Evidence of the water-cage effect on the photolysis of NO₃^- and FeOH²⁺, and its implications for the photochemistry at the air-water interface of atmospheric droplets

Abstract. Experiments are conducted to determine the photolysis quantum yields of nitrate, FeOH²⁺, and H₂O₂ in the bulk and at the surface layer of water. Results show that the quantum yields of nitrate and FeOH²⁺ are enhanced at the surface compared to the bulk due to a reduced water-cage surrounding the photo-fragments (^•OH+^•NO₂ and Fe²⁺+^•OH, respectively). However, no evidence is found for an enhanced quantum yield for H₂O₂ at the surface. The photolysis rate constant distribution within nitrate, FeOH²⁺, and H₂O₂ aerosols is calculated by combining the quantum yield data with Mie theory calculations of light intensity. Values for the photolysis rate constant of nitrate and FeOH²⁺ are significantly higher at the surface than in the bulk due to enhanced quantum yields at the surface. The results concerning the rates of photolysis of these photoactive species are applied to the assessment of the reaction between benzene and ^•OH in the presence of ^•OH scavengers in an atmospherically relevant scenario. For a droplet of 1μm radius, a large fraction of the total ^•OH-benzene reaction (15% for H₂O₂, 20% for nitrate, and 35% for FeOH²⁺) occurs in the surface layer, which accounts for just 0.15% of the droplet volume. By neglecting the surface effects on photochemistry, the rate of the important reactions could be underestimated by a considerable amount.