ACPAtmospheric Chemistry and PhysicsACPAtmos. Chem. Phys.1680-7324Copernicus PublicationsGöttingen, Germany10.5194/acp-16-1603-2016Aqueous-phase mechanism for secondary organic aerosol formation from
isoprene: application to the southeast United States and co-benefit of
SO2 emission controlsMaraisE. A.emarais@seas.harvard.eduhttps://orcid.org/0000-0001-5477-8051JacobD. J.JimenezJ. L.https://orcid.org/0000-0001-6203-1847Campuzano-JostP.https://orcid.org/0000-0003-3930-010XDayD. A.https://orcid.org/0000-0003-3213-4233HuW.https://orcid.org/0000-0002-3485-6304KrechmerJ.https://orcid.org/0000-0003-3642-0659ZhuL.https://orcid.org/0000-0002-3919-3095KimP. S.MillerC. C.FisherJ. A.https://orcid.org/0000-0002-2921-1691TravisK.https://orcid.org/0000-0003-1628-0353YuK.HaniscoT. F.https://orcid.org/0000-0001-9434-8507WolfeG. M.ArkinsonH. L.PyeH. O. T.https://orcid.org/0000-0002-2014-2140FroydK. D.LiaoJ.McNeillV. F.School of Engineering and Applied Sciences, Harvard University,
Cambridge, MA, USAEarth and Planetary Sciences, Harvard University, Cambridge, MA, USACooperative Institute for Research in Environmental Sciences,
University of Colorado, Boulder, CO, USADepartment of Chemistry and Biochemistry, University of Colorado,
Boulder, CO, USASchool of Chemistry and School of Earth and Environmental Sciences,
University of Wollongong, Wollongong, New South Wales, AustraliaAtmospheric Chemistry and Dynamics Lab, NASA Goddard Space Flight
Center, Greenbelt, MD, USAJoint Center for Earth Systems Technology, University of Maryland
Baltimore County, Baltimore, MD, USADepartment of Atmospheric and Oceanic Science, University of Maryland,
College Park, MD, USANational Exposure Research Laboratory, US EPA, Research Triangle Park,
NC, USAChemical Sciences Division, Earth System Research Laboratory, NOAA,
Boulder, CO, USADepartment of Chemical Engineering, Columbia University, New York,
NY, USAE. A. Marais (emarais@seas.harvard.edu)11February20161631603161821October201513November201525January201631January2016This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://acp.copernicus.org/articles/16/1603/2016/acp-16-1603-2016.htmlThe full text article is available as a PDF file from https://acp.copernicus.org/articles/16/1603/2016/acp-16-1603-2016.pdf
Isoprene emitted by vegetation is an important precursor of secondary organic
aerosol (SOA), but the mechanism and yields are uncertain. Aerosol is
prevailingly aqueous under the humid conditions typical of isoprene-emitting
regions. Here we develop an aqueous-phase mechanism for isoprene SOA
formation coupled to a detailed gas-phase isoprene oxidation scheme. The
mechanism is based on aerosol reactive uptake coefficients (γ) for
water-soluble isoprene oxidation products, including sensitivity to aerosol
acidity and nucleophile concentrations. We apply this mechanism to simulation
of aircraft (SEAC4RS) and ground-based (SOAS) observations over the
southeast US in summer 2013 using the GEOS-Chem chemical transport model.
Emissions of nitrogen oxides (NOx≡ NO + NO2) over
the southeast US are such that the peroxy radicals produced from isoprene
oxidation (ISOPO2) react significantly with both NO (high-NOx
pathway) and HO2 (low-NOx pathway), leading to different suites of
isoprene SOA precursors. We find a mean SOA mass yield of 3.3 % from
isoprene oxidation, consistent with the observed relationship of total fine
organic aerosol (OA) and formaldehyde (a product of isoprene oxidation).
Isoprene SOA production is mainly contributed by two immediate gas-phase
precursors, isoprene epoxydiols (IEPOX, 58 % of isoprene SOA) from the
low-NOx pathway and glyoxal (28 %) from both low- and high-NOx
pathways. This speciation is consistent with observations of IEPOX SOA from
SOAS and SEAC4RS. Observations show a strong relationship between IEPOX
SOA and sulfate aerosol that we explain as due to the effect of sulfate on
aerosol acidity and volume. Isoprene SOA concentrations increase as NOx
emissions decrease (favoring the low-NOx pathway for isoprene
oxidation), but decrease more strongly as SO2 emissions decrease (due to
the effect of sulfate on aerosol acidity and volume). The US Environmental
Protection Agency (EPA) projects 2013–2025 decreases in anthropogenic
emissions of 34 % for NOx (leading to a 7 % increase in isoprene
SOA) and 48 % for SO2 (35 % decrease in isoprene SOA). Reducing
SO2 emissions decreases sulfate and isoprene SOA by a similar magnitude,
representing a factor of 2 co-benefit for PM2.5 from SO2 emission
controls.
Introduction
Isoprene emitted by vegetation is a major source of secondary organic
aerosol (SOA) (Carlton et al., 2009, and references therein) with effects on
human health, visibility, and climate. There is large uncertainty in the
yield and composition of isoprene SOA (Scott et al., 2014; McNeill et al.,
2014), involving a cascade of species produced in the gas-phase oxidation of
isoprene and their interaction with pre-existing aerosol (Hallquist et al.,
2009). We develop here a new aqueous-phase mechanism for isoprene SOA
formation coupled to gas-phase chemistry, implement it in the GEOS-Chem
chemical transport model (CTM) to simulate observations in the southeast US,
and from there derive new constraints on isoprene SOA yields and the
contributing pathways.
Organic aerosol is ubiquitous in the atmosphere, often dominating fine
aerosol mass (Zhang et al., 2007), including in the southeast US where it
accounts for more than 60 % in summer (Attwood et al., 2014). It may be
directly emitted by combustion as primary organic aerosol (POA), or produced
within the atmosphere as SOA by oxidation of volatile organic compounds
(VOCs). Isoprene (C5H8) from vegetation is the dominant VOC
emitted globally, and the southeast US in summer is one of the largest
isoprene-emitting regions in the world (Guenther et al., 2006). SOA yields
from isoprene are low compared with larger VOCs (Pye et al., 2010), but
isoprene emissions are much higher. Kim et al. (2015) estimated that
isoprene accounts for 40 % of total organic aerosol in the southeast US in
summer.
