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Abstract. Triplet excited states of organic matter are formed when colored organicmatter (i.e., brown carbon) absorbs light. While these “triplets” can beimportant photooxidants in atmospheric drops and particles (e.g., theyrapidly oxidize phenols), very little is known about their reactivity towardmany classes of organic compounds in the atmosphere. Here we measure thebimolecular rate constants of the triplet excited state of benzophenone(³BP^∗), a model species, with 17 water-solubleC₃–C₆ alkenes that have either been found in theatmosphere or are reasonable surrogates for identified species. Measured rateconstants ($k_{\mathrm{ALK}+\mathrm{3}{\mathrm{BP}}^{\ast }}$) vary by a factor of 30 and are in therange of (0.24–7.5) ×10⁹ M⁻¹ s⁻¹. Biogenic alkenesfound in the atmosphere – e.g., cis-3-hexen-1-ol, cis-3-hexenyl acetate, andmethyl jasmonate – react rapidly, with rate constants above 1×10⁹ M⁻¹ s⁻¹. Rate constants depend on alkene characteristicssuch as the location of the double bond, stereochemistry, and alkylsubstitution on the double bond. There is a reasonable correlation between$k_{\mathrm{ALK}+\mathrm{3}{\mathrm{BP}}^{\ast }}$ and the calculated one-electron oxidation potential(OP) of the alkenes (R²=0.58); in contrast, rate constants are notcorrelated with bond dissociation enthalpies, bond dissociation freeenergies, or computed energy barriers for hydrogen abstraction. Using the OPrelationship, we estimate aqueous rate constants for a number of unsaturatedisoprene and limonene oxidation products with ³BP^∗: values are inthe range of (0.080–1.7) ×10⁹ M⁻¹ s⁻¹, withgenerally faster values for limonene products. Rate constants with lessreactive triplets, which are probably more environmentally relevant, arelikely roughly 25 times slower. Using our predicted rate constants, alongwith values for other reactions from the literature, we conclude thattriplets are probably minor oxidants for isoprene- and limonene-relatedcompounds in cloudy or foggy atmospheres, except in cases in which the tripletsare very reactive.

 

Abstract. While photooxidants are important in atmospheric condensed phases, there arevery few measurements in particulate matter (PM). Here we measure lightabsorption 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 formedfrom photoexcitation of brown carbon (BrC), have not been previously measuredin PM. In the extracts, mass absorption coefficients for dissolved organiccompounds (MACDOC) at 300 nm range between 13 000 and30 000 cm2 (g C)−1 are approximately twice ashigh as previous values in Davis fogs. The average (±1σ)⚫OH steady-state concentration in particle extracts is4.4(±2.3)×10-16 M, which is very similar to previous valuesin fog, cloud, and rain: although our particle extracts are moreconcentrated, the resulting enhancement in the rate of ⚫OHphotoproduction is essentially canceled out by a corresponding enhancement inconcentrations 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 theparticle extracts compared to Davis fogs, a result of higher concentrationsof dissolved organic carbon and faster rates of light absorption in theextracts. The average 1O2* concentration in the PM extractsis 1.6(±0.5)×10-12 M, 7 times higher than past fogmeasurements, 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 sunlightabsorption. Since we cannot experimentally measure photooxidants under ambient particlewater conditions, we measured the effect of PM dilution on oxidantconcentrations and then extrapolated to ambient particle conditions. As theparticle mass concentration in the extracts increases, measuredconcentrations of ⚫OH remain relatively unchanged,1O2* increases linearly, and 3C* concentrations increase lessthan linearly, likely due to quenching by dissolved organics. Based on ourmeasurements, and accounting for additional sources and sinks that should beimportant under PM conditions, we estimate that [⚫OH] inparticles is somewhat lower than in dilute cloud/fog drops, while [3C*]is 30 to 2000 times higher in PM than in drops, and [1O2*] isenhanced by a factor of roughly 2400 in PM compared to drops. Because ofthese enhancements in 1O2* and 3C* concentrations,the lifetimes of some highly soluble organics appear to be much shorter inparticle liquid water than under foggy/cloudy conditions. Based onextrapolating our measured rates of formation in PM extracts, BrC-derivedsinglet molecular oxygen and triplet excited states are overall the dominantsinks for organic compounds in particle liquid water, with an aggregate rateof reaction for each oxidant that is approximately 200–300 times higherthan the aggregate rate of reactions for organics with ⚫OH. Forindividual, highly soluble reactive organic compounds it appears that1O2* is often the major sink in particle water, which is a newfinding. Triplet excited states are likely also important in the fate ofindividual particulate organics, but assessing this requires additionalmeasurements of triplet interactions with dissolved organic carbon innatural samples.