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

The aqueous-phase photoformation of hydroxyl radical ( OH) and singlet molecular oxygen (O2(1Δg) or 1O*2) was characterized in winter fog waters collected in Davis, California. All of the samples studied formed OH and 1O*2 upon illumination with simulated sunlight. Nitrite photolysis was a major source of OH in these samples, accounting for 47–100% of OH photoformation. Compared to calculated rates of gas-to-drop partitioning, in situ photoformation was a significant source of OH to all but the smallest fog drops, and was the dominant source of aqueous-phase 1O*2. Measured lifetimes of OH in the fog drops ranged from 0.38 to 1.5 μs. These values are significantly shorter than those predicted based on known drop constituents, indicating that uncharacterized compounds – likely organic – are significant sinks for OH in fog waters. Based on measured steady-state concentrations, both OH and 1O*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.