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.
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Kinetics and Mass Yields of Aqueous Secondary Organic Aerosol from Highly Substituted Phenols Reacting with a Triplet Excited State
Biomass burning emits large amounts of phenols, which can partition into cloud/fog drops and aerosol liquid water (ALW) and react to form aqueous secondary organic aerosol (aqSOA). Triplet excited states of organic compounds (3C*) are a likely oxidant, but there are no rate constants with highly substituted phenols that have high Henry’s law constants (KH) and are likely important in ALW. To address this gap, we investigated the kinetics of six highly substituted phenols with the triplet excited state of 3,4-dimethoxybenzaldehyde. Second-order rate constants at pH 2 are all fast, (2.6 - 4.6)E9 M-1 s-1, while values at pH 5 are 2 to 5 times smaller. Rate constants are reasonably described by a quantitative structure-activity relationship with phenol oxidation potentials, allowing rate constants of other phenols to be predicted. Triplet-phenol kinetics are unaffected by ammonium sulfate, sodium chloride, galactose (a biomass-burning sugar), or Fe(III). In contrast, ammonium nitrate increases the rate of phenol loss by making hydroxyl radical, while Cu(II) inhibits phenol decay. Mass yields of aqueous SOA from triplet reactions are large and range from 59 to 99%. Calculations using our data along with previous oxidant measurements indicate that phenols with high KH can be an important source of aqSOA in ALW, with 3C* typically the dominant oxidant.
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- Award ID(s):
- 1649212
- NSF-PAR ID:
- 10221755
- Date Published:
- Journal Name:
- Environmental science technology
- ISSN:
- 1520-5851
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract Chemical processes in clouds and fogs can substantially alter atmospheric oxidant budgets and lead to aerosol mass formation. However, many regional and global models do not include detailed aqueous‐phase chemical mechanisms due to the (a) lack of complete understanding of the chemical processes and (b) computational burden of adding constituents. Current gas‐aqueous chemistry 0‐dimensional models were evaluated in a cloud‐chemistry box model intercomparison based on a mid‐September 2016 cloud chemistry event at Whiteface Mountain, New York. Multiphase mechanisms in the five participating models ranged from those appropriate for 3‐d models to highly complex with thousands of reactions. This study focused on oxidant levels in both phases and aqueous‐phase sulfate and organic acid formation. Comparison of gas‐phase‐only chemistry gives very similar oxidant predictions at night but shows significant differences during daytime with the hydroxyl radical (OH) variability of about an order of magnitude. The variability in the model results increases substantially with aqueous chemistry due to different Henry's Law constants, aqueous‐phase reaction rate constants, and chemical mechanisms. Using a prescribed liquid water content and pH value of 4.5, modeled aqueous OH, aldehyde, and organic acid concentrations differ by over an order of magnitude in daytime. Simulations were also conducted at a pH = 5.1, predicted variable pH, and with added transition metal ion chemistry. While we compare predicted and measured inorganic anions and water‐soluble organic carbon, we cannot do so for aqueous‐phase oxidant concentrations due to the lack of measurements. We highlight a need for recommended equilibrium and aqueous‐phase rate constants.
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Blum, Joel (Ed.)Atmospheric oxidation of isoprene yields large quantities of highly water-soluble isoprene epoxydiols (IEPOX) that partition into fogs, clouds, and wet aerosols. In aqueous aerosols, the acid-catalyzed ring-opening of IEPOX followed by nucleophilic addition of inorganic sulfate or water forms organosulfates and 2-methyltetrols, respectively, contributing substantially to secondary organic aerosol (SOA). However, the fate of IEPOX in clouds, fogs, and evaporating hydrometeors is not well understood. Here we investigate the rates, product branching ratios, and stereochemistry of organosulfates from reactions of dilute IEPOX (5–10 mM) under a range of sulfate concentrations (0.3–50 mM) and pH values (1.83–3.38) in order to better understand the fate of IEPOX in clouds and fogs. From these aqueous dark reactions of β-IEPOX isomers (trans- and cis-2-methyl-2,3-epoxybutane-1,4-diols), which are the predominant IEPOX isomers, products were identified and quantified using hydrophilic interaction liquid chromatography coupled to an electrospray ionization high-resolution quadrupole time-of-flight mass spectrometer operated in negative ion mode (HILIC/(−)ESI-HR-QTOFMS). We found that the regiochemistry and stereochemistry were affected by pH, and the tertiary methyltetrol sulfate (C5H12O7S) was promoted by increasing solution acidity. Furthermore, the rate constants for the reaction of IEPOX under cloud-relevant conditions are up to 1 order of magnitude lower than reported in the literature for aerosol-relevant conditions due to a markedly different solution activities. Nevertheless, the contribution of cloud and fog water reactions to IEPOX SOA may be significant in cases of lower aqueous-phase pH (model estimate) or during droplet evaporation (not studied).more » « less
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1021/acs.est.1c00575
