ACPAtmospheric Chemistry and PhysicsACPAtmos. Chem. Phys.1680-7324Copernicus PublicationsGöttingen, Germany10.5194/acp-18-10433-2018Observations of sesquiterpenes and their oxidation products in central
Amazonia during the wet and dry seasonsObservations of sesquiterpenes and their oxidation productsYeeLindsay D.lindsay.yee@berkeley.eduhttps://orcid.org/0000-0001-8965-9319Isaacman-VanWertzGabrielWernisRebecca A.MengMengRiveraVenturaKreisbergNathan M.https://orcid.org/0000-0001-5440-1342HeringSusanne V.https://orcid.org/0000-0001-6536-310XBeringMads S.GlasiusMariannehttps://orcid.org/0000-0002-4404-6989UpshurMary AliceGray BéArianaThomsonRegan J.https://orcid.org/0000-0001-5546-4038GeigerFranz M.OffenbergJohn H.https://orcid.org/0000-0002-0213-4024LewandowskiMichaelKourtchevIvanKalbererMarkushttps://orcid.org/0000-0001-8885-6556de SáSuzaneMartinScot T.AlexanderM. LizabethPalmBrett B.https://orcid.org/0000-0001-5548-0812HuWeiweihttps://orcid.org/0000-0002-3485-6304Campuzano-JostPedrohttps://orcid.org/0000-0003-3930-010XDayDouglas A.https://orcid.org/0000-0003-3213-4233JimenezJose L.https://orcid.org/0000-0001-6203-1847LiuYingjunhttps://orcid.org/0000-0001-6659-3660McKinneyKarena A.https://orcid.org/0000-0003-1129-1678ArtaxoPaulohttps://orcid.org/0000-0001-7754-3036ViegasJuarezManziAntonioOliveiraMaria B.de SouzaRodrigoMachadoLuiz A. T.https://orcid.org/0000-0002-8243-1706LongoKarlaGoldsteinAllen H.https://orcid.org/0000-0003-4014-4896Department of Environmental Science, Policy, and Management,
University of California, Berkeley, Berkeley, California 94720, USADepartment of Civil and Environmental Engineering, University of
California, Berkeley, Berkeley, California 94720, USADepartment of Chemical Engineering, University of California, Berkeley,
Berkeley, California 94720, USAAerosol Dynamics Inc., Berkeley, California 94710, USADepartment of Chemistry, Aarhus University, 8000 Aarhus C, DenmarkDepartment of Chemistry, Northwestern University, Evanston, Illinois 60208, USANational Exposure Research Laboratory, Exposure Methods and
Measurements Division, United States Environmental Protection Agency,
Research Triangle Park, North Carolina 27711, USADepartment of Chemistry, University of Cambridge, Cambridge, CB2 1EW,
UKSchool of Engineering and Applied Sciences, Harvard University,
Cambridge, Massachusetts 02138, USADepartment of Earth and Planetary Sciences, Harvard University,
Cambridge, Massachusetts 02138, USAEnvironmental Molecular Sciences Laboratory, Pacific Northwest
National Laboratory, Richland, Washington 99352, USADept. of Chemistry and Cooperative Institute for Research in
Environmental Sciences (CIRES), University of Colorado, Boulder, Colorado 80309, USADepartment of Applied Physics, University of São Paulo, SP,
BrazilInstituto Nacional de Pesquisas da Amazonia, Manaus, AM, BrazilUniversidade do Estado do Amazonas, Manaus, AM, BrazilInstituto Nacional de Pesquisas Espiacais, São José dos
Campos, SP, BrazilInstituto Nacional de Pesquisas Espiacais, Cachoeira Paulista, SP,
Brazilnow at: Department of Civil and Environmental Engineering, Virginia Tech,
Blacksburg, Virginia 24061, USAnow at: Department of Chemical Engineering and Applied
Chemistry, University of Toronto, Toronto, CA, USAnow at: Department of Environmental Science, Policy, and Management,
University of California, Berkeley, Berkeley, California 94720, USAnow at: Department of Chemistry, Colby College, Waterville, Maine 04901,
USALindsay D. Yee (lindsay.yee@berkeley.edu)23July20181814104331045720February201826February201827June201829June2018This work is licensed under the Creative Commons Attribution 4.0 International License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/4.0/This article is available from https://acp.copernicus.org/articles/18/10433/2018/acp-18-10433-2018.htmlThe full text article is available as a PDF file from https://acp.copernicus.org/articles/18/10433/2018/acp-18-10433-2018.pdf
Biogenic volatile organic compounds (BVOCs) from the Amazon forest region
represent the largest source of organic carbon emissions to the atmosphere
globally. These BVOC emissions dominantly consist of volatile and
intermediate-volatility terpenoid compounds that undergo chemical
transformations in the atmosphere to form oxygenated condensable gases and
