We investigated the photosensitizing properties of secondary organic aerosol (SOA) formed during the hydroxyl radical (OH) initiated oxidation of naphthalene. This SOA was injected into an aerosol flow tube and exposed to UV radiation and gaseous volatile organic compounds or sulfur dioxide (SO2). The aerosol particles were observed to grow in size by photosensitized uptake of d‐limonene and β‐pinene. In the presence of SO2, a photosensitized production (0.2–0.3 µg m−3 h−1) of sulfate was observed at all relative humidity (RH) levels. Some sulfate also formed on particles in the dark, probably due to the presence of organic peroxides. The dark and photochemical pathways exhibited different trends with RH, unraveling different contributions from bulk and surface chemistry. As naphthalene and other polycyclic aromatics are important SOA precursors in the urban and suburban areas, these dark and photosensitized reactions are likely to play an important role in sulfate and SOA formation.
The Caltech Photooxidation Flow Tube reactor: design, fluid dynamics and characterization
Flow tube reactors are widely employed to study gas-phase atmospheric chemistry and secondary organic aerosol (SOA) formation. The development of a new laminar-flow tube reactor, the Caltech Photooxidation Flow Tube (CPOT), intended for the study of gas-phase atmospheric chemistry and SOA formation, is reported here. The present work addresses the reactor design based on fluid dynamical characterization and the fundamental behavior of vapor molecules and particles in the reactor. The design of the inlet to the reactor, based on computational fluid dynamics (CFD) simulations, comprises a static mixer and a conical diffuser to facilitate development of a characteristic laminar flow profile. To assess the extent to which the actual performance adheres to the theoretical CFD model, residence time distribution (RTD) experiments are reported with vapor molecules (O3) and submicrometer ammonium sulfate particles. As confirmed by the CFD prediction, the presence of a slight deviation from strictly isothermal conditions leads to secondary flows in the reactor that produce deviations from the ideal parabolic laminar flow. The characterization experiments, in conjunction with theory, provide a basis for interpretation of atmospheric chemistry and SOA studies to follow. A 1-D photochemical model within an axially dispersed plug flow reactor (AD-PFR) framework is formulated to evaluate the oxidation level in the reactor. The simulation indicates that the OH concentration is uniform along the reactor, and an OH exposure (OHexp) ranging from ∼ 109 to ∼ 1012 molecules cm−3 s can be achieved from photolysis of H2O2. A method to calculate OHexp with a consideration for the axial dispersion in the present photochemical system is developed.
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- Award ID(s):
- 1523500
- NSF-PAR ID:
- 10057671
- Date Published:
- Journal Name:
- Atmospheric Measurement Techniques
- Volume:
- 10
- Issue:
- 3
- ISSN:
- 1867-8548
- Page Range / eLocation ID:
- 839 to 867
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract -
Acid-driven multiphase chemistry of isoprene epoxydiols (IEPOX) with inorganic sulfate aerosols contributes substantially to formation of secondary organic aerosol (SOA), which constitutes a large mass fraction of atmospheric fine particulate matter (PM2.5). However, atmospheric chemical sinks of freshly generated IEPOX-SOA particles remain unclear. We examined the role of heterogeneous oxidation of freshly-generated IEPOX-SOA particles by gas-phase hydroxyl radical (•OH) under dark conditions as one potential atmospheric sink. After 4 h of gas-phase •OH exposure (~3x108 molecules cm-3), chemical changes in smog chamber-generated IEPOX-SOA particles were assessed by hydrophilic interaction liquid chromatography coupled with electrospray ionization high-resolution quadrupole time-of-flight mass spectrometry (HILIC/ESI-HR-QTOFMS). Comparison of molecular-level compositional changes in IEPOX-SOA particles during aging with or without •OH revealed that decomposition of oligomers by heterogeneous •OH oxidation acts as a sink for •OH and maintains a reservoir of low-volatility compounds including monomeric sulfate esters and oligomer fragments. We propose tentative structures and formation mechanisms for previously uncharacterized SOA constituents in PM2.5. Our results suggest that this •OH-driven renewal of low-volatility products may extend atmospheric lifetimes of IEPOX-SOA particles by slowing production of low-molecular weight, high-volatility organic fragments, and likely contributes to large quantities of 2-methyltetrols and methyltetrol sulfates reported in PM2.5.more » « less
