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1. Brown Carbon Aerosol Formation by Multiphase Catechol Photooxidation in the Presence of Soluble Iron

   https://doi.org/10.1021/acsestair.4c00045  

   De_Haan, David O; Hawkins, Lelia N; Weber, Jacob A; Moul, Benjamin T; Hui, Samson; Cox, Santeh A; Esse, Jennifer U; Skochdopole, Nathan R; Lynch, Carys P; De_Haan, Audrey C; et al (May 2024, ACS ES&T Air)

   Catechol (1,2-benzenediol), a common phenolic species emitted during biomass burning, is both redox active and metal chelating. When oxidized by OH radicals in the aqueous phase, it rapidly forms brown carbon (BrC). Here, we report chamber studies of the multiphase chemistry of catechol using HOOH as an OH radical source, soluble iron, simulated sunlight, and either deliquesced or solid-phase seed particles. BrC of remarkable similarity (MAC365 = 1.7 ±0.2 m2 g-1, “medium-BrC” category) was produced whenever gas-phase catechol was photolyzed in the chamber, with or without the presence of an OH radical source, soluble iron, or deliquesced aerosol. The speed and quantity of BrC formation varied, however. While BrC production was slower in the absence of an OH radical source, multiple lines of evidence suggest that OH generation via photosensitization by surface-adsorbed catechol can still generate BrC. Fenton chemistry actively occurred in surface-adsorbed water layers even below the seed particle deliquescence point, leading to significant production of gas-phase benzoquinone. Ratios of BrC and secondary organic aerosol (SOA) relative to catechol concentrations were highest in the presence of trace amounts of soluble iron, HOOH, and simulated sunlight, indicating that photo-Fenton chemistry contributed substantially to BrC and SOA formation by catechol. Finally, we observed that BrC and SOA formation by catechol / photo-Fenton chemistry can occur efficiently even at 40% RH. These results are consistent with catechol being a major source of secondary BrC in biomass burning plumes, even at moderate relative humidity. 

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2. Secondary organic aerosol from chlorine-initiated oxidation of isoprene

   https://doi.org/10.5194/acp-17-13491-2017  

   Wang, Dongyu S.; Ruiz, Lea Hildebrandt (January 2017, Atmospheric Chemistry and Physics)

   Recent studies have found concentrations of reactive chlorine species to be higher than expected, suggesting that atmospheric chlorine chemistry is more extensive than previously thought. Chlorine radicals can interact with hydroperoxy (HO_x) radicals and nitrogen oxides (NO_x) to alter the oxidative capacity of the atmosphere. They are known to rapidly oxidize a wide range of volatile organic compounds (VOCs) found in the atmosphere, yet little is known about secondary organic aerosol (SOA) formation from chlorine-initiated photooxidation and its atmospheric implications. Environmental chamber experiments were carried out under low-NO_x conditions with isoprene and chlorine as primary VOC and oxidant sources. Upon complete isoprene consumption, observed SOA yields ranged from 7 to 36 %, decreasing with extended photooxidation and SOA aging. Formation of particulate organochloride was observed. A high-resolution time-of-flight chemical ionization mass spectrometer was used to determine the molecular composition of gas-phase species using iodide–water and hydronium–water cluster ionization. Multi-generational chemistry was observed, including ions consistent with hydroperoxides, chloroalkyl hydroperoxides, isoprene-derived epoxydiol (IEPOX), and hypochlorous acid (HOCl), evident of secondary OH production and resulting chemistry from Cl-initiated reactions. This is the first reported study of SOA formation from chlorine-initiated oxidation of isoprene. Results suggest that tropospheric chlorine chemistry could contribute significantly to organic aerosol loading. 

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3. Secondary brown carbon from photooxidation of 1-methylnaphthalene and longifolene

   https://doi.org/10.6086/D1R38Z  

   Bahreini, Roya (January 2022, Dryad)

   An improved understanding of the optical properties of secondary organic aerosol (SOA) particles is needed to better predict their climate impacts. Here, SOA was produced by reacting 1-methylnaphthalene or longifolene with hydroxyl radicals (OH) under variable ammonia (NH3), nitrogen oxide (NOx), and relative humidity (RH) conditions. In the presence of NH3 and NOx, longifolene-derived aerosols had relatively high single scattering albedo (SSA) values and low absorption coefficients at 375 nm independent of RH, suggesting that the longifolene SOA is mostly scattering. In 1-methylnaphthalene experiments, the resulting SSA and SOA mass absorption coefficient (MACorg) values suggest the formation of light-absorbing SOA, and the addition of high NOx and high NH3 enhanced the SOA absorption. Under intermediate-NOx dry conditions, the MACorg values increased from 0.13 m2 g−1 in NH3-free conditions to 0.28 m2 g−1 in high-NH3 conditions. Under high-NH3 conditions, the MACorg value further increased to 0.36 m2 g−1 with an increase in RH. Under dry high-NOx conditions, the MACorg value increased from 0.42 to 0.67 m2 g−1 with the addition of NH3, while with elevated RH, the MACorg value reached 0.70 m2 g−1. The time series of MACorg showed increasing trends only in the presence of NH3. Composition analysis of SOA suggests that organonitrates, nitroorganics, and other nitrogen-containing organic compounds (NOCs) are potential chromophores in the 1-methylnaphthalene SOA. Significant formation of NOCs was observed in the presence of high-NOx and NH3 and was enhanced under elevated RH. The data have been collected in an environmental chamber, oxidizing 30-100 ppbv of 1-methylnaphthalene and 70-100 ppbv of longifolene under different levels of nitrogen oxides, ammonia, and relative humidity. Microphysical properties (size distribution and absorption and scattering coefficients) of SOA along with its composition were monitored throughout the experiment. Submitted data include the time series of the calculated mass absorption coefficient and single scattering albedo at 375 nm, derived real and imaginary components of the refractive index, organic nitrate to organic ratio, and organic and nitrate mass concentrations. The data have been analyzed using Wavemetrics Igor Pro, specifically the software written specifically to analyze the data obtained from Aerodyne's aerosol mass spectrometers (i.e., Squirrel and Pika). Other calculations were also carried out in Igor Pro. More details are provided in the related manuscripts. Please see README file. 

