Chemistry of fog waters in California's Central Valley: 1. In situ photoformation of hydroxyl radical and singlet molecular oxygen

The aqueous-phase photoformation of hydroxyl radical ((OH)-O-.) and singlet molecular oxygen (O-2((1)Delta (g)) or O-1(2)*) was characterized in winter fog waters collected in Davis, California. All of the samples studied formed (OH)-O-. and O-1(2)* upon illumination with simulated sunlight. Nitrite photolysis was a major source of (OH)-O-. in these samples, accounting for 47-100% of (OH)-O-. photoformation. Compared to calculated rates of gas-to-drop partitioning, in situ photoformation was a significant source of (OH)-O-. to all but the smallest fog drops, and was the dominant source of aqueous-phase 1O(2)(*). Measured lifetimes of (OH)-O-. in the fog drops ranged from 0.38 to 1.5 mus. These values are significantly shorter than those predicted based on known drop constituents, indicating that uncharacterized compounds - likely organic - are significant sinks for (OH)-O-. in fog waters, Based on measured steady-state concentrations, both (OH)-O-. and O-1(2)* are likely to play significant roles in the transformations of trace species in fog drops. Hydroxyl radical appears to be a relatively significant sink for refractory compounds and a minor sink for reactive trace species. Conversely, 1O(2)(*) will be a minor sink for refractory compounds but a significant sink for certain electron-rich reactive trace species. (C) 2001 Elsevier Science Ltd. All rights reserved.