secondary organic aerosol (SOA). We collected quartz filter samples with
12 h time resolution and performed hourly in situ measurements with a
semi-volatile thermal desorption aerosol gas chromatograph (SV-TAG) at a
rural site (“T3”) located to the west of the urban center of Manaus, Brazil
as part of the Green Ocean Amazon (GoAmazon2014/5) field campaign to measure
intermediate-volatility and semi-volatile BVOCs and their oxidation products
during the wet and dry seasons. We speciated and quantified 30 sesquiterpenes
and 4 diterpenes with mean concentrations in the range 0.01–6.04 ng m-3
(1–670 ppqv). We estimate that sesquiterpenes contribute
approximately 14 and 12 % to the total reactive loss of O3 via
reaction with isoprene or terpenes during the wet and dry seasons,
respectively. This is reduced from ∼ 50–70 % for within-canopy
reactive O3 loss attributed to the ozonolysis of highly reactive
sesquiterpenes (e.g., β-caryophyllene) that are reacted away before
reaching our measurement site. We further identify a suite of their oxidation
products in the gas and particle phases and explore their role in biogenic
SOA formation in the central Amazon region. Synthesized authentic standards
were also used to quantify gas- and particle-phase oxidation products derived
from β-caryophyllene. Using tracer-based scaling methods for these
products, we roughly estimate that sesquiterpene oxidation contributes at
least 0.4–5 % (median 1 %) of total submicron OA mass. However, this
is likely a low-end estimate, as evidence for additional unaccounted
sesquiterpenes and their oxidation products clearly exists. By comparing our
field data to laboratory-based sesquiterpene oxidation experiments we confirm
that more than 40 additional observed compounds produced through sesquiterpene
oxidation are present in Amazonian SOA, warranting further efforts towards
more complete quantification.
Introduction
The emission of biogenic volatile organic compounds (BVOCs) from terrestrial
vegetation represents a large source of organic carbon to Earth's atmosphere.
These emissions comprise a wide array of chemical species, including known
terpenoids: isoprene (C5), monoterpenes (C10),
sesquiterpenes (C15), and diterpenes (C20). Isoprene,
2-methyl-1,3-butadiene (C5H8), is a hemi-terpene and the most
dominant non-methane hydrocarbon emitted to the atmosphere at levels of
∼ 500 TgC yr-1
globally (Guenther et al., 2006, 2012). Global emissions of terpenes are
estimated for monoterpenes (C10H16) at
∼ 160 TgC yr-1, sesquiterpenes (C15H24) at
∼ 30 Tg yr-1, and global emission estimates for diterpenes
(C20H42) have yet to be made (Guenther et al., 2012).
In the atmosphere, such compounds undergo chemical transformations that lead
to the formation of biogenic secondary organic aerosol (BSOA) and affect
local radical budgets (Kesselmeier et al., 2013; Lelieveld et al., 2008,
2016) and carbon cycling (Bouvier-Brown et al., 2012; Guenther, 2002).
Globally, the majority of organic aerosols stem from the oxidation of biogenic
carbon, yet their role in affecting Earth's radiative balance remains unclear
(Committee on the Future of Atmospheric Chemistry Research et al., 2016). This is largely due to limited observations of the speciated
precursors and identification of their oxidation products, which are crucial
for understanding their chemical pathways and fate in Earth's atmosphere
(Worton et al., 2012).
While laboratory measurements simulating the oxidation of BVOCs provide insight
into atmospheric chemistry, challenges still exist for making ambient, high-time-resolution,
speciated measurements of the higher-carbon-number terpenes
and their oxidation. These compounds present measurement challenges for
several reasons. First, their relatively low vapor pressure makes sample
collection more challenging due to line losses. Second, they tend to be
present at lower ambient concentrations due to lower emission and higher
reactivity, therefore requiring very sensitive detection methods and/or lower
time resolution. These measurement challenges have resulted in more research
focused on highly volatile and higher-concentration BVOCs (i.e., isoprene,
monoterpenes), but there is less understanding of sesquiterpenes
(Bouvier-Brown et al., 2009a, b; Chan et al., 2016) and almost no data on
concentrations or chemical reaction pathways of diterpenes.
