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 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 4.4(±2.3)×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 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., 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, 7 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. 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, 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 somewhat lower than in dilute cloud/fog drops, while [3C*] is 30 to 2000 times higher in PM than in drops, and [1O2*] is enhanced by a factor of roughly 2400 in PM compared to drops. 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 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 1O2* 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.

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Photooxidants chemically transform organic compounds in atmospheric drops and particles. Photooxidants such as hydroxyl radical (OH) and singlet molecular oxygen (O*) have been characterized in cloud and fog drops, but there are no measurements of the triplet excited states of organic matter (C*). 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 C* are (0.70-15) × 10 M with an average (±σ) value of 5.0 (±5.1) × 10 M. The average C* photoformation rate is 130 (±130) μM h, while the average quantum yield is 3.7 (±4.5)%. Based on our previous measurements of OH andO* in the same fog samples, the ratio of the steady-state concentrations for O*:C*: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.

Abstract. Triplet excited states of organic matter are formed when colored organic matter (i.e., brown carbon) absorbs light. While these “triplets” can be important photooxidants in atmospheric drops and particles (e.g., they rapidly oxidize phenols), very little is known about their reactivity toward many classes of organic compounds in the atmosphere. Here we measure the bimolecular rate constants of the triplet excited state of benzophenone (3BP∗), a model species, with 17 water-soluble C3–C6 alkenes that have either been found in the atmosphere or are reasonable surrogates for identified species. Measured rate constants (kALK+3BP∗) vary by a factor of 30 and are in the range of (0.24–7.5) ×109 M−1 s−1. Biogenic alkenes found in the atmosphere – e.g., cis-3-hexen-1-ol, cis-3-hexenyl acetate, and methyl jasmonate – react rapidly, with rate constants above 1×109 M−1 s−1. Rate constants depend on alkene characteristics such as the location of the double bond, stereochemistry, and alkyl substitution on the double bond. There is a reasonable correlation between kALK+3BP∗ and the calculated one-electron oxidation potential (OP) of the alkenes (R2=0.58); in contrast, rate constants are not correlated with bond dissociation enthalpies, bond dissociation free energies, or computed energy barriers for hydrogen abstraction. Using the OP relationship, we estimate aqueous rate constants for a number of unsaturated isoprene and limonene oxidation products with 3BP∗: values are in the range of (0.080–1.7) ×109 M−1 s−1, with generally faster values for limonene products. Rate constants with less reactive triplets, which are probably more environmentally relevant, are likely roughly 25 times slower. Using our predicted rate constants, along with values for other reactions from the literature, we conclude that triplets are probably minor oxidants for isoprene- and limonene-related compounds in cloudy or foggy atmospheres, except in cases in which the triplets are very reactive.

<p><strong>Abstract.</strong> Triplet excited states of organic matter, a.k.a. <q>triplets</q>, are formed when brown carbon absorbs light. While triplets can be important photooxidants in atmospheric drops and particles (e.g., they rapidly oxidize phenols), very little is known about their reactivity toward many classes of organic compounds in the atmosphere. Here we measure the bimolecular rate constants of the triplet excited state of benzophenone (<sup>3</sup>BP<sup>*</sup>), a model species, with 17 water-soluble C<sub>3</sub>–C<sub>6</sub> alkenes that have either been found in the atmosphere or are reasonable surrogates for identified species. Measured rate constants (<i>k</i><sub>ALK+<sup>3</sup>BP<sup>*</sup></sub>) vary by a factor of 30 and are in the range of (0.24–7.5) × 10<sup>9</sup> M<sup>−1</sup> s<sup>−1</sup>. Biogenic alkenes found in the atmosphere – e.g., cis-3-hexen-1-ol, cis-3-hexenyl acetate, and methyl jasmonate – react rapidly, with rate constants above 1 × 10<sup>9</sup> M<sup>−1</sup> s<sup>−1</sup>. Rate constants depend on alkene characteristics such as the location of the double bond, stereochemistry, and alkyl substitution on the double bond. There is a reasonable correlation between <i>k</i><sub>ALK+<sup>3</sup>BP<sup>*</sup></sub> and the calculated one-electron oxidation potential (OP) of the alkenes (<i>R</i><sup>2</sup> = 0.58); in contrast, rate constants are not correlated with bond dissociation enthalpies, bond dissociation free energies, or computed energy barriers for hydrogen abstraction. Using the OP relationship, we estimate aqueous rate constants for a number of unsaturated isoprene and limonene oxidation products with <sup>3</sup>BP<sup>*</sup>: values are in the range of (0.080–1.7) × 10<sup>9</sup> M<sup>−1</sup> s<sup>−1</sup>, with generally faster values for limonene products. Using our predicted rate constants, along with values for other reactions from the literature, we conclude that triplets are probably minor oxidants for isoprene and limonene-related compounds in cloudy or foggy atmospheres, except in cases where the triplets are very reactive.</p>