Isotopic evidence of multiple controls on atmospheric oxidants over climate transitions

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. Atmospheric oxidant abundance in changing climates The abundance of tropospheric oxidants, such as ozone (O 3 ) and the hydroxyl radical (OH), determines the lifetimes of reduced trace gases such as methane, but the response of tropospheric oxidants to climate change is poorly constrained. This paper reports isotopic data from a Greenland ice core covering the most recent glacial–interglacial cycle that demonstrate the sensitivity of tropospheric oxidants to climate change. The authors observe an increase in the O 3 /HO x ratio in cold climates, the opposite of current expectations, and suggest that this increase might be driven by enhanced reactive halogen chemistry, enhanced stratosphere-to-troposphere transport of O 3 , or a combination of the two. These processes require further consideration when assessing global methane variations during periods of abrupt climate change. The abundance of tropospheric oxidants, such as ozone (O 3 ) and hydroxyl (OH) and peroxy radicals (HO 2 + RO 2 ), 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 climate 1 and owing to a lack of palaeo-records with which to constrain levels of atmospheric oxidants during past climate transitions 2 . At present, it is thought that temperature-dependent emissions of tropospheric O 3 precursors and water vapour abundance determine the climate response of oxidants, resulting in lower tropospheric O 3 in cold climates while HO x (= OH + HO 2 + RO 2 ) remains relatively buffered 3 . Here we report observations of oxygen-17 excess of nitrate (a proxy for the relative abundance of atmospheric O 3 and HO x ) 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 O 3 /HO x ratio in cold climates, the opposite of current expectations. We hypothesize that the observed increase in O 3 /HO x in cold climates is driven by enhanced stratosphere-t