Modelling photochemical reactions in atmospheric water droplets: An assessment of the importance of surface processes

The relative importance of surface versus bulk processes was computed in spherical water drops, based on the model photochemical reaction yielding phenol from benzene via the •OH radicals, formed on UV photolysis of nitrate. The coadsorption of benzene and nitrate at the droplet surface was taken into account by means of the Wagner–Onsager–Samaras interaction model. The results indicate that the surface accumulation of benzene and the coadsorption of benzene and nitrate, which substantially increase the value of the concentration product of the model species at the droplet surface compared to the bulk, would cause surface processes to play a very significant role, in particular in small drops (1 μm radius, as can be found in haze and mist). Under such circumstances, over 15% of phenol photoformation would take place in just 0.1% of the drop volume. Similar results are expected in the case of benzene hydroxylation and nitration on nitrite photolysis.