Yields of secondary organic aerosol (SOA) from isoprene oxidation as
reported by chamber studies in the literature and plotted as a function of
the initial NO concentration and relative humidity (RH). Yields are defined
as the mass of SOA produced per unit mass of isoprene oxidized. For studies
with no detectable NO we plot the NO concentration as half the reported
instrument detection limit, and stagger points as needed for clarity. Data
are colored by relative humidity (RH). The thick gray line divides the
low-NOx and high-NOx pathways as determined by the fate of the
ISOPO2 radical (HO2 dominant for the low-NOx pathway, NO
dominant for the high-NOx pathway). The transition between the two
pathways occurs at a higher NO concentration than in the atmosphere because
HO2 concentrations in the chambers are usually much higher. Also shown
as a dashed line is the mean atmospheric yield of 3.3 % for the southeast
US determined in our study.
Formation of OA from oxidation of isoprene depends on local concentrations
of nitrogen oxide radicals (NOx≡ NO + NO2) and
pre-existing aerosol. NOx concentrations determine the fate of organic
peroxy radicals originating from isoprene oxidation (ISOPO2), leading
to different cascades of oxidation products in the low-NOx and
high-NOx pathways (Paulot et al., 2009a, b). Uptake of isoprene
oxidation products to the aerosol phase depends on their vapor pressure
(Donahue et al., 2006), solubility in aqueous media (Saxena and Hildeman,
1996), and subsequent condensed-phase reactions (Volkamer et al., 2007).
Aqueous aerosol provides a medium for reactive uptake (Eddingsaas et al.,
2010; Surratt et al., 2010) with dependences on acidity (Surratt et al.,
2007a), concentration of nucleophiles such as sulfate (Surratt et al.,
2007b), aerosol water (Carlton and Turpin, 2013), and organic coatings
(Gaston et al., 2014).
We compile in Fig. 1 the published laboratory yields of isoprene SOA as a
function of initial NO concentration and relative humidity (RH). Here and
elsewhere, the isoprene SOA yield is defined as the mass of SOA produced per
unit mass of isoprene oxidized. Isoprene SOA yields span a wide range, from
< 0.1 % to > 10 %, with no systematic difference
between low-NOx and high-NOx pathways. Yields tend to be higher in
dry chambers (RH < 10 %). Under such dry conditions isoprene SOA
is expected to be solid (Virtanen et al., 2010; Song et al., 2015). At humid
conditions more representative of the summertime boundary layer, aerosols
are likely aqueous (Bateman et al., 2014). Standard isoprene SOA mechanisms
used in atmospheric models assume reversible partitioning onto pre-existing
organic aerosol, fitting the dry chamber data (Odum et al., 1996). However,
this may not be appropriate for actual atmospheric conditions where
aqueous-phase chemistry with irreversible reactive uptake of water-soluble
gases is likely the dominant mechanism (Ervens et al., 2011; Carlton and
Turpin, 2013). Several regional/global models have implemented mechanisms
for aqueous-phase formation of isoprene SOA (Fu et al., 2008, 2009; Carlton
et al., 2008; Myriokefalitakis et al., 2011; Liu et al., 2012; Pye et al.,
2013; Lin et al., 2014).
Here we present a mechanism for irreversible aqueous-phase isoprene SOA
formation integrated within a detailed chemical mechanism for isoprene
gas-phase oxidation, thus linking isoprene SOA formation to gas-phase
chemistry and avoiding more generic volatility-based parameterizations that
assume dry organic aerosol (Odum et al., 1996; Donahue et al., 2006). We use
this mechanism in the GEOS-Chem CTM to simulate observations from the SOAS
(surface) and SEAC4RS (aircraft) field campaigns over the southeast US in summer 2013, with focus on isoprene SOA components and on the
relationship between OA and formaldehyde (HCHO). HCHO is a high-yield
oxidation product of isoprene (Palmer et al., 2003) and we use the OA–HCHO
relationship as a constraint on isoprene SOA yields. SOAS measurements were
made at a ground site in rural Centreville, Alabama (Hu et al., 2015;
http://soas2013.rutgers.edu/). SEAC4RS measurements were made from the
NASA DC-8 aircraft with extensive boundary-layer coverage across the
southeast (Toon and the SEAC4RS science team, 2016; SEAC4RS Archive, 2015).
Gas-phase isoprene oxidation cascade in GEOS-Chem leading to
secondary organic aerosol (SOA) formation by irreversible aqueous-phase
chemistry. Only selected species relevant to SOA formation are shown.
Immediate aerosol precursors are indicated by dashed boxes. Branching ratios
and SOA yields (aerosol mass produced per unit mass isoprene reacted) are
mean values from our GEOS-Chem simulation for the southeast US boundary
layer in summer. The total SOA yield from isoprene oxidation is 3.3 % and
the values shown below the dashed boxes indicate the contributions from the
different immediate precursors adding up to 3.3 %. Contributions of high-
and low-NOx isoprene oxidation pathways to glyoxal are indicated.
Chemical mechanism for isoprene SOA formation
The default treatment of isoprene SOA in GEOS-Chem at the time of this work
(v9-02; http://geos-chem.org) followed a standard parameterization operating
independently from the gas-phase chemistry mechanism and based on reversible
partitioning onto pre-existing OA of generic semivolatile products of
isoprene oxidation by OH and NO3 radicals (Pye et al., 2010). Here we
implement a new mechanism for reactive uptake by aqueous aerosols of species
produced in the isoprene oxidation cascade of the GEOS-Chem gas-phase
mechanism. This couples SOA formation to the gas-phase chemistry and is in
accord with increased evidence for a major role of aqueous aerosols in
isoprene SOA formation (Ervens et al., 2011).
The standard gas-phase isoprene oxidation mechanism in GEOS-Chem v9-02 is
described in Mao et al. (2013) and is based on best knowledge at the time
building on mechanisms for the oxidation of isoprene by OH (Paulot et al.,
2009a, b) and NO3 (Rollins et al., 2009). Updates implemented in
this work are described below and in companion papers applying GEOS-Chem to
simulation of observed gas-phase isoprene oxidation products over the
southeast US in summer 2013 (Fisher et al., 2016; Travis et al., 2016). Most
gas-phase products of the isoprene oxidation cascade in GEOS-Chem have high
dry deposition velocity, competing in some cases with removal by oxidation
and aerosol formation (Nguyen et al., 2015a; Travis et al., 2016).
