First Measurements of Organic Triplet Excited States in Atmospheric Waters

Photooxidants chemically transform organic compounds in atmospheric drops and particles. Photooxidants such as hydroxyl radical (•OH) and singlet molecular oxygen (1O2*) have been characterized in cloud and fog drops, but there are no measurements of the triplet excited states of organic matter (3C*). These “triplets”, which are formed from excitation of chromophoric dissolved organic matter (CDOM), i.e., brown carbon, are difficult to measure because they are a mixture of species instead of a single entity. Here, we use a two-probe technique to measure the steady-state concentrations, rates of photoformation, and quantum yields of oxidizing triplet states during simulated-sunlight illumination of bulk fog waters. Concentrations of 3C* are (0.70–15) × 10–14 M with an average (±σ) value of 5.0 (±5.1) × 10–14 M. The average 3C* photoformation rate is 130 (±130) μM h–1, while the average quantum yield is 3.7 (±4.5)%. Based on our previous measurements of •OH and 1O2* in the same fog samples, the ratio of the steady-state concentrations for 1O2*:3C*:•OH is approximately 3:1:0.04, respectively. At our measured concentrations, triplet excited states can be the dominant aqueous oxidants for organic compounds such as phenols from biomass combustion.