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Simulating Aqueous-Phase Isoprene-Epoxydiol (IEPOX) Secondary Organic Aerosol Production During the 2013 Southern Oxidant and Aerosol Study (SOAS) Sri Hapsari Budisulistiorini, Athanasios Nenes, Ann Marie Grover Carlton, Jason Douglas Surratt, V. Faye McNeill, and Havala Olson Taylor Pye Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b05750 • Publication Date (Web): 10 Apr 2017 Downloaded from http://pubs.acs.org on April 10, 2017
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Environmental Science & Technology
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Simulating Aqueous-Phase Isoprene-Epoxydiol (IEPOX) Secondary Organic
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Aerosol Production During the 2013 Southern Oxidant and Aerosol Study
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(SOAS)
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Sri Hapsari Budisulistiorini†,∞,#, Athanasios Nenes‡,§,¶,ƒ, Annmarie G. Carlton∫, Jason D. Surratt∞, V. Faye McNeill◊,*, Havala O. T. Pye†,*
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†
National Exposure Research Laboratory, US EPA, Research Triangle Park, North Carolina 27711, United States
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∞
Department of Environmental Sciences and Engineering, Gillings School of Global Public Health, University of North Carolina, Chapel Hill, North Carolina 27599, United States
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#
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‡
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§
16 17
¶
18 19
ƒ
Institute of Environmental Research and Sustainable Development, National Observatory of Athens, Palea Penteli, GR-15236, Greece
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∫
21 22
◊
Department of Chemical Engineering, Columbia University, New York, New York 10027, United States
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*
Earth Observatory of Singapore, Nanyang Technological University, Singapore 639798, Singapore School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, Georgia 30322, United States School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, Atlanta, Georgia 30322, United States Institute of Chemical Engineering Sciences, Foundation for Research and Technology - Hellas, GR-26504 Patras, Greece
Department of Chemistry, University of California, Irvine, California, 92617, United States
To whom correspondence should be addressed
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For submission to Environmental Science and Technology
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Abstract
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The lack of statistically robust relationships between IEPOX (isoprene epoxydiol)-derived SOA
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(IEPOX SOA) and aerosol liquid water and pH observed during the 2013 Southern Oxidant and
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Aerosol Study (SOAS) emphasizes the importance of modeling the whole system to understand
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the controlling factors governing IEPOX SOA formation. We present a mechanistic modeling
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investigation predicting IEPOX SOA based on Community Multiscale Air Quality (CMAQ)
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model algorithms and a recently introduced photochemical box model, simpleGAMMA. We aim
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to 1) simulate IEPOX SOA tracers from the SOAS Look Rock ground site, 2) compare the two
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model formulations, 3) determine the limiting factors in IEPOX SOA formation, and 4) test the
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impact of a hypothetical sulfate reduction scenario on IEPOX SOA. The estimated IEPOX SOA
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mass variability is in similar agreement (r2 ~ 0.6) with measurements. Correlations of the
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estimated and measured IEPOX SOA tracers with observed aerosol surface area (r2 ~ 0.5–0.7),
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rate of particle-phase reaction (r2 ~ 0.4–0.7), and sulfate (r2 ~ 0.4–0.5) suggest an important role
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of sulfate in tracer formation via both physical and chemical mechanisms. A hypothetical 25%
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reduction of sulfate results in ~70% reduction of IEPOX SOA formation, reaffirming the
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importance of aqueous phase chemistry in IEPOX SOA production.
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Environmental Science & Technology
Introduction
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Particles with aerodynamic diameters less than 2.5 µm (PM2.5) are known to affect human
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health and regional climate forcing. 1 Organic aerosol (OA) constitutes a large fraction of PM2.5
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mass.
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plays an important role in the spatial and temporal distribution of aerosol optical thickness,
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potentially causing regional cooling over the southeastern United States. 3-5
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In summer, biogenic secondary organic aerosol (SOA) is a major fraction of OA and
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Isoprene (2-methyl-1,3-butadiene) emission is estimated to be 600 Tg yr-1, 6 which makes it the
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most abundant non-methane volatile organic compound (VOC) emitted globally. It also plays a
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substantial role in SOA production. 7 On average, isoprene-derived SOA tracers account for up to
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20% of total OA observed in the southeastern U.S. (SE US), and compounds derived from
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isoprene epoxydiol (IEPOX) chemical pathways can account for >95% of isoprene SOA.
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IEPOX-OA, a positive matrix factor linked to IEPOX-derived SOA (IEPOX SOA) tracers,
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comprises one-third of ambient organic aerosol mass in urban and rural areas in the SE US, 9-11
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suggesting a regional source. Among isoprene-derived SOA tracers, 2-methyltetrols (tetrols) and
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their sulfate ester derivatives (organosulfates) contribute ~ 33% and ~ 34% of IEPOX-OA,
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respectively, at the Look Rock (LRK), Tennessee site. 9
8,9
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Inclusion of biogenic and anthropogenic SOA precursors and mechanisms in the Community
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Multiscale Air Quality (CMAQ) model improves the temporal correlation between predicted and
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semi-empirical secondary organic carbon (OC). 12 Addition of the acid-catalyzed reactive uptake
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mechanism of IEPOX into CMAQ further improves predictions of tetrols against field
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measurements.
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reported a strong correlation between predicted and observed IEPOX SOA, although the
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simulations underestimate the measurements by an order of magnitude.
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Using the updated SOA module in the CMAQ model, Karambelas et al.
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McNeill et al.