Isoprene has a laboratory-determined SOA yield of < 6 % (Dommen et
al., 2006; King et al., 2010; Kroll et al., 2005, 2006; Xu et al., 2014) and
an estimated 3.3 % yield in the southeastern US (Marais et al., 2016), but
nevertheless contributes a major fraction of organic aerosol over forested
regions because it is emitted in such high quantities relative to other BVOCs
(Carlton et al., 2009; Hu et al., 2015). Though other terpenes are present in
the atmosphere at much lower concentrations, they generally react faster with
oxidants such as OH, O3, and NO3 (Atkinson, 1997) and have
higher SOA yields at typical atmospheric OA concentrations:
∼ 5–10 % for monoterpenes as reported in Griffin et al. (1999a, b)
and ∼ 20–70 % for sesquiterpenes as reported in Chen et
al. (2012), Griffin et al. (1999b), Hoffmann et al. (1997), Jaoui et
al. (2013), and Lee et al. (2006a, b). SOA yields from diterpenes have not yet
been quantified, though they are likely higher than those of sesquiterpenes
due to their higher carbon number and lower volatility. While emitted mass
generally decreases with decreasing volatility (and increasing carbon
number), the concomitant rise in the sheer possible number of compounds from
C5 to C10 to C15 to C20 backbones and
the associated SOA yields and oxidant reactivity indicates that lower-volatility
terpenes may have an important impact on regional chemistry and BSOA
formation.
Using the BVOC emission model, MEGAN2.1, it is predicted that
∼ 80 % of terpenoid emissions come from tropical trees that cover
about 20 % of the global land surface (Guenther et al., 2012), yet very
few observations of sesquiterpenes exist from these regions. Chemical
characterization of tropical plant tissue shows the presence of an abundance
of sesquiterpenes (F. Chen et al., 2009; Courtois et al., 2012) and suggests
their widespread emission from such vegetation (F. Chen et al., 2009;
Courtois et al., 2009). Previous branch enclosure measurements of native
Amazon saplings found that sesquiterpene emission was below detection limits
(Bracho-Nuñez et al., 2013) even though sesquiterpenes have been observed
within and outside the canopy of the central Amazon (Alves et al., 2016;
Jardine et al., 2012). Further, F. Chen et al. (2009) observed higher
sesquiterpene emissions from wounded seedlings of the tropical tree
Copaifera officinalis, with similar composition and quantity to that
in the leaves. However, while ambient concentrations of sesquiterpenes have
been measured by field-deployable mass spectrometers such as proton transfer
time-of-flight mass spectrometry (Jordan et al., 2009), previous measurements
have provided little or no separation of isomers, which can vary
substantially (orders of magnitude) in their reactivity and SOA yields, so a
significant knowledge gap remains regarding the contribution of these
compounds to organic aerosol.
Aerosols play a critical role in cloud formation and the hydrologic cycle of
the Brazilian Amazon (Fan et al., 2018; Pöschl et al., 2010; Wang et al.,
2016), which is also one of the major source regions of global BVOCs.
Previous studies have found the aerosols over this region (Martin et al.,
2010) to be primarily composed of organic material (Artaxo et al., 2013)
derived from BVOCs (Q. Chen et al., 2009, 2015). While isoprene oxidation has
been estimated to contribute ∼ 50 % of OA in Amazonia (Q. Chen et al.,
2015), of which a large portion is attributed to the uptake of
isoprene-epoxydiols (Hu et al., 2015; de Sá et al., 2017), the remaining
contribution to OA from other BVOC precursors such as monoterpenes and
sesquiterpenes remains largely unconstrained. Khan et al. (2017) report that
including updated sesquiterpene emissions and SOA pathways (all represented
by the β-caryophyllene mechanism) in the STOCHEM global chemical transport
model led to a 48 % increase in global SOA burden and 57 % increase
in SOA production rate. In addition, the highest concentrations of
sesquiterpene-derived SOA were modeled to be present over central Africa and
South America. However, the general lack of available time-resolved
measurements of speciated sesquiterpenes and their oxidation products in
either the gas or particle phase has precluded fully constraining their
contribution to BSOA formation. For terpenes in general there are even
greater measurement challenges associated with the observation of tracers of
their oxidation: the gas-phase component of these semi-volatile compounds may
condense in sample lines, be lost by wet or dry deposition, or quickly react
to form compounds sufficiently ubiquitous to be disconnected from a specific
precursor. Particle-phase composition is therefore critical for studying the
importance of individual terpenes and terpene classes, but the dynamic
gas–particle partitioning of these semi-volatile products requires the
contemporaneous measurement of the difficult-to-measure gas-phase components.
