Isoprene is one of the most common biogenic volatile organic compounds (BVOC) in the atmosphere, produced by many plants. Isoprene undergoes oxidation to form gaseous isoprene epoxydiols (IEPOX) under low-NOx conditions, which can lead to the formation of secondary organic aerosol (SOA) particles. SOA-containing particles affect climate by scattering and absorbing solar radiation or acting as cloud condensation nuclei (CCN). High concentrations of SOA are also associated with adverse health impacts in people. While in the atmosphere, IEPOX SOA particles continue to undergo reactions with atmospheric oxidants, including hydroxyl radical (OH).
To isolate and probe this process, we studied atmospheric chemical processes in an aerosol chamber to better understand the evolution of heterogeneous OH oxidation of IEPOX-derived SOA particles. Since very little is understood about the structural and spectroscopic properties because of the complexity of their many sources and atmospheric processing, individual particle measurements are necessary to provide better understanding of the composition of IEPOX SOA.
We injected particles composed of mixtures of ammonium sulfate and sulfuric acid across a range of acidities(PH = 0.5 – 2.5) and gas-phase IEPOX into the chamber to generate SOA. The SOA particles were then sent to an oxidation flow reactor, and exposed to different OH concentrations representative of aging of a number of days. We kept relative humidity (RH) constant at ~65%, the temperature was ~23 °C, and levels of oxidation were controlled by adjusting lamp intensity. After oxidized SOA was impacted on quartz substrates, we used single-particle Raman microspectroscopy to identify their functional group compositions. From the Raman vibrational spectra of submicron particles (~500-1000 nm aerodynamic diameter), we observed a distinct difference in core-shell morphology and composition: an organic outer layer and an aqueous-inorganic core. The core also has significantly more CH-stretch than the shell. Small changes were also observed with increasing oxidation, which are important to consider when predicting SOA particle evolution in the atmosphere.
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Decrease in sulfate aerosol light backscattering by reactive uptake of isoprene epoxydiols
Sulfate aerosol is responsible for a net cooling of the Earth's atmosphere due to its ability to backscatter light. Through atmospheric multiphase chemistry, it reacts with isoprene epoxydiols leading to the formation of aerosol and organic compounds, including organosulfates and high-molecular weight compounds. In this study, we evaluate how sulfate aerosol light backscattering is modified in the presence of such organic compounds. Our laboratory experiments show that reactive uptake of isoprene epoxydiols on sulfate aerosol is responsible for a decrease in light backscattering compared to pure inorganic sulfate particles of up to – 12% at 355 nm wavelength and – 21% at 532 nm wavelength. Moreover, while such chemistry is known to yield a core–shell structure, the observed reduction in the backscattered light intensity is discussed with Mie core–shell light backscattering numerical simulations. We showed that the observed decrease can only be explained by considering effects from the complex optical refractive index. Since isoprene is the most abundant hydrocarbon emitted into the atmosphere, and isoprene epoxydiols are the most important isoprene secondary organic aerosol precursors, our laboratory findings can aid in quantifying the direct radiative forcing of sulfates in the presence of organic compounds, thus more clearly resolving the impact of such aerosol particles on the Earth's climate.
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- Award ID(s):
- 1703535
- NSF-PAR ID:
- 10226171
- Date Published:
- Journal Name:
- Physical Chemistry Chemical Physics
- Volume:
- 23
- Issue:
- 10
- ISSN:
- 1463-9076
- Page Range / eLocation ID:
- 5927 to 5935
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Oxidation of isoprene, the biogenic volatile organic compound with the highest emissions globally, is a large source of secondary organic aerosol (SOA) in the atmosphere. Organosulfates, particularly methyltetrol sulfates, formed from acid-driven reactions of isoprene epoxydiols (IEPOX), a key oxidation product, are important contributors to SOA mass. To date, most studies have focused on organosulfate formation on ammonium sulfate particles at low pH. However, recent work has shown sea spray aerosol (SSA) in the accumulation mode (~100 nm) is often quite acidic (pH ~ 2) and IEPOX-derived organosulfates have been identified in marine environments. Herein, we demonstrate that substantial SOA, including organosulfates, are formed on acidic sodium sulfate particles (pH = 1.3), representative of marine aerosol heterogeneously reacting with H2SO4 to form Na2SO4. For acidic sodium and ammonium sulfate particles, 31 and 28% (±1%), respectively, of inorganic sulfate is incorporated into organosulfate species, even though acidic particles with sodium versus ammonium as the primary cation formed 5% (±0.2) less SOA volume and 45% (±6%) less methyltetrol sulfates, suggesting other organosulfates may form. Even though both exhibited core-shell morphology after IEPOX uptake, physicochemical differences were observed via Raman microspectroscopy, with organosulfates identified in both the core and shell of acidic ammonium sulfate SOA particles, but only in the core for acidic sodium sulfate SOA via Raman microspectroscopy. Our results suggest that isoprene-derived SOA formed on aged SSA is potentially an important, but underappreciated, source of SOA and organosulfates in marine and coastal regions and could modify SOA budgets in these environments.more » « less
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Kinetics and Products of Heterogeneous Hydroxyl Radical Oxidation of Isoprene Epoxydiol‐Derived SOA.In isoprene‐rich regions, acid‐catalyzed multiphase reactions of isoprene epoxydiols (IEPOX) with inorganic sulfate (Sulfinorg) particles form secondary organic aerosol (IEPOX‐SOA), extensively converting Sulfinorg to lowervolatility particulate organosulfates (OSs), including 2‐ methyltetrol sulfates (2‐MTSs) and their dimers. Recently, we showed that heterogeneous hydroxyl radical (OH) oxidation of particulate 2‐MTSs generated multifunctional OS products. However, atmospheric models assume that OS‐rich IEPOX‐SOA particles remain unreactive towards heterogeneous OH oxidation, and limited laboratory studies have been conducted to examine the heterogeneous OH oxidation kinetics of full IEPOX‐SOA mixtures. Hence, this study investigated the kinetics and products resulting from heterogeneous OH oxidation of freshly‐generated IEPOXSOA in order to help derive model‐ready parameterizations. First, gas‐phase IEPOX was reacted with acidic Sulfinorg particles under dark conditions in order to form fresh IEPOX‐SOA particles. These particles were then subsequently aged at RH of 56% in an oxidation flow reactor at OH exposures ranging from 0~15 days of equivalent atmospheric exposure. Aged IEPOX‐SOA particles were sampled by an online aerosol chemical speciation monitor (ACSM) and collected onto Teflon filters for off‐line molecular‐level chemical analyses by hydrophilic liquid interaction chromatography method interfaced to electrospray ionization high‐resolution quadrupole time‐offlight mass spectrometry (HILIC/ESI‐HR‐QTOFMS). Our results show that heterogeneous OH oxidation only caused a 7% decay of IEPOX‐SOA by 10 days exposure, likely owing to the inhibition of reactive uptake of OH as fresh IEPOXSOA particles have an inorganic core‐organic shell morphology. A significantly higher fraction of IEPOX‐SOA (~37%) decayed by 15 days exposure, likely due to the increasing reactive uptake of OH as IEPOX‐SOA become more liquid‐like with aging. Freshly‐generated IEPOX‐SOA constituents exhibited varying degrees of aging with 2‐MTSdimers being the most reactive, followed by 2‐MTSs and 2‐ methyltetrols (2‐MTs), respectively. Notably, extensive amounts of previously characterized particle‐phase products in ambient fine aerosols were detected in our laboratory‐aged IEPOX‐SOA samples.more » « less
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/d0cp05468b
