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′17O0.5305), 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.
Sun et al. (2026) studied this question.