Measurements of gas–particle partitioning and concentrations of biogenic OA
tracers from isoprene and monoterpene oxidation have been reported previously
as part of the Green Ocean Amazon (GoAmazon2014/5) field campaign
(Isaacman-VanWertz et al., 2016), but few, if any, sesquiterpene oxidation
products were identified in the dataset at that time. To fully characterize
the sources of OA in the region, molecular-level and chemically specific
signatures of oxidation products from a more complete range of BVOC
precursors need to be identified and quantified.
Here we report the first speciated measurement of 30 sesquiterpenes and four
diterpenes in the central Amazon and assess their role in reactive oxidant
losses during the wet and dry seasons through in situ observations with a
semi-volatile thermal desorption aerosol gas chromatograph (SV-TAG) and
off-line measurements of quartz-filter-collected aerosol samples. We further
report measurements of the specific oxidation products of the sesquiterpene
β-caryophyllene that were synthesized in the laboratory and attribute
more than 40 additional species observed on a representative filter sample
from the wet season to sesquiterpene oxidation based on comparison to
products found in laboratory-based oxidation experiments. Finally, we
provide a rough low-end estimate of the contribution of sesquiterpene
oxidation to OA in the region.
Experimental sectionGreen Ocean Amazon (GoAmazon2014/5) field campaign: “T3” site
description
Measurements were conducted as part of the Green Ocean Amazon
(GoAmazon2014/5) field campaign, for which the scientific objectives and
measurement sites are described in an overview paper by Martin et al. (2016).
We present observations from the wet and dry seasons of 2014, respectively
referred to as intensive operating periods 1 and 2 (IOP1 and IOP2). Wet
season measurements were taken 1 February to 31 March 2014 and dry season
measurements were taken 15 August to 15 October 2014 at the “T3” Manacapuru
rural site, located 70 km downwind of urban Manaus. The T3 site was located
on a cleared pasture site (-3.2133, -60.5987∘) 2 km
north of the nearest heavily traveled road, which connects Manaus to
Manacapuru. The immediate surrounding forest (∼ 1 km away) consisted
primarily of secondary forest, and the prevailing meteorological conditions
(northeasterly winds) resulted in the regional transport of clean air from the
primary forest north and east of the site mixed with the outflow of Manaus
pollution from the east.
Several instruments for investigating gas- and particle-phase chemistry were
housed at T3 alongside the instrument suite of the United States Department
of Energy Atmospheric Radiation Measurement (ARM) Climate Research Facility.
ARM trailers consisted of the Atmospheric Radiation Measurement Facility One
(AMF-1) and Mobile Aerosol Observing System (MAOS; Mather and Voyles, 2013).
Here we focus on measurements conducted using a semi-volatile thermal
desorption aerosol gas chromatograph (SV-TAG) and quartz filters collected
using a custom-designed sequential filter sampler.