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introduced GAMMA (Gas-Aerosol Model for Mechanism Analysis), a
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detailed photochemical box model of aqueous-aerosol SOA (aaSOA) formation. A version of
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GAMMA with simplified aqueous reaction mechanism, simpleGAMMA, has been shown to be
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computationally efficient and suitable for coupling with large-scale atmospheric chemistry
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models. 16 The aqueous aerosol phase mechanism in simpleGAMMA treats SOA formation from
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glyoxal and IEPOX.
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IEPOX-derived organosulfates (IEPOXOS) and tetrols, which have been quantified in 17,18
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laboratory experiments,
have been shown to contribute up to half of all known IEPOX-
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derived aerosol-phase species in the southeastern U.S.
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IEPOX SOA is through acid-catalyzed ring-opening reactions of isoprene epoxides in concert
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with nucleophilic addition in aqueous particles. 18-20 An airborne study over the southeastern US
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found a strong dependency between IEPOXOS and acidic sulfate aerosol. 21 However, ground-
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based studies in the eastern US suggest weak or no correlation between IEPOX SOA and aerosol
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pH or liquid water content (LWC), but a positive correlation with sulfate.
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preliminary modeling study employing simpleGAMMA found good correlation (r2 ~ 0.6)
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between estimated IEPOX SOA and measurements made during the 2013 Southern Oxidant
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Aerosol Study (SOAS) campaign. 9
8,9
The accepted formation pathway for
9,11
Additionally, a
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This study aims to investigate the formation of two IEPOX SOA tracers, IEPOXOS and
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tetrols, using CMAQ and simpleGAMMA algorithms, and evaluate the model response to
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perturbations that replicate atmospheric conditions. Predictions from these two different
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algorithms are compared with measurement results from the 2013 SOAS campaign. Only two
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IEPOX SOA tracers are predicted and investigated by these algorithms since they are the most
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abundant tracers during the entire field observation. 9 Limiting factors in IEPOX SOA formation
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are investigated and the IEPOX SOA response due to changes in SOx emission were estimated.
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Method
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Two different representations of IEPOX SOA formation were implemented in box-model
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form, namely CMAQ-box and simpleGAMMA, using conditions representative of the SOAS
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Look Rock (LRK) site.
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CMAQ-box. CMAQ-box is based on the algorithms in the CMAQ model, which are detailed 13
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in the work of Pye et al.
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uptake of gaseous IEPOX into the aerosol phase as a first order reaction:
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Briefly, CMAQ treats IEPOX SOA formation as heterogeneous
→
(R1)
Governed by a first order heterogeneous rate constant, ,22 such that:
= −
(1)
where
=
!
(2)
" #$
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SA is the aerosol surface area (µm2 cm-3) upon which uptake occurs, % is the mean molecular
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speed (m s-1) in the gas phase, and &' is the effective particle radius (cm). ( is the diffusivity of
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IEPOX in the gas phase, which is estimated as ( = 1.9 ,-. / 01 23
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molecular weight (g mol-1).
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uptake coefficient (6) calculated with the following. 23,24
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0.5 4,
1 7
1
= 8 + ;< ∗ >?
:
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with MW as
The heterogeneous reaction is parameterized using a reactive
1
@AB C B DEFGH IJKL M015M
(3)
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Here, N is the accommodation coefficient (0.02) 15, O ∗ is the Henry’s Law coefficient (M atm-1),
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R is the gas constant, T is temperature (K), (P is diffusivity in the aerosol phase (1 × 10-9 m2 s-1),
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rate constant (s-1) for reaction of IEPOX in the aerosol phase. This formulation allows for the
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model to predict whether the particle-phase reaction will occur throughout the particle, or as a
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surface process based on the relative timescales for reaction Q1⁄ 'PRSTU W vs. aerosol-phase
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diffusion Q&' . ⁄(P W. The Henry’s law coefficient was set to 3 × 107 M atm-1 based on the value
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measured by Nguyen et al. and used in CMAQ starting with v5.226 and simpleGAMMA v1.0. 16
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q is the diffuso-reactive parameter Q&' @ 'PRSTU ⁄(P W, and 'PRSTU is the pseudo-first order
simpleGAMMA. A detailed description of simpleGAMMA is available in Woo and McNeill.
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16
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aqueous-phase reaction23 to form IEPOX SOA consisting of tetrols and IEPOXOS: → PM →
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Briefly, simpleGAMMA represents IEPOX SOA in terms of aqueous uptake followed by
(R2)
Formation of the aaSOA of IEPOX SOA (mol L-1 of aerosol) is given by:
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=
CXD,Z[\]^ >?
−
CXD,Z[\]^ < ∗ >?
_PM ` + ∑C &C,PM
(4)
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where, is the gas-phase partial pressure of species IEPOX. &C,PM are the rates of formation
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or
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(&C,PM = 'PRSTU _PM `). b, is the gas-to-aerosol mass transfer coefficient of
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IEPOX, which can be described as follows: 16
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decomposition
b, =
c d
of
IEPOX
due
to
chemical
reactions
1 !
in
aerosol
phase
(5)
" d#e
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Particle-phase Reaction Parameters. The particle-phase reaction rate constant for IEPOX is
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calculated assuming protonation of the epoxide oxygen and nucleophilic addition in both CMAQ
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and simpleGAMMA following an A2 mechanism by Eddingsaas et al. 19 While CMAQ does not
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treat NH4+ as an acid for epoxide uptake purposes, we implemented 'PRSTU such that it was the
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same in both simpleGAMMA and CMAQ-box calculations using the most recent values from
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literature which included updating the organosulfate formation rate constant following Riedel et
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al. 27 Thus, the particle-phase rate constant for both CMAQ and simpleGAMMA simulations in
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this study is:
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'PRSTU = < f ,gPR h< f i3j +