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4. Secondary organic aerosol formation from the β-pinene+NO₃ system: effect of humidity and peroxy radical fate

   https://doi.org/10.5194/acp-15-7497-2015  

   Boyd, C. M.; Sanchez, J.; Xu, L.; Eugene, A. J.; Nah, T.; Tuet, W. Y.; Guzman, M. I.; Ng, N. L. (January 2015, Atmospheric Chemistry and Physics)

   Abstract. The formation of secondary organic aerosol (SOA) from the oxidation of β-pinene via nitrate radicals is investigated in the Georgia Tech Environmental Chamber (GTEC) facility. Aerosol yields are determined for experiments performed under both dry (relative humidity (RH) < 2 %) and humid (RH = 50 % and RH = 70 %) conditions. To probe the effects of peroxy radical (RO₂) fate on aerosol formation, "RO₂ + NO₃ dominant" and "RO₂ + HO₂ dominant" experiments are performed. Gas-phase organic nitrate species (with molecular weights of 215, 229, 231, and 245 amu, which likely correspond to molecular formulas of C₁₀H₁₇NO₄, C₁₀H₁₅NO₅, C₁₀H₁₇NO₅, and C₁₀H₁₅NO₆, respectively) are detected by chemical ionization mass spectrometry (CIMS) and their formation mechanisms are proposed. The NO⁺ (at m/z 30) and NO₂⁺ (at m/z 46) ions contribute about 11 % to the combined organics and nitrate signals in the typical aerosol mass spectrum, with the NO⁺ : NO₂⁺ ratio ranging from 4.8 to 10.2 in all experiments conducted. The SOA yields in the "RO₂ + NO₃ dominant" and "RO₂ + HO₂ dominant" experiments are comparable. For a wide range of organic mass loadings (5.1–216.1 μg m^&minus;3), the aerosol mass yield is calculated to be 27.0–104.1 %. Although humidity does not appear to affect SOA yields, there is evidence of particle-phase hydrolysis of organic nitrates, which are estimated to compose 45–74 % of the organic aerosol. The extent of organic nitrate hydrolysis is significantly lower than that observed in previous studies on photooxidation of volatile organic compounds in the presence of NO_x. It is estimated that about 90 and 10 % of the organic nitrates formed from the β-pinene+NO₃ reaction are primary organic nitrates and tertiary organic nitrates, respectively. While the primary organic nitrates do not appear to hydrolyze, the tertiary organic nitrates undergo hydrolysis with a lifetime of 3–4.5 h. Results from this laboratory chamber study provide the fundamental data to evaluate the contributions of monoterpene + NO₃ reaction to ambient organic aerosol measured in the southeastern United States, including the Southern Oxidant and Aerosol Study (SOAS) and the Southeastern Center for Air Pollution and Epidemiology (SCAPE) study. 

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5. Effect of relative humidity on the composition of secondary organic aerosol from the oxidation of toluene

   https://doi.org/10.5194/acp-18-1643-2018  

   Hinks, Mallory L.; Montoya-Aguilera, Julia; Ellison, Lucas; Lin, Peng; Laskin, Alexander; Laskin, Julia; Shiraiwa, Manabu; Dabdub, Donald; Nizkorodov, Sergey A. (January 2018, Atmospheric Chemistry and Physics)

   The effect of relative humidity (RH) on the chemical composition of secondary organic aerosol (SOA) formed from low-NO_x toluene oxidation in the absence of seed particles was investigated. SOA samples were prepared in an aerosol smog chamber at &lt;&thinsp;2 % RH and 75 % RH, collected on Teflon filters, and analyzed with nanospray desorption electrospray ionization high-resolution mass spectrometry (nano-DESI&ndash;HRMS). Measurements revealed a significant reduction in the fraction of oligomers present in the SOA generated at 75 % RH compared to SOA generated under dry conditions. In a separate set of experiments, the particle mass concentrations were measured with a scanning mobility particle sizer (SMPS) at RHs ranging from &lt;&thinsp;2 to 90 %. It was found that the particle mass loading decreased by nearly an order of magnitude when RH increased from &lt;&thinsp;2 to 75&ndash;90 % for low-NO_x toluene SOA. The volatility distributions of the SOA compounds, estimated from the distribution of molecular formulas using the molecular corridor approach, confirmed that low-NO_x toluene SOA became more volatile on average under high-RH conditions. In contrast, the effect of RH on SOA mass loading was found to be much smaller for high-NO_x toluene SOA. The observed increase in the oligomer fraction and particle mass loading under dry conditions were attributed to the enhancement of condensation reactions, which produce water and oligomers from smaller compounds in low-NO_x toluene SOA. The reduction in the fraction of oligomeric compounds under humid conditions is predicted to partly counteract the previously observed enhancement in the toluene SOA yield driven by the aerosol liquid water chemistry in deliquesced inorganic seed particles. 

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