Evidence of the water-cage effect on the photolysis of NO3- and [FeOH]2+. Implications of this effect and of H2O2 surface accumulation on photochemistry at the air-water interface of atmospheric droplets.

Experiments are conducted to determine the effect of a cage ofwater molecules on the photolysis quantum yields of nitrate, FeOH2+, and H2O2. Results suggest that the quantum yields of nitrate and FeOH2+ are decreased by the recombination of photofragments ( °OH + °NO2 and Fe2+ + °OH, respectively) before they leave the surrounding cage of water molecules. However, no evidence is found for an enhanced quantum yield for H2O2. Therefore, the photolysis of nitrate and FeOH2+ could be enhanced if the cage of the solvent molecules is incomplete, as is the case at the air-water interface of atmospheric droplets. The photolysis rate constant distribution within nitrate, FeOH2+, and H2O2 aerosols is calculated by combining the expected quantum yield data in the bulk and at the interface with Mie theory calculations of light intensity. The photolysis rate constant of nitrate and FeOH2+ would be significantly higher at the surface than in the bulk if quantum yields are enhanced at the surface. In the case of H2O2, the photolysis rate constant would be enhanced by surface accumulation. The results concerning the expected 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 1 micrometer radius, a large fraction of the total °OH-benzene reaction (15% for H2O2, 20% for nitrate, and 35% for FeOH2+) would occur in the surface layer, which accounts for just 0.15% of the droplet volume.