Short-lived halogens, or SLH, are a deceptively simple family of compounds with outsized consequences. The group includes organic very short-lived substances such as bromoform, dibromomethane and methyl iodide, which are emitted largely by biologically active oceans as by-products of algae and phytoplankton metabolism, as well as inorganic halogen families — collectively labelled Cly, Bry and Iy — that cycle rapidly between reactive and reservoir forms. Unlike the infamous chlorofluorocarbons that linger for decades and only break down in the stratosphere, these short-lived species are destroyed quickly, releasing chlorine, bromine and iodine atoms low in the atmosphere where they catalytically attack ozone, alter the abundance of the hydroxyl radical that acts as the atmosphere’s detergent, and reshape the partitioning of nitrogen oxides that govern smog formation.

The new release is technically ambitious. CESM2-SLH embeds offline emission inventories for nine oceanic halocarbons derived from SeaWiFS chlorophyll-a climatology, anthropogenic emissions of dichloromethane and perchloroethylene based on recent inventories, and — crucially — online, interactive sources of inorganic halogens. These online sources include ozone-driven oxidation of iodide at the sea surface, which releases hypoiodous acid and molecular iodine and accounts for roughly three-quarters of all atmospheric iodine, and sea-salt aerosol dehalogenation, in which oxidised halogen reservoirs collide with airborne sea salt and liberate molecular chlorine, bromine and mixed halogens. The team had to retune accommodation coefficients — reducing them by factors of three to eight — because CESM2 represents sea-salt aerosol with a different modal scheme than its predecessor, producing up to ten times larger aerosol surfaces in the boundary layer.

Chemistry itself was expanded dramatically. The mechanism adds twelve chlorine, nine bromine and nineteen iodine species participating in more than two hundred new reactions spanning photolysis, gas-phase kinetics, heterogeneous recycling on aerosols, ice crystals and cloud droplets, and stratospheric activation pathways. Rate constants were updated to the latest Jet Propulsion Laboratory and IUPAC recommendations, and the photolysis of higher-order iodine oxides — species whose inclusion proved essential to reproduce upper-tropospheric iodine observations — is now handled explicitly. The computational price is a twenty to twenty-five percent increase in model runtime, a modest cost for what the chemistry delivers.

The headline result is striking: including short-lived halogens cuts global surface ozone by twenty-one to twenty-eight percent and tropospheric ozone by seventeen to twenty-two percent, with stratospheric columns dropping by up to three percent. Hydroxyl radical abundances fall by two to nine percent and nitrogen dioxide by one to ten percent, depending on configuration and resolution. Because ozone is both a pollutant and a greenhouse gas, and because OH controls the lifetime of methane, these numbers ripple through every projection of future air quality and climate forcing made with the model. The authors emphasise that halogen-driven ozone destruction in the troposphere reaches roughly 660 teragrams per year, dominated — somewhat surprisingly to non-specialists — by iodine chemistry, which alone accounts for up to thirty percent of ozone loss in the tropical free troposphere.

Validation against observations is encouraging. Simulated vertical profiles of organic source gases in the tropical tropopause layer fall within the ranges assessed by the most recent World Meteorological Organization ozone assessment, and the model reproduces the split between source gas injection and product gas injection that determines how efficiently halogens reach the stratosphere. Bromine from short-lived sources adds about five parts per trillion to the stratospheric loading — a twenty-five percent enhancement over long-lived sources — while iodine reaches the stratosphere almost entirely in inorganic form at roughly 0.7 parts per trillion. Comparisons with ship-based campaigns measuring iodine monoxide across the Pacific and Atlantic, and with coastal bromine observations at Cape Verde, show the model capturing magnitudes and day-to-day variability, if not always exact timing.

Perhaps the most consequential finding concerns the Antarctic ozone hole. When short-lived bromine and iodine chemistry is included, springtime ozone destruction over the polar vortex deepens by more than twenty Dobson units — about fourteen percent — in October, confirming earlier CESM1 results with the new model generation. Because the radiative balance of the climate system is exquisitely sensitive to ozone changes in the lowermost stratosphere, this region matters far beyond its thin vertical extent. Previous work by the same groups has shown that these halogens exert an overall cooling influence on climate through coupled effects on ozone, methane, aerosols and stratospheric water vapour, meaning that models lacking this chemistry may systematically misjudge both air quality trajectories and radiative forcing.

The release is deliberately user-friendly. Six new model compsets span nudged and free-running configurations at coarse and fine horizontal resolutions, from 1.9 by 2.5 degrees down to 0.9 by 1.25 degrees, and from low-top configurations reaching about forty kilometres to whole-atmosphere WACCM setups extending into the lower thermosphere. A new namelist section exposes scaling factors that let users adjust sea-salt dehalogenation and washout efficiencies when switching resolutions or meteorological datasets, and a technical appendix walks newcomers through spin-up requirements, emission specifiers, deposition lists and cross-section files. The authors recommend that new users start with the coarse, nudged configuration, where transport is consistent across experiments and chemical signals can be isolated cleanly from dynamical noise.

Not every development from the CESM1 era made the cut. Emission-driven methane simulations, continental inorganic halogen emissions from coal burning, polar sea-ice halogen sources, iodine release from dust, and the expanded sulphur chemistry involving methanethiol and hydroperoxymethyl thioformate are all absent from this first release, flagged for future work. The authors are candid that these omissions mean the simulated halogen influence is, in most respects, a lower limit. Even so, the message to the community is unambiguous: any serious simulation of past, present or future atmospheric composition — from urban smog to the ozone hole — should now include the reactive halogens that the ocean, sea salt and human industry continuously pump into the air. With CESM2-SLH freely available, there is little excuse left to leave them out.

Subject of Research: Implementation of short-lived halogen sources and chemistry in the Community Earth System Model v2

Article Title: Short-lived halogen sources and chemistry in the Community Earth System Model v2 (CESM2-SLH)

Article References: Fernandez, R. P., Cuevas, C. A., Villamayor, J., Feinberg, A., Kinnison, D. E., Vitt, F., Bossolasco, A., Barrera, J. A., Reynoso, A., Tomazzeli, O. G., Li, Q., & Saiz-Lopez, A. (2026). Short-lived halogen sources and chemistry in the Community Earth System Model v2 (CESM2-SLH). Geoscientific Model Development, 19(19), 9325-9376. https://doi.org/10.5194/gmd-19-9325-2026

Image Credits: AI Generated

DOI: 10.5194/gmd-19-9325-2026

Keywords: short-lived halogens, CESM2, atmospheric chemistry, ozone depletion, iodine emissions, bromine, chlorine, sea-salt aerosol, stratosphere, tropospheric ozone, hydroxyl radical, climate modelling