AQUEOUS-PHASE PHOTOCHEMICAL FORMATION OF HYDROXYL RADICAL IN AUTHENTIC CLOUDWATERS AND FOGWATERS

Aqueous-phase photochemical reactions initiated by simulated sunlight and 313-nm light form hydroxyl radical (.OH) in authentic cloud water and fogwater samples from various locations in the U.S. The quantum yields for .OH formation at 313 nm and the photoformation rates of .OH under sunlight conditions were quantified by the formation of phenol from the .OH-mediated oxidation of added benzene. Based on this technique, the measured photo formation rates of .OH in several cloudwaters and fog-waters ranged from 0.32 to 3.0 muM h-1 under midday equinox sunlight conditions. These rates of aqueous-phase .OH photoformation are of the same magnitude as previously published estimated rates of .OH in corporation from gas-to-drop partitioning of .OH and from the aqueous-phase decomposition Of O3 transfered from the gas to the drop. The quantum yields for .OH photoformation at 313 nm ranged from 4.6 X 10(-4) to 0.010 for cloudwaters, based on the total absorbance of the cloudwater at 313 nm, using the benzene-scavenging technique. Aqueous-phase photochemical reactions are a significant, and in some cases dominant, source of 'OH to cloud and fog drops, and this represents the first report of 'OH photoformation in any authentic condensed phase of the atmosphere.