Figure 2 shows the isoprene oxidation cascade in GEOS-Chem leading to SOA
formation. Reaction pathways leading to isoprene SOA precursors are
described below. Yields are in mass percent, unless stated otherwise.
Reactive ISOPO2 isomers formed in the first OH oxidation step react
with NO, the hydroperoxyl radical (HO2), other peroxy radicals
(RO2), or undergo isomerization (Peeters et al., 2009). The NO reaction
pathway (high-NOx pathway) yields C5 hydroxy carbonyls, methyl
vinyl ketone, methacrolein, and first-generation isoprene nitrates (ISOPN).
The first three products go on to produce glyoxal and methylglyoxal, which
serve as SOA precursors. The overall yield of glyoxal from the high-NOx
pathway is 7 mol % (yield on a molar basis). Oxidation of ISOPN by OH and
O3 is as described by Lee et al. (2014). Reaction of ISOPN with OH
produces saturated dihydroxy dinitrates (DHDN), 21 and 27 mol % from the
beta and delta channels respectively (Lee et al., 2014), and 10 mol %
isoprene epoxydiols (IEPOX) from each channel (Jacobs et al., 2014). We also
adopt the mechanism of Lin et al. (2013) to generate C4 hydroxy epoxides
(methacrylic acid epoxide and hydroxymethylmethyl-α-lactone, both
denoted MEPOX) from OH oxidation of a peroxyacyl nitrate, formed when
methacrolein reacts with OH followed by NO2. Only
hydroxymethylmethyl-α-lactone is shown in Fig. 2.
The HO2 reaction pathway for ISOPO2 leads to formation of
hydroxyhydroperoxides (ISOPOOH) that are oxidized to IEPOX (Paulot et al.,
2009b) and several low-volatility products, represented here as C5-LVOC
(Krechmer et al., 2015). The kinetics of IEPOX oxidation by OH is uncertain,
and experimentally determined IEPOX lifetimes vary from 8 to 28 h for an OH
concentration of 1 × 106 molecules cm-3 (Jacobs et al.,
2013; Bates et al., 2014). In GEOS-Chem we apply the fast kinetics of Jacobs
et al. (2013) and reduce the yield of IEPOX from ISOPOOH from 100 to
75 %, within the range observed by St. Clair et al. (2015), to address a
factor of 4 overestimate in simulated IEPOX pointed out by Nguyen et
al. (2015a). The IEPOX discrepancy could alternatively be addressed with an
order-of-magnitude increase in uptake by aerosol (see below) but the model
would then greatly overestimate the observed IEPOX SOA concentrations in SOAS
and SEAC4RS (Sect. 4).
IEPOX oxidizes to form glyoxal and methylglyoxal (Bates et al., 2014). The
overall glyoxal yield from the ISOPO2+ HO2 pathway is 6 mol %.
Krechmer et al. (2015) report a 2.5 mol % yield of C5-LVOC
from ISOPOOH but we reduce this to 0.5 mol % to reproduce surface
observations of the corresponding aerosol products (Sect. 4). Methyl vinyl
ketone and methacrolein yields from the ISOPO2+ HO2 pathway are
2.5 and 3.8 mol %, respectively (Liu et al., 2013), sufficiently low that
they do not lead to significant SOA formation.
Minor channels for ISOPO2 are isomerization and reaction with RO2.
Isomerization forms hydroperoxy aldehydes (HPALD) that go on to photolyze,
but products are uncertain (Peeters and Müller, 2010). We assume
25 mol % yield each of glyoxal and methylglyoxal from HPALD photolysis in
GEOS-Chem following Stavrakou et al. (2010). Reaction of ISOPO2 with
RO2 leads to the same suite of C4–C5 carbonyls as reaction
with NO (C5 hydroxy carbonyls, methacrolein, and methyl vinyl ketone)
and from there to glyoxal and methylglyoxal.
Constants for reactive uptake of isoprene SOA
precursorsa.
a Effective Henry's law constants H∗ and
aqueous-phase rate constants used to calculate reactive uptake coefficients
γ for isoprene SOA precursors IEPOX, ISOPNβ, ISOPNδ, and DHDN following Eqs. (1) and (2). Calculation of γ for other
isoprene SOA precursors in Fig. 2 is described in the text. b See
Fig. 2 for definition of acronyms. c Best fit to SOAS and
SEAC4RS IEPOX SOA and consistent with Nguyen et al. (2014).
d Cole-Filipiak et al. (2010). e Eddingsaas et
al. (2010). f ISOPN species formed from the beta and delta isoprene
oxidation channels (Paulot et al., 2009a) are treated separately in
GEOS-Chem. g By analogy with 4-nitrooxy-3-methyl-2-butanol (Rollins
et al., 2009). h Jacobs et al. (2014). i Assumed the same
as for ISOPNδ (Hu et al., 2011).
Immediate aerosol precursors from the isoprene + OH oxidation cascade are
identified in Fig. 2. For the high-NOx pathway (ISOPO2+ NO
channel) these include glyoxal and methylglyoxal (McNeill et al., 2012),
ISOPN (Darer et al., 2011; Hu et al., 2011), DHDN (Lee et al., 2014), MEPOX
(Lin et al., 2013), and IEPOX (Jacobs et al., 2014). For the low-NOx
pathway (ISOPO2+ HO2 channel) aerosol precursors are IEPOX
(Eddingsaas et al., 2010), C5-LVOC (Krechmer et al., 2015, in which the
aerosol-phase species is denoted ISOPOOH-SOA), glyoxal, and methylglyoxal.
Glyoxal and methylglyoxal are also produced from the ISOPO2+ RO2 and ISOPO2 isomerization channels.
Ozonolysis and oxidation by NO3 are additional minor isoprene reaction
pathways (Fig. 2). The NO3 oxidation pathway is a potentially important
source of isoprene SOA at night (Brown et al., 2009) from the irreversible
uptake of low-volatility second-generation hydroxynitrates (NT-ISOPN) (Ng et
al., 2008; Rollins et al., 2009). We update the gas-phase chemistry of
Rollins et al. (2009) as implemented by Mao et al. (2013) to include
formation of 4 mol % of the aerosol-phase precursor NT-ISOPN from
first-generation alkyl nitrates (Rollins et al., 2009). Ozonolysis products
are volatile and observed SOA yields in chamber studies are low
(< 1 %; Kleindienst et al., 2007). In GEOS-Chem only methylglyoxal is
an aerosol precursor from isoprene ozonolysis.