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Chemical mechanisms play an important role in simulating the atmospheric chemistry of volatile organic compound oxidation. Comparison of mechanism simulations with laboratory chamber data tests our level of understanding of the prevailing chemistry as well as the dynamic processes occurring in the chamber itself. α-Pinene photooxidation is a well-studied system experimentally, for which detailed chemical mechanisms have been formulated. Here, we present the results of simulating low-NO α-pinene photooxidation experiments conducted in the Caltech chamber with the Generator for Explicit Chemistry and Kinetics of Organics in the Atmosphere (GECKO-A) under varying concentrations of seed particles and OH levels. Unexpectedly, experiments conducted at low and high OH levels yield the same secondary organic aerosol (SOA) growth, whereas GECKO-A predicts greater SOA growth under high OH levels. SOA formation in the chamber is a result of a competition among the rates of gas-phase oxidation to low-volatility products, wall deposition of these products, and condensation into the aerosol phase. Various processes – such as photolysis of condensed-phase products, particle-phase dimerization, and peroxy radical autoxidation – are explored to rationalize the observations. In order to explain the observed similar SOA growth at different OH levels, we conclude that vapor wall loss in the Caltech chamber is likely of order 10−5 s−1, consistent with previous experimental measurements in that chamber. We find that GECKO-A tends to overpredict the contribution to SOA of later-generation oxidation products under high-OH conditions. Moreover, we propose that autoxidation may alternatively resolve some or all of the measurement–model discrepancy, but this hypothesis cannot be confirmed until more explicit mechanisms are established for α-pinene autoxidation. The key role of the interplay among oxidation rate, product volatility, and vapor–wall deposition in chamber experiments is illustrated.more » « less
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We recently demonstrated that the heterogeneous hydroxyl radical (OH) oxidation is an important aging process for isoprene epoxydiol-derived secondary organic aerosol (IEPOX-SOA) that alters its chemical composition, and thus, aerosol physicochemical properties. Notably, dimeric species in IEPOX-SOA were found to heterogeneously react with OH at a much faster rate than monomers, suggesting that the initial oligomeric content of freshly-generated IEPOX-SOA particles may affect its subsequent atmospheric oxidation. Aerosol acidity could in principle influence this aging process by enhancing the formation of sulfated and non-sulfated oligomers in freshly-generated IEPOX-SOA. Many multifunctional organosulfate (OS) products derived from heterogeneous OH oxidation of sulfur-containing IEPOX-SOA have been observed in cloud water residues and ice nucleating particles and could affect the ability of aged IEPOX-SOA particles to act as cloud condensation nuclei. Hence, this study systematically investigated the effect of aerosol acidity on the kinetics and products resulting from heterogeneous OH oxidation of IEPOX-SOA particles. We reacted gas-phase IEPOX with inorganic sulfate particles of varying pH (0.5 to 2.5) in an indoor smog chamber operated under dark, steady-state conditions to form freshly-generated IEPOX-SOA particles. These particles were aged at a relative humidity of 65% in an oxidation flow reactor (OFR) for 0-21 days of equivalent atmospheric OH exposure. Through molecular-level chemical analyses by hydrophilic interaction liquid chromatography method interfaced to electrospray ionization high-resolution quadrupole time-of-flight mass spectrometry (HILIC/ESI-HR-QTOFMS), we observed that highly acidic aerosol has higher oligomer ratio and exhibit much slower mass decay with OH oxidation (pH=0.5, lifetime = 56 days) as compared to less acidic aerosols (pH=2.5, lifetime=17 days). Based on atomic force microscopy (AFM) analysis, aerosol acidity could also affect the morphology and viscosity of IEPOX-SOA during OH oxidation process.more » « less
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We recently demonstrated that the heterogeneous hydroxyl radical (OH) oxidation is an important aging process for isoprene epoxydiol-derived secondary organic aerosol (IEPOX-SOA) that alters its chemical composition, and thus, aerosol physicochemical properties. Notably, dimeric species in IEPOX-SOA were found to heterogeneously react with OH at a much faster rate than monomers, suggesting that the initial oligomeric content of freshly-generated IEPOX-SOA particles may affect its subsequent atmospheric oxidation. Aerosol acidity could in principle influence this aging process by enhancing the formation of sulfated and non-sulfated oligomers in freshly-generated IEPOX-SOA. Many multifunctional organosulfate (OS) products derived from heterogeneous OH oxidation of sulfur-containing IEPOX-SOA have been observed in cloud water residues and ice nucleating particles and could affect the ability of aged IEPOX-SOA particles to act as cloud condensation nuclei. Hence, this study systematically investigated the effect of aerosol acidity on the kinetics and products resulting from heterogeneous OH oxidation of IEPOX-SOA particles. We reacted gas-phase IEPOX with inorganic sulfate particles of varying pH (0.5 to 2.5) in an indoor smog chamber operated under dark, steady-state conditions to form freshly-generated IEPOX-SOA particles. These particles were aged at a relative humidity of 65% in an oxidation flow reactor (OFR) for 0-21 days of equivalent atmospheric OH exposure. Through molecular-level chemical analyses by hydrophilic interaction liquid chromatography method interfaced to electrospray ionization high-resolution quadrupole time- of-flight mass spectrometry (HILIC/ESI-HR-QTOFMS), we observed that highly acidic aerosol has higher oligomer ratio and exhibit much slower mass decay with OH oxidation (pH=0.5, lifetime = 56 days) as compared to less acidic aerosols (pH=2.5, lifetime=17 days). Based on atomic force microscopy (AFM) analysis, aerosol acidity could also affect the morphology and viscosity of IEPOX-SOA during OH oxidation process.more » « less
- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.5194/amt-10-839-2017
