Atmospheric Chemistry of Organic Acids

Organic acids, principally formic and acetic acids, pervade the troposphere influencing the acidity of rain, aerosol growth and cloud formation. Sources are direct emissions from vegetation, biomass burning and anthropogenic activities and secondary production via gas-phase oxidation of volatile organic compounds (VOCs) such as isoprene, formaldehyde and methanol. These acids partake in multiphase processes including uptake into aqueous aerosols and cloud droplets, where photochemical and radical-driven reactions alter their lifetimes and yields. The hydroxyl radical (OH) initiates oxidation pathways in both gas and aqueous phases, producing carboxylic acids that modulate the oxidative capacity of the atmosphere. In cloud water, reversible hydration of aldehydes establishes diol-mediated pathways that may greatly enhance formic acid formation relative to gas-phase sources alone. Organic acids contribute to secondary organic aerosol (SOA) mass, affect nucleation of cloud condensation nuclei (CCN) through acid–base interactions and adjust pH, thereby altering aerosol optical properties and heterogeneous reaction rates.

Research from Nature Portfolio

Recent studies have uncovered rapid multiphase routes to organic acids that challenge existing budgets. Chamber experiments have demonstrated that formaldehyde dissolved in warm cloud droplets hydrates to methanediol, which outgasses and oxidises efficiently to gaseous formic acid at rates that may quadruple all known gas-phase sources combined. This diol mechanism reconciles long-standing discrepancies between measured and modelled formic acid levels and highlights the role of aqueous-phase chemistry in atmospheric budgets. Foundational work has also revealed that the reaction between methyl peroxy radicals and hydroxyl radicals proceeds at unexpectedly high rates, forming methanol through a stabilised methylhydrotrioxide intermediate. Modelling predicts that this pathway contributes an annual methanol source comparable to terrestrial emissions, with significant implications for radical cycling and secondary organic acid formation in remote regions.

Atmospheric Chemistry of Organic Acids publication trend

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

Technical terms

Secondary organic aerosol (SOA): Particulate matter formed from the atmospheric oxidation of volatile organic compounds, often enriched in organic acids.

Multiphasic pathway: Reaction sequence involving both gas and liquid phases, such as oxidation within cloud droplets followed by gas-phase outgassing.

Hydroxyl radical (OH): A highly reactive atmospheric oxidant initiating the oxidation of VOCs to produce organic acids.

Cloud condensation nuclei (CCN): Aerosol particles that serve as seeds for cloud droplet formation, influenced by acid–base chemistry and hygroscopicity.

References

1. Ubiquitous atmospheric production of organic acids mediated by cloud droplets. Nature (2021).
2. The reaction of methyl peroxy and hydroxyl radicals as a major source of atmospheric methanol. Nature Communications (2016).
3. Measurement report: Enhanced photochemical formation of formic and isocyanic acids in urban regions aloft – insights from tower-based online gradient measurements. Atmospheric Chemistry and Physics (2024).
4. Emission of volatile organic compounds from residential biomass burning and their rapid chemical transformations. The Science of The Total Environment (2023).