18.4±7.4 mW m^-2 per DU, in 2011–2016, and 31.6±20.3 mW m^-2 per DU, in 2019. About 84–85 % of the tropospheric ozone RF occurs in the longwave, with ~4–6 % larger values of this proportion in the tropics than in the extra-tropics. Our estimates are 60–90 % larger than the most recent global average tropospheric ozone RF estimates with online modelling. Our study underlines the importance of the evolution of ozone vertical profiles for the tropospheric ozone RF.