We implement uptake of isoprene oxidation products to aqueous aerosols using
laboratory-derived reactive uptake coefficients (γ) as given by
Anttila et al. (2006) and Gaston et al. (2014):
γ=1α+3ω4rRTH∗kaq-1.
Here α is the mass accommodation coefficient (taken as 0.1 for all
immediate SOA precursors in Fig. 2), ω is the mean gas-phase
molecular speed (cm s-1), r is the aqueous particle radius (cm), R
is the universal gas constant (0.08206 L atm K-1 mol-1), T is
temperature (K), H∗ is the effective Henry's Law constant
(M atm-1) accounting for any fast dissociation equilibria in the
aqueous phase, and kaq is the pseudo-first-order aqueous-phase
reaction rate constant (s-1) for conversion to non-volatile products.
Precursors with epoxide functionality, IEPOX and MEPOX, undergo
acid-catalyzed epoxide ring opening and nucleophilic addition in the aqueous
phase. The aqueous-phase rate constant formulation is from Eddingsaas et
al. (2010),
kaq=kH+H++knucnucH++kHSO4-HSO4-,
and includes three channels: acid-catalyzed ring opening followed by
nucleophilic addition of H2O (kH+ in M-1 s-1)
leading to methyltetrols, acid-catalyzed ring opening followed by
nucleophilic addition of sulfate and nitrate ions (nuc ≡ SO42-+ NO3-, knuc in M-2 s-1)
leading to organosulfates and organonitrates, and concerted protonation and
nucleophilic addition by bisulfate, HSO4- (kHSO4- in
M-1 s-1), leading to organosulfates.
Precursors with nitrate functionality (-ONO2), ISOPN and DHDN, hydrolyze
to form low-volatility polyols and nitric acid (Hu et al., 2011; Jacobs et
al., 2014), so kaq in Eq. (1) is the hydrolysis rate constant.
Glyoxal and methylglyoxal form SOA irreversibly by surface uptake followed by
aqueous-phase oxidation and oligomerization to yield non-volatile products
(Liggio et al., 2005; Volkamer et al., 2009; Nozière et al., 2009; Ervens
et al., 2011; Knote et al., 2014). Glyoxal forms SOA with higher yields
during the day than at night due to OH aqueous-phase chemistry (Tan et al.,
2009; Volkamer et al., 2009; Sumner et al., 2014). We use a daytime γ
of 2.9 × 10-3 for glyoxal from Liggio et al. (2005) and a
nighttime γ of 5 × 10-6 (Waxman et al., 2013; Sumner et
al., 2014). The SOA yield of methylglyoxal is small compared with that of
glyoxal (McNeill et al., 2012). A previous GEOS-Chem study by Fu et
al. (2008) used the same γ (2.9 × 10-3) for glyoxal and
methylglyoxal. Reaction rate constants are similar for aqueous-phase
processing of glyoxal and methylglyoxal (Buxton et al., 1997; Ervens et al.,
2003), but H∗ of glyoxal is about 4 orders of magnitude higher. Here
we scale the γ for methylglyoxal to the ratio of effective Henry's
law constants: H∗= 3.7 × 103 M atm-1 for
methylglyoxal (Tan et al., 2010) and H∗= 2.7 × 107 M atm-1 for glyoxal (Sumner et al.,
2014). The resulting uptake of methylglyoxal is very slow and makes a
negligible contribution to isoprene SOA.
The species C5-LVOC from ISOPOOH oxidation and NT-ISOPN from isoprene
reaction with NO3 have very low volatility and are assumed to condense
to aerosols with a γ of 0.1 limited by mass accommodation. Results
are insensitive to the precise value of γ since uptake by aerosols
is the main sink for these species in any case.
Mean reactive uptake coefficients γ of isoprene SOA
precursorsa.
a Mean values computed in GEOS-Chem for the southeast US in summer as
sampled along the boundary-layer (< 2 km) SEAC4RS aircraft
tracks and applied to aqueous aerosol. The reactive uptake coefficient
γ is defined as the probability that a gas molecule colliding with
an aqueous aerosol particle will be taken up and react in the aqueous phase
to form non-volatile products.
b See Fig. 2 for definition of acronyms.
cγ for IEPOX and MEPOX are continuous functions of pH (Eq. 2).
Values shown here are averages for different pH ranges sampled along
the SEAC4RS flight tracks. Aqueous aerosol pH is calculated locally in
GEOS-Chem using the ISORROPIA thermodynamic model (Fountoukis and Nenes,
2007).
d Daytime value. Nighttime value is 5 × 10-6.
Table 1 gives input variables used to calculate γ for IEPOX, ISOPN,
and DHDN by Eqs. (1) and (2). Rate constants are from experiments in
concentrated media, representative of aqueous aerosols, so no activity
correction factors are applied. Reported experimental values of
kH+ vary by an order of magnitude from
1.2 × 10-3 M-1 s-1 (Eddingsaas et al., 2010) to
3.6 × 10-2 M-1 s-1 (Cole-Filipiak et al., 2010).
Values of knuc vary by 3 orders of magnitude from
2 × 10-4 M-2 s-1 (Eddingsaas et al., 2010) to
5.2 × 10-1 M-2 s-1 (Piletic et al., 2013). Reported
values of IEPOX H∗ vary by 2 orders of magnitude (Eddingsaas et
al., 2010; Nguyen et al., 2014). We chose values of kH+,
knuc, and H∗ to fit the SOAS and SEAC4RS observations
of total IEPOX SOA and IEPOX organosulfates, as discussed in Sect. 4.
Table 2 lists average values of γ for all immediate aerosol
precursors in the southeast US boundary layer in summer as simulated by
GEOS-Chem (Sect. 3). γ for IEPOX is a strong function of pH and
increases from 1 × 10-4 to 1 × 10-2 as pH
decreases from 3 to 0. Gaston et al. (2014) reported order-of-magnitude
higher values of γ for IEPOX, reflecting their use of a higher
H∗, but this would lead in our model to an overestimate of IEPOX SOA
observations (Sect. 4). The value of γ for MEPOX is assumed to be
30 times lower than that of IEPOX when the aerosol is acidic (pH < 4),
due to slower acid-catalyzed ring opening (Piletic et al., 2013; Riedel et
al., 2015). At pH > 4 we assume that γ for IEPOX and MEPOX are
the same (Riedel et al., 2015), but they are then very low.
