Radiative Forcing From Halogen Reservoir and Halocarbon Breakdown Products

Gillian D. Thornhill, Lucy A. Smith, Keith P. Shine

https://doi.org/10.1029/2024JD040912

Congratulations to our InHALE colleagues, Gill Thornhill, Lucy Smith and Keith Shine, at the University of Reading, who published their paper entitled “Radiative Forcing from halogen reservoir and halocarbon breakdown products” in the Journal of Geophysical Research, Atmospheres.

In their work radiative forcing of four halogen reservoir or halocarbon species were quantified using satellite derived distributions and found a global mean stratospherically adjusted radiative forcing of 7 mW m⁻². This is a total halocarbon enhancement radiative forcing enhancement of 3%.

Abstract

https://doi.org/10.1029/2024JD040912

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.

The InHALE project held its Kick-Off meeting on the 26th and 27th of January at the University of Bristol. This was held as a hybrid meeting with a group of six people joining online and 26 in person.

Over two days we heard a range of engaging talks from the work package leaders and science talks from other parts of the project. Each day talks were interspersed with a lab tour. On Thursday, small group tours were made of the Spectroscopy Facilities in the Chemistry Department which will be used in work package 4. After a day full of science on Thursday, meeting delegates enjoyed a relaxed meal at a local independent restaurant walking distance from the University in Cotham, Bristol.

Fridays lab tour of the Atmospheric Chemistry Research Group was very popular with the measurement community. The meeting was concluded with compelling keynote talk by Susan Solomon from Massachusetts Institute of Technology, thank you Susan for getting up at 6am to share your work with us!