Photoformation of Triplet Excited States and Other Oxidants in Fog Waters and Their Impact on Fog Processing of Organic Compounds

Abstract:

Reactions in fog and cloud drops are important for a number of processes, such as formation of secondary organic aerosol (SOA), cycling of organic carbon and nitrogen, and determining the lifetimes of pollutants. The rates of these processes depend upon the steady-state concentrations of the major photooxidants, namely, hydroxyl radical (^.OH), singlet molecular oxygen (¹O₂*) and triplet excited states of organic compounds (³C*). While there are some past measurements of ^.OH and ¹O₂* concentrations in fog and cloud drops, there are no data for the concentrations of triplet excited states. However, there is increasing evidence that triplets might be important for the processing of organics in a cloudy or foggy atmosphere.

To address this question, we collected fog water samples from Davis, CA and Baton Rouge, LA, illuminated them with simulated sunlight, and measured the steady-state concentrations of ^.OH , ¹O₂* and ³C* . To understand the relative importance of these photooxidants, we also measured the photochemical loss of two added model organic compounds in the illuminated fog waters - syringol (a biomass burning phenol) and methyl jasmonate (a green leaf volatile). Our results show that triplet excited states can play a major role in oxidizing the model compounds, typically accounting for 30 – 90% of the loss of both model compounds. Given that atmospheric triplets are relatively less understood, our results highlight the importance of deeper investigation into their nature.