The influence of short-lived halogens on atmospheric chemistry and climate
Article metadata
Article Date: 2025-12-10
Article URL: https://www.nature.com/articles/s41586-025-09753-x
Article Image: (not provided)
Summary
This article reviews and synthesises evidence that short-lived halogens (SLHs) — reactive chlorine, bromine and iodine compounds with lifetimes from hours to months — have major, multifaceted impacts on atmospheric chemistry and climate. It covers observational advances (satellite, aircraft, ground, balloon), laboratory and field studies of sources and formation pathways (natural ocean emissions, biogenic sources, volcanic injection, heterogeneous reactions on sea spray/dust, and anthropogenic emissions), and modelling work that quantifies effects on ozone (tropospheric and stratospheric), oxidants (OH), methane lifetime, aerosol formation and radiative forcing. Recent findings show increasing atmospheric iodine and new anthropogenic growth in certain SLH emissions, and demonstrate links to mercury oxidation, air quality, aerosol nucleation and modulation of climate forcing.
Key Points
- SLHs (Cl, Br, I) substantially deplete ozone in both the troposphere and stratosphere via catalytic cycles; bromine and iodine are particularly efficient per atom.
- Observations (satellite, in situ, balloon) and ice/ice-core/tree-ring records show rising iodine levels since the mid-20th century and growing anthropogenic sources of chlorine-containing SLHs (e.g. dichloromethane, chloroform).
- Natural oceanic emissions and biogenic iodine drive particle nucleation (iodine oxoacids) and marine aerosol formation, with potential cooling effects on climate through increased aerosol burdens and cloud interactions.
- Anthropogenic SLHs and pollution-enhanced halogen activation (including from wildfire/biomass aerosol and coastal pollution) alter tropospheric oxidants, increasing methane lifetime and changing radiative forcing.
- SLHs convert elemental mercury to oxidised forms, increasing deposition and human exposure — a linkage between halogen chemistry and mercury risk.
- Volcanic eruptions and wildfire plumes can inject halogens or activate halogen chemistry in the upper atmosphere, amplifying ozone depletion under certain conditions.
- Heterogeneous chemistry on sea spray, mineral dust and snow/ice surfaces is a major source and activation pathway for SLHs in polar and midlatitude regions.
- Inclusion of SLHs in chemistry–climate models materially changes projections of ozone, oxidants, methane lifetime and radiative forcing; omission biases air-quality and climate assessments.
Content summary
The paper compiles decades of research demonstrating that SLHs originate from multiple natural and anthropogenic sources and undergo rapid reactive cycles that control oxidant budgets and ozone. Satellite records and ground/balloon observations have mapped BrO and IO distributions, while laboratory work and field campaigns have elucidated emission mechanisms — for example iodide/ozone chemistry at the sea surface, photochemical release from snow/sea-ice, and heterogeneous formation of reactive chlorine in polluted and biomass-burning plumes.
Modelling studies show SLHs influence both local air quality (ozone and particulate pollution) and global climate via indirect pathways: by depleting ozone (affecting UV and radiative forcing), altering OH (hence methane lifetime), and nucleating aerosols (particularly from iodine oxoacids) that modify cloud–radiation interactions. Recent observational trends point to increasing iodine emissions and rising anthropogenic emissions of some very short-lived chlorocarbons, which together may strengthen halogen-driven impacts in the 21st century.
Context and relevance
The role of SLHs intersects with major environmental themes: ozone recovery, climate forcing, air quality regulation and pollutant–ecosystem interactions (e.g. mercury). For climate modellers and air-quality scientists, the article underscores that neglecting SLHs produces sizable biases in projections of ozone, methane lifetime and aerosol forcing. For policymakers, it highlights that both natural feedbacks (ocean emissions influenced by ozone/pollution and climate) and anthropogenic emissions matter — so mitigation strategies should consider SLH sources alongside traditional greenhouse gases and ozone-depleting substances.
Author style
Punchy. This is important — not niche. The review bundles observational proof, mechanistic chemistry and model results to show SLHs are a major player across air quality and climate systems. If you care about ozone recovery, methane budgets or aerosol–cloud effects, the details here matter; skim at your peril.
Why should I read this?
Look — if you want to understand why ozone, methane and aerosols might not behave the way your current model expects, this piece brings together the missing halogen angle. It saves you trawling 150 references: sources, observations and models that show halogens change air quality, mercury exposure and climate signals. Quick, relevant and a bit alarming — in a useful way.