Microdroplet-Assisted Sulfate Formation Incorporating Versatile Oxygen Sources

Microdroplet-assisted oxidation of atmospheric SO₂ helps to explain the rapid sulfate formation observed during haze episodes. Competing oxidation pathways (by O₂ or by NO₂) have been proposed, both supported by field observations, chamber experiments, and modeling studies. Current models predict 25% of sulfate oxygen coming from the O₂ and 75% from water via the O₂ pathway, whereas 100% from water via the NO₂ pathway. Since O₂ and water oxygen sources can be distinguished by their triple oxygen isotope composition (Δ'¹⁷O_0.5305), the two pathways could be disentangled. Here, we conducted chamber experiments exposing SO₂ to a natural or Δ'¹⁷O-labeled microdroplet environment containing O₂ and/or NO₂. Under mixed O₂/NO₂ conditions, sulfate Δ'¹⁷O closely matched O₂-only conditions, suggesting O₂ being the main oxidant. However, the sulfate Δ'¹⁷O deviates from the prediction, indicating that oxygen in sulfate is also sourced from SO₂. A possible mechanism is the direct oxidation of SO₂ by reactive oxygen species generated at the microdroplet environment. The finding challenges the current atmospheric sulfur chemistry models.