Evidence on Interfacial Reaction Governing NO2 Hydrolysis in Deliquesced Aerosol Particles

Heterogeneous NO₂ hydrolysis forms nitrate and nitrous acid but is believed to proceed slowly in the atmosphere. Accelerated reactions in microdroplets have gained significant attention, but whether or not NO₂ hydrolysis is accelerated in deliquesced particles remains unclear. We address the gap by measuring size-dependent NO₂ hydrolysis rates in sulfate- or halide-containing droplets. Results show that the reaction rates in Na₂SO₄ droplets increased by 25-fold as the particle radius decreased from ∼40 to ∼4 μm. An even higher enhancement of ∼100 times was observed in NaCl and NaI particles, likely due to the NO₂–halide interactions. The enhancement in NaBr particles, however, was comparable to that in Na₂SO₄ particles. Kinetic modeling results illustrate that the accelerated reactions are due to ∼6 orders of magnitude enhancement factor (EF) of surface reaction rates over bulk-phase reaction rates. Compared to Na₂SO₄ particles, the surface reaction rates increase by factors of 2.3, 1.5, and 4.4 in NaCl, NaBr, and NaI particles, respectively. Under ambient conditions, EFs can increase by up to 10⁸, corresponding to the ambient nitrate production rates of >1 μg m^–3 h^–1. The rates are comparable to those of N₂O₅ hydrolysis and OH + NO₂ reaction pathways, making NO₂ hydrolysis a crucial source of reactive nitrogen species.

The significantly enhanced heterogeneous NO₂ hydrolysis at the droplet surface renders it a vital source of reactive nitrogen species.