🌔 Nocturnal oxidation: a missing link in climate models

Scientists have discovered that humidity alone, even in darkness, is sufficient to trigger the oxidation of organic aerosols suspended in the atmosphere. This transformation accelerates their aging and could upend our understanding of climate and air pollution, especially at night.

Organic aerosols—airborne particles from vegetation, combustion, or chemicals—play a key role in air quality, cloud formation, and climate. It is commonly accepted that light activates the oxidation of these aerosols by highly reactive oxidizing agents such as ozone or hydroxyl radicals (OH).


These reactions must be taken into account in climate prediction models. A recent study reveals a new scenario in which this "aging" of aerosols continues even at night in the presence of humidity and oxygen.

To demonstrate this phenomenon, an international team of scientists from the Institute for Research on Catalysis and the Environment in Lyon (CNRS/Claude Bernard Lyon 1 University) and the University of Toronto in Canada reproduced common atmospheric aerosols containing organic acids (citric, maleic, aconitic) in the laboratory.

Placed in a closed, perfectly opaque reactor with a controlled atmosphere, these particles exposed to different humidity levels transformed even in the absence of light. Humidity alone triggers the spontaneous production of OH radicals on the particle surfaces, which then initiate oxidation. The more humid the air and the smaller the particles, the more efficient the reaction.

The presence of oxygen remains crucial: without O₂, the reaction weakens considerably. Theoretical calculations show that these oxidation processes are thermodynamically favored and can occur very rapidly, even under realistic atmospheric conditions.

This phenomenon, previously overlooked, could have major consequences for our understanding of aerosol aging. Indeed, in areas where the air is less polluted and OH concentrations are low, this interfacial oxidation could become the main driver of organic particle transformation.

The study, published in the journal Science Advances, will need to be incorporated into global atmospheric models, alongside other photo-induced degradation processes, to better assess the effects of atmospheric chemistry on climate and air quality.