Isoprene SOA formation in clouds is not considered here. Acid-catalyzed
pathways would be slow. Observations show that the isoprene SOA yield in the
presence of laboratory-generated clouds is low (0.2–0.4 %;
Brégonzio-Rozier et al., 2015). Wagner et al. (2015) found no significant
production of SOA in boundary-layer clouds over the southeast US during
SEAC4RS.
GEOS-Chem simulation and isoprene SOA yields
Several companion papers apply GEOS-Chem to interpret SEAC4RS and
surface data over the southeast US in summer 2013 including Kim et al. (2015)
for aerosols, Fisher et al. (2016) for organic nitrates, Travis et al. (2016)
for ozone and NOx, and Zhu et al. (2016) for HCHO. These studies use a
model version with 0.25∘× 0.3125∘ horizontal
resolution over North America, nested within a
4∘× 5∘ global simulation. Here we use a
2∘× 2.5∘ global GEOS-Chem simulation with no
nesting. Yu et al. (2016) found little difference between
0.25∘× 0.3125∘ and
2∘× 2.5∘ resolutions in simulated regional
statistics for isoprene chemistry.
The reader is referred to Kim et al. (2015) for a general presentation of the
model, the treatment of aerosol sources and sinks, and evaluation with
southeast US aerosol observations; and to Travis et al. (2016) and Fisher et
al. (2016) for presentation of gas-phase chemistry and comparisons with
observed gas-phase isoprene oxidation products. Isoprene emission is from the
MEGAN v2.1 inventory (Guenther et al., 2012). The companion papers decrease
isoprene emission by 15 % from the MEGAN v2.1 values to fit the HCHO data
(Zhu et al., 2016), but this is not applied here.
Our SOA simulation differs from that of Kim et al. (2015). They assumed fixed
3 and 5 % mass yields of SOA from isoprene and monoterpenes,
respectively, and parameterized SOA formation from anthropogenic and open
fire sources as a kinetic irreversible process following Hodzic and
Jimenez (2011). Here we use our new aqueous-phase mechanism for isoprene SOA
coupled to gas-phase chemistry as described in Sect. 2, and otherwise use the
semivolatile reversible partitioning scheme of Pye et al. (2010) for
monoterpene, anthropogenic, and open fire SOA. Kim et al. (2015) found no
systematic bias in detailed comparisons to OA measurements from SEAC4RS
and from surface networks. We find a low bias, as shown below, because the
reversible partitioning scheme yields low anthropogenic and open fire SOA
concentrations.
Organic aerosol and sulfate contribute most of the aerosol mass over the
southeast US in summer, while nitrate is negligibly small (Kim et al.,
2015). GEOS-Chem uses the ISORROPIA thermodynamic model (Fountoukis and
Nenes, 2007) to simulate sulfate–nitrate–ammonium (SNA) aerosol composition,
water content, and acidity as a function of local conditions. Simulated
aerosol pH along the SEAC4RS flight tracks in the southeast US boundary
layer averages 1.3 (interquartiles 0.92 and 1.8). The aerosol pH remains
below 3 even when sulfate aerosol is fully neutralized by ammonia (Guo et
al., 2015).
We consider that the aqueous aerosol population where isoprene SOA formation
can take place is defined by the sulfate aerosol population. This assumes
that all aqueous aerosol particles contain some sulfate, and that all sulfate
is aqueous. Clear-sky RH measured from the aircraft in the southeast US
boundary layer during SEAC4RS averaged 72 ± 17 %, and the
corresponding values in GEOS-Chem sampled along the flight tracks averaged
66 ± 16 %. These RHs are sufficiently high that sulfate aerosol can
reliably be expected to be aqueous (Wang et al., 2008). The rate of gas
uptake by the sulfate aerosol is computed with the pseudo-first-order
reaction rate constant khet (s-1) (Schwartz, 1986; Jacob,
2000):
khet=∫0∞4πr2rDg+4γω-1n(r)dr,
where Dg is the gas-phase diffusion constant (taken to be
0.1 cm2 s-1) and n(r) is the number size distribution of
sulfate aerosol (cm-4). The first and second terms in parentheses
describe the limitations to gas uptake from gas-phase diffusion and
aqueous-phase reaction, respectively.
The sulfate aerosol size distribution including RH-dependent hygroscopic
growth factors is from the Global Aerosol Data Set (GADS) of Koepke et
al. (1997), as originally implemented in GEOS-Chem by Martin et al. (2003)
and updated by Drury et al. (2010). The GADS size distribution compares well
with observations over the eastern US in summer (Drury et al., 2010),
including for SEAC4RS (Kim et al., 2015). We compute n(r) locally in
GEOS-Chem by taking the dry SNA mass concentration, converting from mass to
volume with a dry aerosol mass density of 1700 kg m-3 (Hess et al.,
1998), applying the aerosol volume to the dry sulfate size distribution in
GADS, and then applying the GADS hygroscopic growth factors. We verified that
the hygroscopic growth factors from GADS agree within 10 % with those
computed locally from ISORROPIA.
Figure 2 shows the mean branching ratios for isoprene oxidation in the
southeast US boundary layer as calculated by GEOS-Chem. 87 % of isoprene
reacts with OH, 8 % with ozone, and 5 % with NO3. Oxidation of
isoprene by OH produces ISOPO2 of which 51% reacts with NO
(high-NOx pathway), 35 % reacts with HO2, 8 % isomerizes,
and 6 % reacts with other RO2 radicals.
Glyoxal is an aerosol precursor common to all isoprene + OH pathways in
our mechanism with yields of 7 mol % from the ISOPO2+ NO pathway,
6 mol % from ISOPO2+ HO2, 11 mol % from ISOPO2+ RO2,
and 25 mol % from ISOPO2 isomerization. For the southeast
US conditions we thus find that 44% of glyoxal is from the ISOPO2+ NO
pathway, 24 % from ISOPO2+ HO2, 8 % from ISOPO2+ RO2, and 24 % from ISOPO2 isomerization.
