Evidence on Interfacial Reaction Governing NO₂ Hydrolysis in Deliquesced Aerosol Particles

Area of Science:

• Atmospheric Chemistry
• Environmental Science
• Chemical Kinetics

Background:

• Heterogeneous nitrogen dioxide (NO2) hydrolysis forms nitrate and nitrous acid but is considered slow in the atmosphere.
• Accelerated reactions in microdroplets are gaining attention, yet NO2 hydrolysis in deliquesced particles remains poorly understood.

Purpose of the Study:

• To investigate the size-dependent NO2 hydrolysis rates in sulfate- and halide-containing deliquesced particles.
• To elucidate the role of particle composition and size in NO2 hydrolysis acceleration.

Main Methods:

• Experimental measurement of size-dependent NO2 hydrolysis rates in Na2SO4, NaCl, NaI, and NaBr droplets.
• Kinetic modeling to determine enhancement factors (EF) for surface versus bulk reaction rates.

Main Results:

• NO2 hydrolysis rates increased significantly with decreasing particle size in Na2SO4 droplets (25-fold).
• Halide-containing particles (NaCl, NaI) showed even greater rate enhancements (∼100-fold), attributed to NO2-halide interactions.
• Kinetic modeling revealed surface reaction rates were enhanced by up to 6 orders of magnitude over bulk rates, with halide particles showing higher surface rate enhancements.

Conclusions:

• NO2 hydrolysis is significantly accelerated in deliquesced atmospheric particles, particularly those with halide content.
• This accelerated pathway is a critical source of reactive nitrogen, comparable to N2O5 hydrolysis and OH + NO2 reactions.
• Under ambient conditions, enhanced NO2 hydrolysis can lead to substantial nitrate production rates, impacting air quality.