Aerosol Acidity and Atmospheric Chemistry

Aerosol acidity refers to the hydrogen ion concentration in atmospheric particles and aqueous phases such as cloud droplets and fog. It governs the partitioning of semi-volatile inorganic compounds (for example nitric acid, ammonia, hydrochloric acid) and organic species between gas and particle phases, directly influencing rates of heterogeneous reactions, secondary aerosol formation and the atmospheric lifetime of pollutants. Fine particles with pH values typically between 0 and 5 catalyse acid-driven processes that affect air quality, human health and climate through modifications of aerosol optical properties and cloud condensation nuclei activity. Acidic aerosols also play a central role in the deposition of reactive nitrogen and sulphur, with consequences for ecosystem nutrient cycling and surface acidification. Accurately characterising aerosol pH requires a combination of in situ measurements, thermodynamic modelling and comprehensive precursor observations, and remains a major challenge owing to variations in chemical composition, humidity and meteorology across regions and seasons.

Research from Nature Portfolio

Recent studies have uncovered a marked shift in the formation regime of secondary inorganic aerosols over rural parts of the United States. A decade of in situ observations reveals that controls on ammonia emissions have become less effective at reducing fine particulate matter, owing to a shortened atmospheric lifetime of reduced nitrogen and a diminished sensitivity of aerosol composition to ammonia fluctuations. This regime shift implies that near emission hotspots nitrogen deposition may actually rise, underlining the need for more collocated chemical and precursor measurements in both rural and urban settings. Another global analysis of aerosol thermodynamics demonstrates that even under unusually high ammonia concentrations, fine particles remain acidic (pH < 6) across diverse environments. This sustained acidity precludes efficient oxidation of sulphur dioxide via nitrogen dioxide pathways, indicating that metal-catalysed and other oxidation routes dominate sulphate production. The findings emphasise that neutralisation by ammonia seldom raises particle pH sufficiently to alter established sulphate formation mechanisms.

Aerosol Acidity and Atmospheric Chemistry publication trend

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

Technical terms

Aerosol acidity: The measure of hydrogen ion concentration (pH) in the aqueous phase of atmospheric particles.

pH: A logarithmic scale expressing the acidity or alkalinity of an aqueous solution; lower values indicate higher acidity.

Secondary inorganic aerosol: Particles formed in the atmosphere by chemical reactions of precursor gases such as SO₂, NOₓ and NH₃, yielding sulphate, nitrate and ammonium salts.

Thermodynamic equilibrium model: A computational tool (for example ISORROPIA) that predicts gas–particle partitioning and aerosol pH based on measured or assumed chemical composition, relative humidity and temperature.

Ammonia (NH₃): A gaseous base emitted largely from agricultural sources; it neutralises acidic species in aerosols and influences particle pH and secondary formation pathways.

Gas–particle partitioning: The distribution of semi-volatile compounds between the gas phase and the condensed aerosol phase, governed by temperature, humidity and acidity.

References

1. Regime shift in secondary inorganic aerosol formation and nitrogen deposition in the rural United States. Nature Geoscience (2024).
2. High levels of ammonia do not raise fine particle pH sufficiently to yield nitrogen oxide-dominated sulfate production. Scientific Reports (2017).
3. Dynamics of water-soluble inorganic ions in Qinhuangdao: Particle size association and influences of environmental conditions. Urban Climate (2025).
4. The acidity of atmospheric particles and clouds. Atmospheric Chemistry and Physics (2020).