Predicting photooxidant concentrations in aerosol liquid water based on laboratory extracts of ambient particles

Aerosol liquid water (ALW) is a unique reaction medium, but its chemistry is poorly understood. For example, little is known of photooxidant concentrations - including hydroxyl radicals ( (OH)-O-circle), singlet molecular oxygen (O-1(2)*), and oxidizing triplet excited states of organic matter (C-3*) - even though they likely drive much of ALW chemistry. Due to the very limited water content of particles, it is difficult to quantify oxidant concentrations in ALW directly. To predict these values, we measured photooxidant concentrations in illuminated aqueous particle extracts as a function of dilution and used the resulting oxidant kinetics to extrapolate to ALW conditions. We prepared dilution series from two sets of particles collected in Davis, California: one from winter (WIN) and one from summer (SUM). Both periods are influenced by biomass burning, with dissolved organic carbon (DOC) in the extracts ranging from 10 to 495 mg C L-1. In the winter sample, the (OH)-O-circle concentration is independent of particle mass concentration, with an average value of 5.0 ( +/- 2.2) x 10(-15) M, while in summer OH increases with DOC in the range (0.4-7.7) x 10(-15) M. In both winter and summer samples, C-3* concentrations increase rapidly with particle mass concentrations in the extracts and then plateau under more concentrated conditions, with a range of (0.2-7) x 10(-13) M. WIN and SUM have the same range of O-1(2)* concentrations, (0.2-8.5) x 10(-12) M, but in WIN the O-1(2)* concentration increases linearly with DOC, while in SUM O-1(2)* approaches a plateau. We next extrapolated the relationships of oxidant formation rates and sinks as a function of particle mass c q oncentration from our dilute extracts to the much more concentrated condition of aerosol liquid water. Predicted OH concentrations in ALW (including mass transport of (OH)-O-circle from the gas phase) are (5-8) x 10(-15) M, similar to those in fog/cloud waters. In contrast, predicted concentrations of C-3* and O-1(2)* in ALW are approximately 10 to 100 times higher than in cloud/fogs, with values of (4-9) x 10(-13)M and (1-5) x 10(-12) M, respectively. Although (OH)-O-circle is often considered the main sink for organic compounds in the atmospheric aqueous phase, the much higher concentrations of C-3* and O-1(2)* in aerosol liquid water suggest these photooxidants will be more important sinks for many organics in particle water.