Secondary Organic Aerosol Formation Mechanisms

Secondary organic aerosol (SOA) forms when volatile organic compounds (VOCs) emitted from both natural and anthropogenic sources undergo atmospheric oxidation, yielding lower‐volatility products that partition into the particle phase. Key oxidative agents include hydroxyl radicals, ozone and nitrate radicals, which attack unsaturated bonds to produce multifunctional organics such as aldehydes, ketones, peroxides and organic acids. These products can undergo further accretion reactions, oligomerisation and heterogeneous chemistry in the aqueous phase or on particle surfaces. Environmental parameters—especially NOx levels, relative humidity and aerosol acidity—influence the balance between fragmentation and functionalisation, thereby controlling SOA yield, composition and volatility. Biogenic precursors such as isoprene and monoterpenes typically generate highly oxygenated molecules through auto-oxidation pathways, whereas anthropogenic aromatics often yield both ring-retaining and ring-opening products. The interplay of gas–particle partitioning, multiphase reactions and reactive uptake drives the global distribution of SOA, with significant implications for climate forcing, air quality and human health. Despite advances, quantitative prediction of SOA burdens remains challenging due to the complexity of reaction networks and variability in atmospheric conditions.

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Secondary Organic Aerosol Formation Mechanisms publication trend

The graph below shows the total number of articles in secondary organic aerosol formation mechanisms across all publications each year (not limited to Nature Index journals).

Technical terms

Volatility: A measure of a compound’s tendency to remain in the gas phase versus partition into particles, often expressed as saturation mass concentration.

Hydroxyl radical (OH): A highly reactive species in the atmosphere that initiates oxidation of volatile organic compounds.

Ozonolysis: The reaction of ozone with unsaturated organic compounds, producing oxygenated fragments that can form SOA.

Gas–particle partitioning: The reversible distribution of semi-volatile species between gas and particle phases based on compound volatility and atmospheric conditions.

Reactive uptake: The process by which gaseous species dissolve or adsorb onto particle surfaces or within aqueous phases and undergo chemical transformation, contributing to SOA mass.

References

1. Characterization of products formed from the oxidation of toluene and m-xylene with varying NOx and OH exposure. Chemosphere (2023).
2. An overlooked oxidation mechanism of toluene: computational predictions and experimental validations. Chemical Science (2023).
3. Modeling daytime and nighttime secondary organic aerosol formation via multiphase reactions of biogenic hydrocarbons. Atmospheric Chemistry and Physics (2023).