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

摘要

Experiments are conducted to determine the photolysis quantum yields ofnitrate, FeOH, and HO in the bulk and at the surfacelayer of water. Results show that the quantum yields of nitrate andFeOH are enhanced at the surface compared to the bulk due to areduced water-cage surrounding the photo-fragments (OH+NO and Fe+OH, respectively).However, no evidence is found for an enhanced quantum yield forHO at the surface. The photolysis rate constant distributionwithin nitrate, FeOH, and HO aerosols is calculated bycombining the quantum yield data with Mie theory calculations of lightintensity. Values for the photolysis rate constant of nitrate andFeOH are significantly higher at the surface than in the bulk due toenhanced quantum yields at the surface. The results concerning the rates ofphotolysis of these photoactive species are applied to the assessment of thereaction between benzene and OH in the presence of OH scavengers in an atmospherically relevant scenario. For adroplet of 1μm radius, a large fraction of the total OH-benzene reaction (15% for HO, 20% for nitrate, and 35% for FeOH) occurs in the surface layer, which accounts for just0.15% of the droplet volume. By neglecting the surface effects onphotochemistry, the rate of the important reactions could be underestimatedby a considerable amount.