The mean total yield of isoprene SOA computed in GEOS-Chem for the southeast
US boundary layer is 3.3 %, as shown in Fig. 2. IEPOX contributes
1.9 % and glyoxal 0.9 %. The low-NOx pathway involving
ISOPO2 reaction with HO2 contributes 73 % of the total isoprene
SOA yield, mostly from IEPOX, even though this pathway is only 35 % of
the fate of ISOPO2. The high-NOx pathway contributes 16 % of
isoprene SOA, mostly from glyoxal. MEPOX contribution to isoprene SOA is
small (2 %) and consistent with a recent laboratory study that finds low
SOA yields from this pathway under humid conditions (Nguyen et al., 2015b).
The minor low-NOx pathways from ISOPO2 isomerization and reaction
with RO2 contribute 8 % of isoprene SOA through glyoxal. The
remainder of isoprene SOA formation (3 %) is from nighttime oxidation by
NO3.
The dominance of IEPOX and glyoxal as precursors for isoprene SOA was
previously found by McNeill et al. (2012) using a photochemical box model.
Both IEPOX and glyoxal are produced photochemically, and both are removed
photochemically in the gas phase by reaction with OH (and photolysis for
glyoxal). The mean lifetimes of IEPOX and glyoxal against gas-phase
photochemical loss average 1.6 and 2.3 h respectively for SEAC4RS
daytime conditions; mean lifetimes against reactive uptake by aerosol are 31
and 20 h, respectively. For both species, aerosol uptake is thus a minor
sink competing with gas-phase photochemical loss. Although we have assumed
here the fast gas-phase kinetics from Jacobs et al. (2013) for the
IEPOX + OH reaction, this result would not change if we used the slower
kinetics from Bates et al. (2014).
Relationship of organic aerosol (OA) and formaldehyde (HCHO)
concentrations over the southeast US in summer. The figure shows scatter
plots of SEAC4RS aircraft observations of OA concentrations in the
boundary layer (< 2 km) vs. HCHO mixing ratios measured from the aircraft
(left), and column HCHO (ΩHCHO) retrieved from OMI satellite
observations (right). Individual points denote data from individual
SEAC4RS flight days (8 August–10 September), averaged on the GEOS-Chem
grid. OMI data are for SEAC4RS flight days and coincident with the
flight tracks. GEOS-Chem is sampled for the corresponding locations and
times. Results from our simulation with aqueous-phase isoprene SOA chemistry
are shown in red, and results from a simulation with the Pye et al. (2010)
semivolatile reversible partitioning scheme are shown in blue. Aerosol
concentrations are per m3 at standard conditions of temperature and
pressure (273 K; 1 atm), denoted sm-3. Reduced major axis (RMA)
regressions are also shown with regression parameters and Pearson's
correlation coefficients given inset. 1σ standard deviations on the
regression slopes are obtained with jackknife resampling.
The dominance of gas-phase loss over aerosol uptake for both IEPOX and
glyoxal implies that isoprene SOA formation is highly sensitive to their
reactive uptake coefficients γ and to the aqueous aerosol mass
concentration (in both cases, γ is small enough that uptake is
controlled by bulk aqueous-phase rather than surface reactions). We find
under SEAC4RS conditions that γ for IEPOX is mainly controlled
by the H+ concentration (kH+[H+] in Eq. 2), with
little contribution from nucleophile-driven and HSO4--driven
channels, although this is based on highly uncertain rate constants
(Sect. 2). Consistency with SOAS and SEAC4RS observations will be
discussed below.
The 3.3 % mean yield of isoprene SOA from our mechanism is consistent
with the fixed yield of 3 % assumed by Kim et al. (2015) in their
GEOS-Chem simulation of the SEAC4RS period, including extensive
comparisons to OA observations that showed a 40 % mean contribution of
isoprene to total OA. We conducted a sensitivity simulation using the default
isoprene SOA mechanism in GEOS-Chem based on reversible partitioning of
semivolatile oxidation products onto pre-existing OA (Pye et al., 2010). The
isoprene SOA yield in that simulation was only 1.1 %. The observed
correlation of OA with HCHO in SEAC4RS supports our higher yield, as
shown below.
Observational constraints on isoprene SOA yields
Isoprene is the largest source of HCHO in the southeast US (Millet et al.,
2006), and we use the observed relationship between OA and HCHO to evaluate
the GEOS-Chem isoprene SOA yields. The SEAC4RS aircraft payload included
measurements of OA from an Aerodyne High-Resolution Time-of-Flight Aerosol Mass Spectrometer (HR-ToF-AMS;
DeCarlo et al., 2006; Canagaratna et al., 2007) concurrent with HCHO from a
laser-induced fluorescence instrument (ISAF; Cazorla et al., 2015). Column
HCHO was also measured during SEAC4RS from the Ozone Monitoring Instrument (OMI) satellite instrument
(González Abad et al., 2015; Zhu et al., 2016), providing a proxy for
isoprene emission (Palmer et al., 2003, 2006).
Figure 3 (left) shows the observed and simulated relationships between OA and
HCHO mixing ratios in the boundary layer. There is a strong correlation in
the observations and in the model (R=0.79 and R=0.82, respectively).
OA simulated with our aqueous-phase isoprene SOA mechanism reproduces the
observed slope (2.8 ± 0.3 µg sm-3 ppbv-1, vs.
3.0 ± 0.4 µg sm-3 ppbv-1 in the observations).
Similarly strong correlations and consistency between model and observations
are found with column HCHO measured from OMI (Fig. 3, right). The estimated
error on individual OMI HCHO observations is about 30 % (Millet et al.,
2006).
Time series of the concentrations of isoprene SOA components at the
SOAS site in Centreville, Alabama (32.94∘ N; 87.18∘ W), in
June–July 2013: measured (black) and modeled (red) IEPOX SOA (top) and
C5-LVOC SOA (bottom) mass concentrations. Means and 1σ standard
deviations are given for the observations and the model.
Also shown in Fig. 3 is a sensitivity simulation with the default GEOS-Chem
mechanism based on reversible partitioning with pre-existing organic aerosol
(Pye et al., 2010) and producing a 1.1 % mean isoprene SOA yield, as
compared to 3.3 % in our simulation with the aqueous-phase mechanism.
That sensitivity simulation shows the same OA-HCHO correlation (R=0.82)
but underestimates the slope
(2.0 ± 0.3 µg sm-3 ppbv-1). The factor of 3
increase in our isoprene SOA yield does not induce a proportional increase in
the slope, as isoprene contributes only ∼ 40 % of OA in the
southeast US. But the slope is sensitive to the isoprene SOA yield, and the
good agreement between our simulation and observations supports our estimate
of a mean 3.3 % yield for the southeast US.
