Radical Chemistry in Atmospheric Systems

Radical species, notably hydroxyl (OH), hydroperoxy (HO2) and organic peroxy (RO2) radicals, occupy a central role in atmospheric oxidation processes. These highly reactive intermediates initiate the degradation of pollutants and volatile organic compounds (VOCs), thereby determining air quality, ozone formation and the self-cleansing capacity of the troposphere. Radical production arises from photolysis of precursors such as ozone, nitrous acid and oxygenated VOCs, while interconversion and termination pathways govern radical lifetimes and propagation. Recent advances reveal unexpected sources and sinks, including autoxidation of carbonyl-containing peroxy radicals and rapid photolysis of hydroperoxyl-carbonyl species. Such mechanisms help resolve longstanding discrepancies between observed radical concentrations and modelled budgets, especially under conditions of low nitrogen oxides and high VOC loads. The interplay of radical chemistry with regional emissions, meteorology and secondary aerosol formation underpins global air-pollution events and feedbacks on climate. Improved characterisation of radical processes informs strategies to mitigate ozone and fine-particle pollution, emphasising the need for integrated field observations, laboratory kinetics and three-dimensional modelling to capture the complexity of atmospheric radical networks across diverse environments.

Research from Nature Portfolio

Recent studies have revealed an unrecognised pathway for hydroxyl regeneration via autoxidation of higher aldehyde-derived peroxy radicals. Quantum mechanical analyses indicate that carbonyl-containing peroxy radicals undergo rapid hydrogen migrations to form hydroperoxyl-carbonyl intermediates. Subsequent photolysis of these species yields new OH radicals at rates that can close the gap between measured and modelled OH in low-NOx, high-VOC regimes. This mechanism may dominate OH production in biogenically enriched and urban–rural transition zones, reshaping our understanding of the tropospheric self-cleansing capacity under future emissions scenarios seeking nitrogen oxide reductions.

Radical Chemistry in Atmospheric Systems publication trend

The graph below shows the total number of articles in radical chemistry in atmospheric systems across all publications each year (not limited to Nature Index journals).

Technical terms

Hydroxyl radical (OH): The primary daytime oxidant in the troposphere, initiating degradation of pollutants and trace gases.

Hydroperoxy radical (HO2): A secondary radical formed from OH reactions, propagating radical chains and interconverting with OH.

Organic peroxy radical (RO2): Derived from VOC oxidation, instrumental in ozone formation and radical propagation.

Photolysis: Light-induced dissociation of molecules, generating radicals such as OH and O(¹D).

Autoxidation: Intramolecular radical migration within peroxy intermediates, leading to rapid formation of new oxidants.

References

1. Reactive aldehyde chemistry explains the missing source of hydroxyl radicals. Nature Communications (2024).
2. Radical chemistry at a rural site (Wangdu) in the North China Plain: observation and model calculations of OH, HO2 and RO2 radicals. Atmospheric Chemistry and Physics (2017).
3. Experimental budgets of OH, HO2, and RO2 radicals and implications for ozone formation in the Pearl River Delta in China 2014. Atmospheric Chemistry and Physics (2019).