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© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 10 Dec 2018

Research article | 10 Dec 2018

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This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).

Photooxidants from Brown Carbon and Other Chromophores in Illuminated Particle Extracts

Richie Kaur1, Jacqueline R. Labins1, Scarlett S. Helbock1, Wenqing Jiang2, Keith Bein3, Qi Zhang2, and Cort Anastasio1 Richie Kaur et al.
  • 1Department of Land, Air and Water Resources, University of California-Davis, One Shields Avenue, Davis, CA 95616-8627, USA
  • 2Department of Environmental Toxicology, University of California-Davis, One Shields Avenue, Davis, CA 95616-8627, USA
  • 3Center for Health and the Environment , University of California-Davis, One Shields Avenue, Davis, CA 995616-8627, USA

Abstract. 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 (1O2*) and oxidizing triplet excited states of organic matter (3C*) – in illuminated aqueous extracts of wintertime particles from Davis, California. 1O2* 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 (MACDOC) at 300 nm range between 13,000–30,000 cm2 g–C–1 and are approximately twice as high as previous values in Davis fogs. The average (± 1σ) OH steady-state concentration in particle extracts is 4.7 (± 1.9) × 10−16 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 cancelled 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., 1O2* 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 1O2* concentration in the PM extracts is 1.6 (± 0.5) × 10−12 M, seven times higher than past fog measurements, while the average concentration of oxidizing triplets is 1.0 (± 0.4) × 10−13 M, nearly double the average Davis fog value. Additionally, the rates of 1O2* and 3C* photoproduction are both well correlated with the rate of sunlight absorption.

While concentrations of 1O2* and 3C* are higher in our PM extracts compared to fog, the extracts are approximately 1000 times more dilute than water-containing ambient PM. Since we cannot experimentally measure photooxidants under these ambient 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, 1O2* 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 essentially the same as in fog waters, [3C*] is higher in PM by nearly a factor of 3, and [1O2*] is enhanced by a factor of roughly 600. Because of these enhancements in 1O2* 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 our extrapolated rates of formation, BrC-derived singlet molecular oxygen and triplet excited states are 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 1O2* is the major sink in particle water. 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.

Richie Kaur et al.
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Richie Kaur et al.
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Short summary
We measured hydroxyl radical (OH), singlet oxygen (1O2*) and organic triplets (3C*) in illuminated aqueous particle extracts. After measuring the impact of dilution on oxidant concentrations, we extrapolated our results to predict them in ambient particles – 1O2* and 3C* concentrations appear to be greatly enhanced, while OH appears largely unchanged. Two of these oxidants (1O2*, 3C*) are not yet included in atmospheric models, and our results make it possible to include them in the future.
We measured hydroxyl radical (OH), singlet oxygen (1O2*) and organic triplets (3C*) in...