Observations from a Greenland ice core reveal that tropospheric oxidants are sensitive to climate-driven changes in reactive halogen chemistry and stratosphere-to-troposphere transport of ozone, in addition to ozone precursor emissions.
The abundance of tropospheric oxidants, such as ozone (O3) and hydroxyl (OH) and peroxy radicals (HO2 + RO2), determines the lifetimes of reduced trace gases such as methane and the production of particulate matter important for climate and human health. The response of tropospheric oxidants to climate change is poorly constrained owing to large uncertainties in the degree to which processes that influence oxidants may change with climate1 and owing to a lack of palaeo-records with which to constrain levels of atmospheric oxidants during past climate transitions2. At present, it is thought that temperature-dependent emissions of tropospheric O3 precursors and water vapour abundance determine the climate response of oxidants, resulting in lower tropospheric O3 in cold climates while HOx (= OH + HO2 + RO2) remains relatively buffered3. Here we report observations of oxygen-17 excess of nitrate (a proxy for the relative abundance of atmospheric O3 and HOx) from a Greenland ice core over the most recent glacial-interglacial cycle and for two Dansgaard-Oeschger events. We find that tropospheric oxidants are sensitive to climate change with an increase in the O3/HOx ratio in cold climates, the opposite of current expectations. We hypothesize that the observed increase in O3/HOx in cold climates is driven by enhanced stratosphere-to-troposphere transport of O3, and that reactive halogen chemistry is also enhanced in cold climates. Reactive halogens influence the oxidative capacity of the troposphere directly as oxidants themselves and indirectly4 via their influence on O3 and HOx. The strength of stratosphere-to-troposphere transport is largely controlled by the Brewer-Dobson circulation5, which may be enhanced in colder climates owing to a stronger meridional gradient of sea surface temperatures6, with implications for the response of tropospheric oxidants7 and stratospheric thermal and mass balance8. These two processes may represent important, yet relatively unexplored, climate feedback mechanisms during major climate transitions.