Microdroplet-Assisted Sulfate Formation Incorporating Versatile Oxygen Sources

Area of Science:

• Atmospheric Chemistry
• Environmental Science
• Geochemistry

Background:

• Haze episodes involve rapid sulfate formation, with proposed oxidation pathways involving O2 or NO2.
• Current models predict distinct oxygen sources for sulfate depending on the pathway.
• Triple oxygen isotopes (Δ'17O) can differentiate oxygen sources (O2 vs. water).

Purpose of the Study:

• To investigate the dominant oxidation pathway of SO2 in microdroplets.
• To determine the oxygen sources for sulfate formation under mixed O2/NO2 conditions.
• To challenge and refine current atmospheric sulfur chemistry models.

Main Methods:

• Chamber experiments exposing SO2 to O2 and/or NO2 in natural or isotopically labeled microdroplets.
• Analysis of sulfate triple oxygen isotope composition (Δ'17O).
• Comparison of experimental results with predictions from existing atmospheric models.

Main Results:

• Under mixed O2/NO2 conditions, sulfate Δ'17O aligned with O2-only oxidation.
• Observed sulfate Δ'17O deviated from model predictions, indicating SO2 as an oxygen source.
• Evidence suggests reactive oxygen species in microdroplets contribute to SO2 oxidation.

Conclusions:

• O2 is the primary oxidant for SO2 in the studied microdroplet environment.
• Current atmospheric sulfur chemistry models may overestimate sulfate oxygen from O2 and water.
• Direct SO2 oxidation by microdroplet-generated reactive oxygen species is a plausible mechanism requiring further investigation.