Photooxidants from brown carbon and other chromophores in illuminated particle extracts

While photooxidants are important in atmospheric condensed phases, there are very few measurements in particulate matter (PM). Here we measure light absorption and the concentrations of three photooxidants – hydroxyl radical (^⚫OH), singlet molecular oxygen (${}^{\mathrm{1}}{{\mathrm{O}}_{\mathrm{2}}}^{*}$), and oxidizing triplet excited states of organic matter (³C^*) – in illuminated aqueous extracts of wintertime particles from Davis, California. ${}^{\mathrm{1}}{{\mathrm{O}}_{\mathrm{2}}}^{*}$ and ³C^*, which are formed from photoexcitation of brown carbon (BrC), have not been previously measured in PM. In the extracts, mass absorption coefficients for dissolved organic compounds (MAC_DOC) at 300 nm range between 13 000 and 30 000 cm² (g C)⁻¹ are approximately twice as high as previous values in Davis fogs. The average (±1σ)^⚫OH steady-state concentration in particle extracts is $\mathrm{4.4}(\pm \mathrm{2.3})\times {\mathrm{10}}^{-\mathrm{16}}$ M, which is very similar to previous values in fog, cloud, and rain: although our particle extracts are more concentrated, the resulting enhancement in the rate of ^⚫OH photoproduction is essentially canceled out by a corresponding enhancement in concentrations of natural sinks for ^⚫OH. In contrast, concentrations of the two oxidants formed primarily from brown carbon (i.e., ${}^{\mathrm{1}}{{\mathrm{O}}_{\mathrm{2}}}^{*}$ and ³C^*) are both enhanced in the particle extracts compared to Davis fogs, a result of higher concentrations of dissolved organic carbon and faster rates of light absorption in the extracts. The average ${}^{\mathrm{1}}{{\mathrm{O}}_{\mathrm{2}}}^{*}$ concentration in the PM extracts is $\mathrm{1.6}(\pm \mathrm{0.5})\times {\mathrm{10}}^{-\mathrm{12}}$ M, 7 times higher than past fog measurements, while the average concentration of oxidizing triplets is $\mathrm{1.0}(\pm \mathrm{0.4})\times {\mathrm{10}}^{-\mathrm{13}}$ M, nearly double the average Davis fog value. Additionally, the rates of ${}^{\mathrm{1}}{{\mathrm{O}}_{\mathrm{2}}}^{*}$ and ³C^* photoproduction are both well correlated with the rate of sunlight absorption.

Since we cannot experimentally measure photooxidants under ambient particle water conditions, we measured the effect of PM dilution on oxidant concentrations and then extrapolated to ambient particle conditions. As the particle mass concentration in the extracts increases, measured concentrations of ^⚫OH remain relatively unchanged, ${}^{\mathrm{1}}{{\mathrm{O}}_{\mathrm{2}}}^{*}$ increases linearly, and ³C^* concentrations increase less than linearly, likely due to quenching by dissolved organics. Based on our measurements, and accounting for additional sources and sinks that should be important under PM conditions, we estimate that [^⚫OH] in particles is somewhat lower than in dilute cloud/fog drops, while [³C^*] is 30 to 2000 times higher in PM than in drops, and [${}^{\mathrm{1}}{{\mathrm{O}}_{\mathrm{2}}}^{*}$] is enhanced by a factor of roughly 2400 in PM compared to drops. Because of these enhancements in ${}^{\mathrm{1}}{{\mathrm{O}}_{\mathrm{2}}}^{*}$ and ³C^* concentrations, the lifetimes of some highly soluble organics appear to be much shorter in particle liquid water than under foggy/cloudy conditions. Based on extrapolating our measured rates of formation in PM extracts, BrC-derived singlet molecular oxygen and triplet excited states are overall the dominant sinks for organic compounds in particle liquid water, with an aggregate rate of reaction for each oxidant that is approximately 200–300 times higher than the aggregate rate of reactions for organics with ^⚫OH. For individual, highly soluble reactive organic compounds it appears that ${}^{\mathrm{1}}{{\mathrm{O}}_{\mathrm{2}}}^{*}$ is often the major sink in particle water, which is a new finding. Triplet excited states are likely also important in the fate of individual particulate organics, but assessing this requires additional measurements of triplet interactions with dissolved organic carbon in natural samples.