Microdroplet-Assisted Sulfate Formation Incorporating Versatile Oxygen Sources

Le résumé fourni par la source

Microdroplet-assisted oxidation of atmospheric SO 2 helps to explain the rapid sulfate formation observed during haze episodes. Competing oxidation pathways (by O 2 or by NO 2 ) have been proposed, both supported by field observations, chamber experiments, and modeling studies. Current models predict 25% of sulfate oxygen coming from the O 2 and 75% from water via the O 2 pathway, whereas 100% from water via the NO 2 pathway. Since O 2 and water oxygen sources can be distinguished by their triple oxygen isotope composition (Δ′ 17 O 0.5305 ), the two pathways could be disentangled. Here, we conducted chamber experiments exposing SO 2 to a natural or Δ′ 17 O-labeled microdroplet environment containing O 2 and/or NO 2 . Under mixed O 2 /NO 2 conditions, sulfate Δ′ 17 O closely matched O 2 -only conditions, suggesting O 2 being the main oxidant. However, the sulfate Δ′ 17 O deviates from the prediction, indicating that oxygen in sulfate is also sourced from SO 2 . A possible mechanism is the direct oxidation of SO 2 by reactive oxygen species generated at the microdroplet environment. The finding challenges the current atmospheric sulfur chemistry models.

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