Figure 3 shows an offset between the model and observations illustrated by
the regression lines. We overestimate HCHO by 0.4 ppbv on average because we
did not apply the 15 % downward correction to MEGAN v2.1 isoprene
emissions (Zhu et al., 2016). We also underestimate total OA measured by the
AMS in the boundary layer by 1.1 µg sm-3 (mean AMS OA is
5.8 ± 4.3 µg sm-3; model OA is
4.7 ± 4.4 µg sm-3). The bias can be explained by our
omission of anthropogenic and open fire SOA, found by Kim et al. (2015) to
account on average for 18 % of OA in SEAC4RS.
Figure 4 shows time series of the isoprene SOA components IEPOX SOA and
C5-LVOC SOA at Centreville, Alabama, during SOAS. AMS observations from
Hu et al. (2015) and Krechmer et al. (2015) are compared to model values.
IEPOX SOA and C5-LVOC SOA are on average 17 and 2 % of total AMS OA,
respectively (Hu et al., 2015; Krechmer et al., 2015). The model reproduces
mean IEPOX SOA and C5-LVOC SOA without bias, supporting the conclusion
that IEPOX is the dominant contributor to isoprene SOA in the southeast US (Fig. 2).
Figure 5 shows the relationships of daily mean IEPOX SOA and sulfate
concentrations at Centreville and in the SEAC4RS boundary layer. The
same factor analysis method was used to derive IEPOX SOA in SEAC4RS as
in SOAS; however, the uncertainty is larger for the aircraft observations due
to the much wider range of conditions encountered. There is a strong
correlation between IEPOX SOA and sulfate, both in observations and the
model, with similar slopes. Correlation between IEPOX SOA and sulfate has
similarly been observed at numerous southeast US monitoring sites
(Budisulistiorini et al., 2013, 2015; Xu et al., 2015; Hu et al., 2015). Xu
et al. (2015) concluded that IEPOX SOA may form by nucleophilic addition of
sulfate (sulfate channels in Eq. 2) leading to organosulfates. However, we
find in our model that the H+-catalyzed channel
(kH+[H+] term in Eq. 2) contributes 90 % of IEPOX SOA
formation throughout the southeast US boundary layer, and that sulfate
channels play only a minor role. The correlation of IEPOX SOA and sulfate in
the model is because increasing sulfate drives an increase in aqueous aerosol
volume and acidity. Although dominance of the H+-catalyzed channel is
sensitive to uncertainties in the rate constants (Sect. 2), measurements from
the PALMS (Particle Analysis by Laser Mass Spectrometry) instrument during SEAC4RS (Liao et al., 2015)
show a mean IEPOX organosulfate concentration of
0.13 µg sm-3, amounting to at most 9 % of total IEPOX
SOA. The organosulfate should be a marker of the sulfate channels because its
hydrolysis is negligibly slow (Hu et al., 2011).
Relationship of IEPOX SOA and sulfate concentrations over the
southeast US in summer. Observed (black) and simulated (red) data are
averages for each campaign day during SOAS (left), and boundary-layer
averages (< 2 km) for 2∘× 2.5∘
GEOS-Chem grid squares on individual flight days during SEAC4RS
(right). RMA regression slopes and Pearson's correlation coefficients are
shown. 1σ standard deviations on the regression slopes are obtained
with jackknife resampling.
Correlation between IEPOX SOA and sulfate is also apparent in the spatial
distribution of IEPOX SOA, as observed by the SEAC4RS aircraft below
2 km and simulated by GEOS-Chem along the aircraft flight tracks (Fig. 6).
The correlation between simulated and observed IEPOX SOA in Fig. 6 is
R= 0.70. Average (mean) IEPOX SOA is
1.4 ± 1.4 µg sm-3 in the observations and
1.3 ± 1.2 µg sm-3 in the model. The correlation between
IEPOX SOA and sulfate is 0.66 in the observations and 0.77 in the model.
IEPOX SOA concentrations are highest in the industrial Midwest and Kentucky,
and in Louisiana–Mississippi, coincident with the highest sulfate
concentrations sampled on the flights. We also see in Fig. 6 frequent
observations of very low IEPOX SOA (less than 0.4 µg sm-3)
that are well captured by the model. These are associated with very low
sulfate (less than 1 µg sm-3).
Spatial distributions of IEPOX SOA and sulfate concentrations in
the boundary layer (< 2 km) over the southeast US during
SEAC4RS (August–September 2013). Aircraft AMS observations of IEPOX SOA
(top left) and sulfate (bottom left) are compared to model values sampled at
the time and location of the aircraft observations (individual points) and
averaged during the SEAC4RS period (background contours). Data are on a
logarithmic scale.
The mean IEPOX SOA concentration simulated by the model for the SEAC4RS
period (background contours in Fig. 6) is far more uniform than IEPOX SOA
simulated along the flight tracks. This shows the importance of day-to-day
variations in sulfate in driving IEPOX SOA variability. IEPOX SOA contributed
on average 24 % of total OA in the SEAC4RS observations, and
28 % in GEOS-Chem sampled along the flight tracks and as a regional mean.
With IEPOX SOA accounting for 58 % of isoprene SOA in the model (Fig. 2),
this amounts to a 41–48 % contribution of isoprene to total OA,
consistent with the previous estimate of 40 % by Kim et al. (2015).
Effect of anthropogenic emission reductions
The Environmental Protection Agency (EPA) projects that US anthropogenic emissions of NOx and SO2
will decrease respectively by 34 and 48 % from 2013 to 2025 (EPA, 2014).
We conducted a GEOS-Chem sensitivity simulation to examine the effect of
these changes on isoprene SOA, assuming no other changes and further assuming
that the emission decreases are uniform across the US.
Figure 7 shows the individual and combined effects of NOx and SO2
emission reductions on the branching pathways for isoprene oxidation, sulfate
mass concentration, aerosol pH, and isoprene SOA in the southeast US boundary
layer in summer. Reducing NOx emission by 34 % decreases the mean NO
concentration by only 23 %, in part because decreasing OH increases the
NOx lifetime and in part because decreasing ozone increases the
NO / NO2 ratio. There is no change in HO2. We find a 10 %
decrease in the high-NOx pathway and a 6 % increase in the
low-NOx pathway involving ISOPO2+ HO2. Aerosol sulfate
decreases by 2 % and there is no change in [H+]. The net effect is a
7 % increase in isoprene SOA, as the major individual components IEPOX
SOA and glyoxal SOA increase by 17 % and decrease by 8 %,
respectively.
