Effect of Organic Coatings on Gas-Phase Nitrogen Dioxide Production from Aqueous Nitrate Photolysis

The influence of stearic acid, octanol, and octanoic acid monolayer coatings on the release of NO2 into the gas phase following aqueous NO3- photolysis was studied using incoherent broadband cavity-enhanced absorption spectroscopy (IBBC-EAS). The different organic compounds, when present at the aqueous surface, had varying effects on the gas-phase NO2 evolved. Stearic acid monolayers lowered the initial rate of appearance of NO2(g), and its steady-state concentration was the same as for uncoated solutions after similar to 50 min. In the presence of octanol monolayers, both the steady-state [NO2(g)] and its rate of appearance decreased. A simple kinetic phase partitioning model suggests that the rate of NO2(g) evaporation from the aqueous surface is physically inhibited by the long uncompressed stearic acid chains, whereas both NO2 evaporation and steady-state NO2(g) concentration decrease when octanol is present at the aqueous surface, due to the enhanced solubility of NO2 in the less polar octanol environment. Despite its structural similarity to octanol, monolayers of octanoic acid showed a different effect and slightly increased the steady-state [NO2(g)]. We propose that octanoic acid enhances NO2(g) production because of an increase in solution acidity, which increases the quantum yield of NO2 production from nitrate photolysis.