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Efficient isoprene secondary organic aerosol formation from a non-IEPOX pathway Jiumeng Liu, Emma L. D'Ambro, Ben Hwan Lee, Felipe Lopez-Hilfiker, Rahul Zaveri, Jean C. Rivera-Rios, Frank N. Keutsch, Siddharth Iyer, Theo Kurtén, Zhenfa Zhang, Avram Gold, Jason Douglas Surratt, John E. Shilling, and Joel A. Thornton Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b01872 • Publication Date (Web): 22 Aug 2016 Downloaded from http://pubs.acs.org on August 23, 2016
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Efficient isoprene secondary organic aerosol formation from a non-IEPOX pathway Jiumeng Liu1†, Emma L. D'Ambro2†, Ben H. Lee3, Felipe D. Lopez-Hilfiker3, Rahul A. Zaveri1, Jean C. Rivera-Rios4, Frank N. Keutsch4, Siddharth Iyer5, Theo Kurten5, Zhenfa Zhang6, Avram Gold6, Jason D. Surratt6, John E. Shilling1*, and Joel A. Thornton2,3* 1
Atmospheric Sciences and Global Change Division, Pacific Northwest National Laboratory Richland, WA 99352, USA. 2
Department of Chemistry, University of Washington, Seattle, WA 98195, USA.
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Department of Atmospheric Sciences, University of Washington, Seattle, WA 98195, USA.
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Paulson School of Engineering and Applied Sciences and Department of Chemistry and Chemical Biology, Harvard University, Cambridge, MA 02138, USA. 5
Department of Chemistry, University of Helsinki, Helsinki FI-00014, Finland.
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Department of Environmental Sciences and Engineering, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599, USA. *Correspondence to: Joel Thornton Phone: 206-543-4010; Email:
[email protected]; Present address: Department of Chemistry, University of Washington, Seattle, WA 98195, USA. John Shilling Phone: 509-375-6874; Email:
[email protected]; Present address: Atmospheric Sciences and Global Change Division, Pacific Northwest National Laboratory Richland, WA 99352, USA. †
Equally contributing authors.
Keywords:
Isoprene secondary organic aerosol, dihydroxydihydroperoxides, Chamber wall losses
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Abstract
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With a large global emission rate and high reactivity, isoprene has a profound effect upon
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atmospheric chemistry and composition. The atmospheric pathways by which isoprene converts
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to secondary organic aerosol (SOA) and how anthropogenic pollutants such as nitrogen oxides
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and sulfur affect this process are a subject of intense research because particles affect Earth’s
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climate and local air quality. In the absence of both nitrogen oxides and reactive aqueous seed
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particles, we measure SOA mass yields from isoprene photochemical oxidation of up to 15%,
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which are factors of 2, or more, higher than those typically used in coupled chemistry-climate
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models. SOA yield is initially constant with the addition of increasing amounts of nitric oxide
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(NO) but then sharply decreases for input concentrations above 50 ppbv. Online measurements
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of aerosol molecular composition show that the fate of second-generation RO2 radicals is key to
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understanding the efficient SOA formation and the NOx dependent yields described here and in
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the literature. These insights allow for improved quantitative estimates of SOA formation in the
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pre-industrial atmosphere and in biogenic-rich regions with limited anthropogenic impacts and
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suggest a more complex representation of NOx dependent SOA yields may be important in
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models.
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Introduction
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Global forests emit 500 to 750 Tg of isoprene per year, the largest flux of biogenic or
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anthropogenic non-methane hydrocarbons to the atmosphere.1 Isoprene-derived secondary
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organic aerosol (iSOA) is predicted to comprise a significant fraction of the organic aerosol (OA)
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burden over large regions of the globe.2-5 Isoprene also influences the oxidative capacity of the
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troposphere, particularly in pristine forested regions,6,
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atmospheric radicals and its effect on the fate of reactive nitrogen oxides, with consequences for
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the abundance and lifetime of greenhouse gases such as methane and tropospheric ozone, and
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likely indirect effects on particle formation, growth,8 and SOA yield from other VOCs.9 There is
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thus great interest in resolving the atmospheric fate of isoprene derived carbon.
