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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 (HOx) radicals and nitrogen oxides (NOx) 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-NOx 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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The fuel of atmospheric chemistry: Toward a complete description of reactive organic carbon
The Earth’s atmosphere contains a multitude of emitted (primary) and chemically formed (secondary) gases and particles that degrade air quality and modulate the climate. Reactive organic carbon (ROC) species are the fuel of the chemistry of the atmosphere, dominating short-lived emissions, reactivity, and the secondary production of key species such as ozone, particulate matter, and carbon dioxide. Despite the central importance of ROC, the diversity and complexity of this class of species has been a longstanding obstacle to developing a comprehensive understanding of how the composition of our atmosphere, and the associated environmental implications, will evolve. Here, we characterize the role of ROC in atmospheric chemistry and the challenges inherent in measuring and modeling ROC, and highlight recent progress toward achieving mass closure for the complete description of atmospheric ROC.
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- Award ID(s):
- 1638672
- PAR ID:
- 10188522
- Date Published:
- Journal Name:
- Science Advances
- Volume:
- 6
- Issue:
- 6
- ISSN:
- 2375-2548
- Page Range / eLocation ID:
- eaay8967
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Reactive organic carbon (ROC) is diverse in its speciation, functionalization, and volatility, with varying implications for ozone production and secondary organic aerosol formation and growth. Chemical ionization mass spectrometry (CIMS) approaches can provide in situ ROC observations, and the CIMS reagent ion controls the detectable ROC species. To expand the range of detectable ROC, we describe a method for switching between the reagent ions NH4+ and H3O+ in a Vocus chemical ionization time-of-flight mass spectrometer (Vocus-CI-ToFMS). We describe optimization of ion–molecule reactor conditions for both reagent ions, at the same temperature, and compare the ability of NH4+ and H3O+ to detect a variety of volatile organic compounds (VOCs) and semi-volatile and intermediate-volatility organic compounds (SVOCs and IVOCs), including oxygenates and organic sulfur compounds. Sensitivities are comparable to other similar instruments (up to ∼5 counts /s /pptv), with detection limits on the order of 1–10 s of pptv (1 s integration time). We report a method for characterizing and filtering periods of hysteresis following each reagent ion switch and compare use of reagent ions, persistent ambient ions, and a deuterated internal standard for diagnosing this hysteresis. We deploy NH4+/H3O+ reagent ion switching in a rural pine forest in central Colorado, US, and use our ambient measurements to compare the capabilities of NH4+ and H3O+ in the same instrument, without interferences from variation in instrument and inlet designs. We find that H3O+ optimally detects reduced ROC species with high volatility, while NH4+ improves detection of functionalized ROC compounds, including organic nitrates and oxygenated SVOCs and IVOCs that are readily fragmented by H3O+.more » « less
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Abstract Number: 99 Working Group: Aerosol Chemistry Abstract Isoprene, the largest non-methane volatile organic species emitted into Earth’s atmosphere, reacts with hydroxyl radicals to initiate formation of secondary organic aerosol (SOA). Under low nitric oxide conditions, the major oxidative pathway proceeds through acid catalyzed reactive uptake of isoprene-epoxydiol isomers (IEPOX). We have recently established the structures of the semivolatile C5H10O3 uptake products (formerly designated “C5-alkene triols) of cis- and trans-β-IEPOX as 3-methylenebutane-1,2,4-triol and isomeric 3-methyltetrahydrofuran-2,4-diols. Importantly, both uptake products showed significant partitioning into the gas phase. Here, we report evidence that the uptake products along with their gas phase oxidation products constitute a hitherto unrecognized source of SOA. We show that partitioning into the gas phase results in further oxidation into low volatility products, including highly oxygenated C5-polyols, organosulfates, and dimers. In the chamber studies, gas phase products were characterized by online by iodide-Chemical Ionization Mass Spectrometry (I-CIMS) and particle phase products by offline analysis of filter extracts by HILIC/(-)ESI-HR-QTOFMS using authentic standards. The chamber studies show the potential for a substantial contribution to SOA from reactive uptake of the second generation gas phase oxidation products onto both acidified and non-acidified ammonium bisulfate seed aerosols. Identification of these previously unrecognized early-generation oxidation products will improve estimates of atmospheric carbon distribution and advance our understanding of the fate of isoprene oxidation products in the atmosphere.more » « less
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1126/sciadv.aay8967
