We have examined aqueous-phase secondary organic aerosol (SOA) and organosulfate (OS) formation in atmospheric aerosols using a photochemical box model with coupled gas-phase chemistry and detailed aqueous aerosol chemistry. SOA formation in deliquesced ammonium sulfate aerosol is highest under low-NO(x) conditions, with acidic aerosol (pH = 1) and low ambient relative humidity (40%). Under these conditions, with an initial sulfate loading of 4.0 μg m(-3), 0.9 μg m(-3) SOA is predicted after 12 h. Low-NO(x) aqueous-aerosol SOA (aaSOA) and OS formation is dominated by isoprene-derived epoxydiol (IEPOX) pathways; 69% or more of aaSOA is composed of IEPOX, 2-methyltetrol, and 2-methyltetrol sulfate ester. 2-Methyltetrol sulfate ester comprises >99% of OS mass (66 ng m(-3) at 40% RH and pH 1). In urban (high-NO(x)) environments, aaSOA is primarily formed via reversible glyoxal uptake, with 0.12 μg m(-3) formed after 12 h at 80% RH, with 20 μg m(-3) initial sulfate. OS formation under all conditions studied is maximum at low pH and lower relative humidities (<60% RH), i.e., when the aerosol is more concentrated. Therefore, OS species are expected to be good tracer compounds for aqueous aerosol-phase chemistry (vs cloudwater processing).

Unconventional natural gas development (UNGD) generates large volumes of wastewater, the detailed composition of which must be known for adequate risk assessment and treatment. In particular, transformation products of geogenic compounds and disclosed additives have not been described. This study investigated six Fayetteville Shale wastewater samples for organic composition using a suite of one- and two-dimensional gas chromatographic techniques to capture a broad distribution of chemical structures. Following the application of strict compound-identification-confidence criteria, we classified compounds according to their putative origin. Samples displayed distinct chemical distributions composed of typical geogenic substances (hydrocarbons and hopane biomarkers), disclosed UNGD additives (e.g., hydrocarbons, phthalates such as diisobutyl phthalate, and radical initiators such as azobis(isobutyronitrile)), and undisclosed compounds (e.g., halogenated hydrocarbons, such as 2-bromohexane or 4-bromoheptane). Undisclosed chloromethyl alkanoates (chloromethyl propanoate, pentanoate, and octanoate) were identified as potential delayed acids (i.e., those that release acidic moieties only after hydrolytic cleavage, the rate of which could be potentially controlled), suggesting they were deliberately introduced to react in the subsurface. In contrast, the identification of halogenated methanes and acetones suggested that those compounds were formed as unintended byproducts. Our study highlights the possibility that UNGD operations generate transformation products and underscores the value of disclosing additives injected into the subsurface.

Unconventional natural gas extraction via hydraulic fracturing coupled with horizontal drilling (HDHF) has generated disruptive growth in the domestic energy sector. Field analyses of residual HDHF fluids have detected halogenated species, potentially the product of unexplored reactions between authigenic halides and HDHF additives. Utilizing a custom high-pressure reactor system, we simultaneously screened 12 frequently disclosed, functionally diverse HDHF additives to uncover transformation chemistry. One emergent pathway, the halogenation of cinnamaldehyde in the presence of ammonium persulfate, demonstrated the potential for oxidative breakers to react with halides to yield reactive halogen species. Halogenated product formation, product distribution, and kinetics were evaluated with respect to shale well subsurface condition, linking transformation risk to measurable well-dependent characteristics (e.g., halide compositions, well temperatures, and pH). In a representative flowback brine, the brominated product dominated on a molar percent basis (6 ± 2%, as normalized by initial cinnamaldehyde loading) over chlorinated (1.4 ± 0.4%) and iodinated forms (2.5 ± 0.9%), reflecting relative halide abundance and propensity for oxidation. This work demonstrates that relevant subsurface reactions between natural brines and hydraulic fracturing additives can result in the unintended formation of halogenated products.