Effect of projected 2013–2025 reductions in US anthropogenic
emissions on the formation of isoprene secondary organic aerosol (SOA).
Emissions of NOx and SO2 are projected to decrease by 34 and
48 %, respectively. Panels show the resulting percentage changes in the
branching of ISOPO2 between the NO and HO2 oxidation channels,
sulfate mass concentration, aerosol [H+] concentration, and isoprene
SOA mass concentration. Values are summer means for the southeast US
boundary layer.
A 48 % decrease in SO2 emissions drives a 36 % reduction in
sulfate mass concentration, leading to a decline in aerosol volume (31 %)
that reduces uptake of all isoprene SOA precursors. The decrease in aerosol
[H+] (26 %) further reduces IEPOX uptake. Decline in aerosol volume
and [H+] have a comparable effect on IEPOX SOA, as the change in each
due to SO2 emission reductions is similar (∼ 30 %) and uptake
of IEPOX SOA is proportional to the product of the two (Sect. 4). IEPOX SOA
and glyoxal SOA decrease by 45 and 26 %, respectively, and total isoprene
SOA decreases by 35 %. Pye et al. (2013) included uptake of IEPOX to
aqueous aerosols in a regional chemical transport model and similarly found
that SO2 emissions are more effective than NOx emissions at
reducing IEPOX SOA in the southeast US. Remarkably, we find that reducing
SO2 emissions decreases sulfate and isoprene SOA with similar
effectiveness (Fig. 7). With sulfate contributing ∼ 30 % of
present-day PM2.5 in the southeast US and isoprene SOA contributing
∼ 25 % (Kim et al., 2015), this represents a factor of 2 co-benefit
on PM2.5 from reducing SO2 emissions.
Conclusions
Standard mechanisms for the formation of isoprene secondary organic aerosol
(SOA) in chemical transport models assume reversible partitioning of
isoprene oxidation products to pre-existing dry OA. This may be appropriate
for dry conditions in experimental chambers but not for typical atmospheric
conditions where the aerosol is mostly aqueous. Here we developed an
aqueous-phase reactive uptake mechanism coupled to a detailed gas-phase
isoprene chemistry mechanism to describe the reactive uptake of
water-soluble isoprene oxidation products to aqueous aerosol. We applied
this mechanism in the GEOS-Chem chemical transport model to simulate surface
(SOAS) and aircraft (SEAC4RS) observations over the southeast US in
summer 2013.
Our mechanism includes different channels for isoprene SOA formation by the
high-NOx pathway, when the isoprene peroxy radicals (ISOPO2) react
with NO, and in the low-NOx pathway where they react mostly with
HO2. The main SOA precursors are found to be isoprene epoxide (IEPOX)
in the low-NOx pathway and glyoxal in the high- and low-NOx
pathways. Both of these precursors have dominant gas-phase photochemical
sinks, and so their uptake by aqueous aerosol is nearly proportional to the
reactive uptake coefficient γ and to the aqueous aerosol mass
concentration. The γ for IEPOX is mostly determined by the rate of
H+-catalyzed ring opening in the aqueous phase.
Application of our mechanism to the southeast US indicates a mean isoprene
SOA yield of 3.3 % on a mass basis. By contrast, a conventional mechanism
based on reversible uptake of semivolatile isoprene oxidation products yields
only 1.1 %. Simulation of the observed relationship of OA with
formaldehyde (HCHO) provides support for our higher yield. We find that the
low-NOx pathway is 5 times more efficient than the high-NOx pathway
for isoprene SOA production. Under southeast US conditions, IEPOX and glyoxal
account respectively for 58 and 28 % of isoprene SOA.
Our model simulates the observations and variability of IEPOX SOA at
the surface and from aircraft well. The observations show a strong correlation
with sulfate that we reproduce in the model. We find this is due to the
effect of sulfate on aerosol pH and volume concentration, increasing IEPOX
uptake by the H+-catalyzed ring-opening mechanism. Low concentrations
of sulfate are associated with very low IEPOX SOA, both in the observations
and the model, and we attribute this to the compounding effects of low
sulfate on aerosol [H+] and on aerosol volume.
The US EPA has projected that US NOx and SO2 emissions will
decrease by 34 and 48 % respectively from 2013 to 2025. We find in our
model that the NOx reduction will increase isoprene SOA by 7 %,
reflecting greater importance of the low-NOx pathway. The SO2
reduction will decrease isoprene SOA by 35 %, due to decreases in both
aerosol [H+] and volume concentration. The combined effect of these two
changes is to decrease isoprene SOA by 32 %, corresponding to a decrease
in the isoprene SOA mass yield from 3.3 to 2.3 %. Decreasing SO2
emissions by 48 % has similar relative effects on sulfate (36 %) and
isoprene SOA (35 %). Considering that sulfate presently accounts for
about 30 % of PM2.5 in the southeast US in summer, while isoprene
SOA contributes 25 %, we conclude that decreasing isoprene SOA represents
a factor of 2 co-benefit when reducing SO2 emissions.
Acknowledgements
We are grateful to the entire NASA SEAC4RS team for their help in the
field, in particular Paul Wennberg, John Crounse, Jason St. Clair, and
Alex Teng for their CIT-CIMS measurements. Thanks also to Jesse Kroll for
assisting in the interpretation of chamber study results. This work was
funded by the NASA Tropospheric Chemistry Program, the NASA Air Quality
Applied Sciences Team, and a South African National Research Foundation
Fellowship and Schlumberger Faculty for the Future Fellowship to E. A. Marais.
W. Hu, J. Krechmer, P. Campuzano-Jost, D. A. Day, and
J. L. Jimenez were supported by NASA NNX12AC03G/NNX15AT96G and NSF
AGS-1243354. J. Krechmer was supported by EPA STAR (FP-91770901-0) and CIRES
Fellowships. J. A. Fisher acknowledges support from a University of
Wollongong Vice Chancellor's Postdoctoral Fellowship. HCHO observations were
acquired with support from NASA ROSES SEAC4RS grant NNH10ZDA001N.
Although this document has been reviewed by US EPA and approved for
publication, it does not necessarily reflect US EPA's policies or
views. Edited by: F. Yu
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