Evidence on Interfacial Reaction Governing NO<sub>2</sub> Hydrolysis in Deliquesced Aerosol Particles
Ruifeng Zhang, Rikuto Minamikawa, Masao Gen, Nitish Singh, Dan Daniel, Yongjie Li, Xuan Wang, Chak K. Chan
Abstract
High Resolution Image Download MS PowerPoint Slide Heterogeneous NO 2 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 NO 2 hydrolysis is accelerated in deliquesced particles remains unclear. We address the gap by measuring size-dependent NO 2 hydrolysis rates in sulfate- or halide-containing droplets. Results show that the reaction rates in Na 2 SO 4 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 NO 2 –halide interactions. The enhancement in NaBr particles, however, was comparable to that in Na 2 SO 4 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 Na 2 SO 4 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 10 8, corresponding to the ambient nitrate production rates of >1 μg m –3 h –1 . The rates are comparable to those of N 2 O 5 hydrolysis and OH + NO 2 reaction pathways, making NO 2 hydrolysis a crucial source of reactive nitrogen species.