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due to its high reactivity towards
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Recently, there has been significant interest in determining whether anthropogenic pollutants,8, 10
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such as NOx and sulfur oxides, may enhance iSOA formation, and thereby increase the climate
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effects of aerosol particles, which directly interact with solar radiation and alter the reflectivity
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and lifecycle of clouds. The magnitude of this anthropogenic forcing of a natural aerosol
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formation process has been difficult to quantify with certainty,11 in part because the iSOA
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formation potential under pre-industrial “pristine” conditions is poorly known. The net
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anthropogenic aerosol forcing of climate is determined by referencing to year 1750 conditions,
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when NOx and sulfur emissions were approximately 4 and 6 times lower, respectively, than
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today.12 Thus, assessing the anthropogenic aerosol forcing is inherently tied to accurately
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quantifying natural aerosol sources under pre-industrial conditions.10 This need, in turn, requires
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a mechanistic-level understanding of the iSOA formation pathways.
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In the atmosphere, isoprene reacts with hydroxyl radicals (OH) to form reactive organic peroxy
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radical intermediates (ISOPO2). The short-lived (~100 s in forests13) ISOPO2 intermediates
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ultimately react with hydroperoxyl radicals (HO2), nitric oxide (NO), or other organic peroxy
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radicals (RO2). The HO2 pathway, estimated to account globally for one half of the reactive fate
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of ISOPO2,11 and possibly more in the pre-industrial atmosphere, produces a hydroxy
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hydroperoxide (ISOPOOH), see Figure 1. In remote regions and in the pre-industrial atmosphere
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where both NO and HOx concentrations are low, the ISOPO2 radical also isomerizes to produce
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hydroperoxyaldehyde species.11, 14
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Most recent experiments have focused on iSOA formed from the multiphase acid-catalyzed
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chemistry of ISOPOOH-derived epoxydiols (IEPOX).12, 15-19 However, measurements of iSOA
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yield under low-NOx conditions, in the absence of reactive aqueous seed particles should be
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revisited, particularly in light of recent findings of significant wall artifacts.20 These conditions
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likely represent much of the pre-industrial atmosphere and remote pristine regions in present-day
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where isoprene is oxidized far from anthropogenic emissions. Moreover, most previous
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experiments studying photochemical iSOA formation, rather than that from IEPOX multiphase
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chemistry, were conducted under conditions where the oxidant field and thus product branching
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is continuously changing and where the important iSOA-forming products are apparently lost to
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the walls of the reaction chamber, and thus undetected.20 In addition, most models that do
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include a NOx dependence of SOA yields classify them as either high-NOx or NOx-free, while
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studies have suggested that NOx dependence for iSOA is more complex.5, 21, 22 Addition of NOx
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has been shown to change the relative branching ratios of isoprene RO2 radicals reacting with
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HO2 and NO, but the dynamic NOx-dependence of iSOA yield is not mechanistically well
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understood and therefore difficult to accurately parameterize for inclusion in models. These
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issues suggest a continued need for additional studies of iSOA formation.
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Here we report results from a series of laboratory studies in which we systematically investigate
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photochemical iSOA formation as a function of the branching between isoprene-derived peroxy
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radical reactions with hydroperoxyl (HO2) and nitric oxide (NO) radicals. Our experiments were
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operated in the continuous-flow mode, which provides an alternative approach to most previous
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studies. We measure iSOA yields that are significantly higher than most literature values and that
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are dependent on both H2O2 and NO concentrations. We also show that this approach can have
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lower wall-loss effects (due to equilibration) and simplifies the kinetic interpretation of the
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experimental results by investigating steady state periods. That is, iSOA yields are obtained for a
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pre-determined extent of aging, set by oxidant concentrations and chamber residence time, as
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opposed to batch-mode experiments where products are continuously passing through several
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photochemical generations which might have significantly different iSOA production and loss
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terms. We use a box model based on a detailed chemical mechanism to investigate the oxidant
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fields and thus infer production pathways of online-measured gas- and condensed-phase
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compounds. Our results demonstrate a pathway for efficient iSOA formation independent of
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IEPOX chemistry and may be important in regions of the atmosphere far removed from
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anthropogenic influence.
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Experimental Section
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Chamber Experiment Overview
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Experiments were conducted in the Pacific Northwest National Laboratory (PNNL) 10.6 m3
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Teflon environmental chamber.23 The chamber was flushed with purified air from AADCO pure
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air generators until the particle concentration was less than 1 particle cm-3 before the start of
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continuous-mode experiments. Isoprene, and/or NO in purified air were injected from calibrated
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gas cylinders with flows regulated by MKS mass flow controllers. H2O2 was introduced into the
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chamber by passing pure air through a gently heated glass bulb, with the H2O2 solution (Sigma-
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Aldrich, 50 wt% in H2O) continuously injected into the bulb using a syringe pump. Humidity
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was controlled at 50%, by passing pure air at a variable flow rate through pure water (18.2 MΩ
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cm,
