Photooxidants drive much of the daytime chemistry in drops and particles, including the oxidation of reduced sulfur and nitrogen compounds as well as the conversion of volatile organic carbon into aqueous secondary organic aerosol (SOA(aq)). The current paradigm in atmospheric condensed phases is that hydroxyl radical, OH, is the dominant oxidant. In contrast, researchers studying surface waters have a fuller picture of the suite of photooxidants that are important in initiating transformations. This suite includes two oxidants that are rarely measured in atmospheric drops and particles: triplet excited states of organic compounds (3C*) and singlet molecular oxygen (1O2*), which are both formed via light absorption by brown carbon (colored dissolved organic matter).  To address this gap, we measured concentrations of OH, 3C*, and 1O2* in fog waters and aqueous extracts of fine particles. There are a number of similarities between our results and past measurements from surface waters, both in terms of oxidant quantum yields as well as their relative concentrations. In fog waters, the average steady-state concentrations of singlet molecular oxygen, triplet excited states, and hydroxyl radical have a ratio of approximately 75 : 25 : 1. Despite its low concentrations, OH is a significant sink for many dissolved organic compounds because of its very high rate constants. However, the higher concentrations of triplet excited states make these important (but generally overlooked) oxidants for some important classes of organic compounds in drops. In particle extracts the behaviors of the three oxidants vary with dilution: while the OH steady-state concentration is relatively unchanged as extracts get more concentrated, the concentration of 1O2* appears to increase approximately linearly and 3C* increases but plateaus. Extrapolating to ambient particle liquid water conditions suggests that 1O2* can be a major oxidant for organics in particulate matter, while OH can be significant in some cases. Finally, we will show examples of the impacts of the three oxidants on organic carbon and SOA(aq) formation in airborne drops and particles.

Photooxidants in atmospheric drops and particles: Moving beyond solely OH

2019 Fall National Meeting

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.