Radiative Forcing From Halogen Reservoir and Halocarbon Breakdown Products

Abstract

The direct radiative forcing (RF) from halocarbons is reasonably well characterized. However, the forcing due to polyatomic halogen reservoir and halocarbon breakdown products has not previously been quantified and it is important to estimate this contribution. Four gases, ClONO2, COCl2, COF2 and COClF, are considered; their stratospheric abundances mostly originate from the breakdown of chlorofluorocarbons, hydrochlorofluorocarbons and CCl4. They have significant mid‐infrared absorption bands and peak stratospheric mole fractions ranging from around 20 ppt to over 1 ppb, which are large compared to typical abundances of many emitted halocarbons. Using satellite observations of stratospheric abundance, observed infrared spectra, and a narrow‐band radiation code, the stratosphere‐adjusted radiative forcings (SARF) is computed. The global‐annual mean SARF is estimated to be 7 ± 0.8 mW m−2 based on measured abundances in the period 2004–2019, with ClONO2 contributing about 50%. Whilst not a major contributor to anthropogenic RF, only six individual halocarbon gases cause a significantly greater forcing. This forcing is then approximately attributed to their source gases; for most, it modestly enhances (by 1%–3%) both their direct RF and their global warming potentials. The most significant enhancement (5%–15%) is to CCl4, the principal source of stratospheric COCl2 and contributor to ClONO2 abundances; disagreement in recent satellite‐based COCl2 retrievals is a significant source of uncertainty. These additional gases enhance the available best estimate of the total forcing due to halocarbon source gases (including e.g., ozone depletion) by about 3%; notably, this is the only identified indirect mechanism that increases, rather than decreases, total halocarbon forcing.