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 NO 2 into the gas phase following aqueous NO 3 – 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 NO 2 evolved. Stearic acid monolayers lowered the initial rate of appearance of NO 2(g), and its steady-state concentration was the same as for uncoated solutions after ∼50 min. In the presence of octanol monolayers, both the steady-state [NO 2(g) ] and its rate of appearance decreased. A simple kinetic phase partitioning model suggests that the rate of NO 2(g) evaporation from the aqueous surface is physically inhibited by the long uncompressed stearic acid chains, whereas both NO 2 evaporation and steady-state NO 2(g) concentration decrease when octanol is present at the aqueous surface, due to the enhanced solubility of NO 2 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 [NO 2(g) ]. We propose that octanoic acid enhances NO 2(g) production because of an increase in solution acidity, which increases the quantum yield of NO 2 production from nitrate photolysis.