Production of Br from NO and Br in Salty and Surfactant-Coated Water Microjets.

Gas-liquid scattering experiments are used to investigate the oxidation-reduction reaction NO(g) + 2Br(aq) → Br(g) + NO(aq) + NO(aq), a model for the nighttime absorption of NO into aerosol droplets containing halide ions. The detection of evaporating Br molecules provides our first observation of a gaseous reaction product generated by a water microjet in vacuum. NO molecules are directed at a 35 μm diameter jet of 6 or 8 LiBr in water at 263 or 240 K, followed by detection of both unreacted NO and product Br molecules by velocity-resolved mass spectrometry. The NO reaction probability at near-thermal collision energy is too small to be measured and likely lies below 0.2. However, the evaporating Br product can be detected and controlled by the presence of surfactants. The addition of 0.02 1-butanol, which creates ∼40% of a compact monolayer, reduces Br production by 35%. Following earlier studies, this reduction may be attributed to surface butanol molecules that block NO entry or alter the near-surface distribution of Br. Remarkably, addition of the cationic surfactant tetrabutylammonium bromide (TBABr) at 0.005 (9% of a monolayer) reduces the Br signal by 85%, and a 0.050 solution (58% of a monolayer) causes the Br signal to disappear entirely. A detailed analysis suggests that TBA efficiently suppresses Br evaporation because it tightly bonds to the Br intermediate formed in the highly concentrated Br solution and thereby hinders the rapid release and evaporation of Br.