Gas-Phase Reactions of Sulfur Dioxide and Water
Summary
Sulfur dioxide (SO₂) engages in a variety of gas-phase interactions with water vapour that underpin key atmospheric processes, from acid formation to new particle nucleation. In the troposphere, SO₂ is oxidised by hydroxyl radicals to form sulphurous acid (H₂SO₃) intermediates, a route whose kinetics are modulated by the presence of one or more water molecules. Photochemical excitation of SO₂ to higher electronic states can lower activation barriers to direct hydration, yielding transient adducts that may serve as condensation nuclei. Water clusters also catalyse the stepwise addition of OH and HO₂ radicals to SO₂, facilitating pathways to H₂SO₄ and ultimately influencing cloud condensation processes. The balance between gas-phase hydrolysis, photolytic dissociation and cluster-mediated reaction channels determines the lifetimes of SO₂-derived species and their propensity to form acid aerosols. Understanding these mechanisms is essential for accurate modelling of acid deposition, aerosol radiative forcing and the fate of volcanic and anthropogenic SO₂ emissions on regional to global scales.
Research from Nature Portfolio
Recent studies have illuminated the role of electronically excited SO₂ in direct gas-phase hydration. Quantum-chemical investigations reveal that excitation to the triplet manifold enables a barrier-lowered reaction with water, producing sulphurous acid under near-UV irradiation. This mechanism contrasts with the endothermic ground-state route and offers a plausible pathway for acid formation in regions of intense solar flux. The identified H₂SO₃ product is highly hygroscopic, promoting the onset of new particle formation without requiring pre-existing aerosol surfaces. These findings unify observed discrepancies between satellite‐measured SO₂ decay and model predictions, highlighting excited‐state chemistry as a missing sink in sulphur budgets.
Gas-Phase Reactions of Sulfur Dioxide and Water publication trend
The graph below shows the total number of articles in gas-phase reactions of sulfur dioxide and water across all publications each year (not limited to Nature Index journals).
Technical terms
Triplet state: An electronically excited state of a molecule with two unpaired electrons of parallel spin.
Intersystem crossing: A radiationless transition between electronic states of different spin multiplicity.
Sulphurous acid (H₂SO₃): A transient gas-phase species formed by addition of water to SO₂, acting as a condensation nucleus.
Activation barrier: The minimum energy required for reactants to transform into products in a chemical reaction.
New particle formation: The initial step of aerosol formation in the atmosphere, often initiated by low-volatility vapours clustering.
References
- Gas-phase hydrolysis of triplet SO2: A possible direct route to atmospheric acid formation. Scientific Reports (2016).
- Gasphasenbildung von Schwefliger Säure (H2SO3) in der Atmosphäre. Angewandte Chemie (2024).
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