Photooxidants from brown carbon and other chromophores in illuminated particle extracts

Abstract

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.

Molecular composition and photochemical lifetimes of brown carbon chromophores in biomass burning organic aerosol

“…Polycyclic aromatic hydrocarbons (PAHs) are known to be products of incomplete combustion, and they have the potential to be long-lived BrC chromophores despite their reactivity (Keyte et al, 2013). PAHs have been observed in pristine environments, and it has been suggested that this is due to phase separation of particles and slow diffusivity of PAHs to surfaces where they react with atmospheric oxidants (Fernández et al, 2002;Keyte et al, 2013;Macdonald et al, 2000;Sofowote et al, 2011;Zhou et al, 2012Zhou et al, , 2019.…”

“…Polycyclic aromatic hydrocarbons (PAHs) are known to be products of incomplete combustion, and they have the potential to be long-lived BrC chromophores despite their reactivity (Keyte et al, 2013). PAHs have been observed in pristine environments, and it has been suggested that this is due to phase separation of particles and slow diffusivity of PAHs to surfaces where they react with atmospheric oxidants (Fernández et al, 2002;Keyte et al, 2013;Macdonald et al, 2000;Sofowote et al, 2011;Zhou et al, 2012Zhou et al, , 2019. In addition to its climatic effects, PAHs are mutagenic and carcinogenic as their metabolites, diol epoxides, bind to guanidine nucleobases in DNA, effectively leading to mutations (Finlayson-Pitts and Pitts, 2000;Moorthy et al, 2015;Wood et al, 1984;Xue and Warshawsky, 2005;Zhou et al, 2017).…”

Asphalt-related emissions are a major missing nontraditional source of secondary organic aerosol precursors

“…These reactive species may then react with larger less-volatile structures (e.g., asphaltenes) to form smaller, emittable compounds (54)(55)(56). Similar photoexcitation of chromophores is known to occur in atmospheric light-adsorbing brown carbon that undergo photoinitiated transformations over time-analogous to the photobleaching observed with real-world asphalt (55,57). Similarly, the generation of other reactive species (e.g., H 2 O 2 ) via irradiation of quinone-like structures known to form from PAH oxidation could also play a role (58)(59)(60)(61).…”

Characteristics, primary sources and secondary formation of water-soluble organic aerosols in downtown Beijing

“…WSOC in aerosols is active in light adsorption (Yan et al, 2015;Geng et al, 2020) and thus may make a significant impact on radiative forcing and global climate change (Andreae and Gelencser, 2006). Meanwhile, the photoexcitation of water-soluble brown carbon (BrC) can generate oxidants in aerosols and cloud/fog droplets (Manfrin et al, 2019;Kaur et al, 2019), which can promote atmospheric chemical reactions and aging processes of organic aerosols. Overall, WSOC is widely involved in cloud processes and heterogeneous reactions due to its surface activity and water solubility (Ervens et al, 2011;George et al, 2015), thus playing a significant role in severe haze episodes (Cheng et al, 2015;Wu Q. Yu et al: Primary sources and secondary formation of WSOC in PM 2.5 in Beijing, China et al, 2019;Ma et al, 2020).…”