Hydraulic fracturing coupled with horizontal drilling (HDHF) involves the deep-well injection of a fracturing fluid composed of diverse and numerous chemical additives designed to facilitate the release and collection of natural gas from shale plays. Analyses of flowback wastewaters have revealed organic contamination from both geogenic and anthropogenic sources. The additional detections of undisclosed halogenated chemicals suggest unintended in situ transformation of reactive additives, but the formation pathways for these are unclear in subsurface brines. To develop an efficient experimental framework for investigating the complex shale-well parameter space, we have reviewed and synthesized geospatial well data detailing temperature, pressure, pH, and halide ion values as well as industrial chemical disclosure and concentration data. Our findings showed subsurface conditions can reach pressures up to 4500 psi (310 bars) and temperatures up to 95 °C, while at least 588 unique chemicals have been disclosed by industry, including reactive oxidants and acids. Given the extreme conditions necessary to simulate the subsurface, we briefly highlighted existing geochemical reactor systems rated to the necessary pressures and temperatures, identifying throughput as a key limitation. In response, we designed and developed a custom reactor system capable of achieving 5000 psi (345 bars) and 90 °C at low cost with 15 individual reactors that are readily turned over. To demonstrate the system's throughput, we simultaneously tested 12 disclosed HDHF chemicals against a radical initiator compound in simulated subsurface conditions, ruling out a dozen potential transformation pathways in a single experiment. This review outlines the dynamic and diverse parameter range experienced by HDHF chemical additives and provides an optimized framework and novel reactor system for the methodical study of subsurface transformation pathways. Ultimately, enabling such studies will provide urgently needed clarity for water treatment downstream or releases to the environment.

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A number of flowback samples derived from horizontally drilled hydraulic fracturing (HDHF) operations reveal consistent detections of halogenated organic species , yet the source of these compounds remains uncertain. Studies simulating subsurface conditions have found that oxidative “breakers” can halogenate certain additives, but these pathways are unverified in the presence of cross-linked-gels, common features of HDHF operations. Using a high-throughput custom reactor system, we implemented a reaction matrix to test the capacity for halogenation of two frequently disclosed compounds with demonstrated halogenation pathways (cinnamaldehyde and citric acid) across guar gels with varied types and concentrations of cross-linkers and oxidative breakers. Cinnamaldehyde halogenation proceeded most readily in borate cross-linked gels at high ammonium persulfate dosages. Citric acid formed trihalomethanes (THMs) broadly across the matrix, generating brominated THMs at higher levels of hypochlorite breaker. Isolated removals of cross-linker or guar enhanced or diminished certain product formations, highlighting additional capacities for relevant ingredients to influence halogenation. Finally, we analyzed flowback samples from the Denver–Julesberg Basin, finding that additions of oxidant enhanced halogenation. As a more realistic subsurface simulation, this work demonstrates strict criteria for the subsurface halogenation of cinnamaldehyde and the broad capacity for THM formation due to systematic oxidant usage as gel breakers in HDHF operations.

The reactive uptake of glyoxal by atmospheric aerosols is believed to be a significant source of secondary organic aerosol (SOA). Several recent laboratory studies have been performed with the goal of characterizing this process, but questions remain regarding the effects of photochemistry on SOA growth. We applied GAMMA (McNeill et al. Environ. Sci. Technol. 2012, 46, 8075-8081), a photochemical box model with coupled gas-phase and detailed aqueous aerosol-phase chemistry, to simulate aerosol chamber studies of SOA formation by the uptake of glyoxal by wet aerosol under dark and irradiated conditions (Kroll et al. J. Geophys. Res. 2005, 110 (D23), 1-10; Volkamer et al. Atmos. Chem. Phys. 2009, 9, 1907-1928; Galloway et al. Atmos. Chem. Phys. 2009, 9, 3331- 306 3345 and Geophys. Res. Lett. 2011, 38, L17811). We find close agreement between simulated SOA growth and the results of experiments conducted under dark conditions using values of the effective Henry's Law constant of 1.3-5.5 × 10(7) M atm(-1). While irradiated conditions led to the production of some organic acids, organosulfates, and other oxidation products via well-established photochemical mechanisms, these additional product species contribute negligible aerosol mass compared to the dark uptake of glyoxal. Simulated results for irradiated experiments therefore fell short of the reported SOA mass yield by up to 92%. This suggests a significant light-dependent SOA formation mechanism that is not currently accounted for by known bulk photochemistry, consistent with recent laboratory observations of SOA production via photosensitizer chemistry.

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