Anion-catalyzed dissolution of NO2 on aqueous microdroplets

Area of Science:

• Atmospheric Chemistry
• Environmental Science
• Physical Chemistry

Background:

• Atmospheric chemical models struggle to predict hydroxyl radical (*OH/HO(2)*) concentrations under nitrogen oxides (NOx)-rich conditions.
• This deficiency stems partly from uncertain rates and mechanisms of nitrogen dioxide (NO2) reactive dissolution in fog and aerosol droplets.
• Current models often use data from "deactivated tunnel wall residue" to parametrize NO2 uptake by atmospheric aerosol.

Discussion:

• Experiments monitored nitrate (NO3-) production on microdroplet surfaces exposed to NO2 gas using thermospray mass spectrometry.
• NO2 uptake coefficients (gamma) on aqueous sodium halide (NaX) microdroplets were measured, showing non-monotonic dependence on electrolyte concentration.
• A peak uptake coefficient (gamma_max) of approximately 10(-4) was observed at [NaX] approximately 1 mM, over 1000 times greater than in deionized water.

Key Insights:

• NO2 does not readily dissolve in pure water but reacts efficiently on aqueous NaX microdroplets, forming nitrate (NO3-) and nitrite (NO2-).
• Halide anions (X-) appear to capture NO2 gas, forming radical anions (X-NO2*-) that react further at the air/water interface.
• The presence of electrolytes dramatically enhances NO2 uptake, resolving discrepancies between previous studies in neat water and electrolyte-seeded clouds.

Outlook:

• These findings provide a general mechanism for the heterogeneous conversion of NO2 gas to nitrate (NO3-) and nitrous acid (HONO) on aqueous surfaces.
• Improved understanding of NO2 uptake mechanisms will enhance the accuracy of atmospheric chemical models.
• Accurate modeling is essential for predicting air quality and understanding the impacts of nitrogen oxides on the environment.