Deployment of a semi-volatile thermal desorption aerosol gas
chromatograph (SV-TAG)
We deployed a semi-volatile thermal desorption aerosol gas chromatograph
(SV-TAG) housed in one of the instrument trailers at T3. Details of the
instrument development and operation have been published previously
(Isaacman-VanWertz et al., 2016; Isaacman et al., 2014; Kreisberg et al.,
2009; Williams et al., 2006; Zhao et al., 2013) and we describe SV-TAG
deployment briefly here. During the campaign, ambient air was pulled through
a 15.24 cm ID duct at ∼ 5 m above ground level. Flow through the
ducting was maintained at laminar flow conditions with minimal residence time
to minimize loss of semi-volatile species from the center line of flow. Air
was then subsampled from the center of the sampling duct at
20 L min-1 for 22 min through a
cyclone (PM1 cut point) to SV-TAG's dual collection cells held at
32 ∘C. Concentrations and gas-particle partitioning were measured
through a combination of particle-only measurements, in which gas-phase
components were removed through a series of two multichannel carbon monolith
denuders (500 channels, 30 mm OD × 40.6 cm; MAST Carbon) upstream
of one collection cell, and total gas-plus-particle measurements with no
removal of the gas-phase component. One collection cell always sampled total
gas-plus-particle concentrations, while the other cell alternated between
particle-only samples and total gas-plus-particle samples, which were used to
remove any bias between collection cells. Particle fraction, Fp,
is calculated by comparing the particle-only signal from the denuded channel to
the gas and particle signal from the other cell. Measurements of
Fp for many tracers of biogenic origin from this campaign were
presented previously in Isaacman-VanWertz et al. (2016) and an
intercomparison of SV-TAG and other instrumentation for measuring
Fp is described in Thompson et al. (2017). We found here that
sesquiterpenes and their oxidation products detected by SV-TAG were
completely in the gas phase (Fp was zero, as discussed in
Sect. 3.1 and 3.2.3), so we do not
present time-dependent Fp measurements for these compounds. As
described in Isaacmann-VanWertz et al. (2016), regular checks of denuder
efficiency were done by inserting a filter upstream of the denuder to remove
particles and sampling the normal volume of air through this “blank”
system so the measured signal would indicate any breakthrough. Any remaining
mass signal was subtracted from the sample mass signal as part of data
correction before quantification. Previous laboratory testing of the denuder
efficiency was also performed by sending gas standards (e.g., the
sesquiterpene longifolene) through the denuder and measuring the
sesquiterpene signal upstream and downstream using
proton-transfer-reaction mass spectrometry. This led to a calculated
penetration value on average of < 5 % for a single denuder and a
predicted penetration of < 0.5 % for the two denuders used in series
on SV-TAG.
After sample collection, material was thermally desorbed from the collection
cells into helium (He) at a rate of 35 ∘C min-1 up to
320 ∘C, taking approximately 8 min. The helium was saturated with a
derivatization agent, N-methyl-N-(trimethylsilyl) trifluoroacetamide
(MSTFA), which converts polar -OH moieties to
-OSi(CH3)3 (trimethylsilyl ester) groups for separation on a gas
chromatography column (Isaacman et al., 2014). Desorbed material was focused
onto a pre-concentrator held at 30 ∘C and then transferred onto a
gas chromatography column (Restek, Rtx-5Sil MS;
20 m × 0.18 mm × 0.18 µm) via a valveless
injector (Kreisberg et al., 2014). Analysis was performed using an Agilent
7890A/5975 gas chromatograph (GC) coupled to a quadrupole mass spectrometer
(MS). The GC program consisted of He flow at 1 sccm and a ramp from
50 to 330 ∘C (ramp rate 23.6 ∘C min-1),
then holding at 330 ∘C for 2.2 min while He flow ramped to 3 sccm.
The use of online derivatization greatly extends the use of SV-TAG for
analysis of the highly oxygenated species typical of BVOC oxidation, but it also
renders more complex chromatograms. For many of the observed sesquiterpenes,
their individual contributions to the total ion signal within a chromatogram
were low. Thus, analysis was occasionally performed (∼ every 13 h)
without derivatization to allow for clearer identification and quantification
of all detected sesquiterpene species. To generate continuous time series of
the total sesquiterpene concentration as presented in Sect. 3.2.1, we assumed that the longer-lived
and regularly detected α-copaene comprised 6 % of total
sesquiterpene concentration at all times, since this was the average %
composition during runs without derivatization (Fig. S1 in the Supplement).
Selected ion chromatograms at sesquiterpene characteristic ion
m/z 161 of ambient air (black) and copaiba essential oil (red). Sesquiterpenes
in ambient air were measured in the gas phase. Copaiba essential oil
was analyzed from direct liquid injection on SV-TAG collection cells. Retention
times for standards analyzed in the field are indicated in blue.
Compound identification
A typical sample total ion chromatogram (TIC) contains hundreds to thousands
of compounds. For peak deconvolution, compound peaks are separated and
quantified using a characteristic (“selected”) ion fragment. Chemical
identification is aided by comparing the peak's background-subtracted mass
spectrum and n-alkane-based retention index to those of authentic standards
run on SV-TAG or presented in the literature and available mass spectral
libraries. The 2014 NIST/EPA/NIH Mass Spectral Library (Stein et al., 2014),
the Adams essential oil library (Adams, 2007), and a proprietary library from
a flavor and fragrance company (MANE) were used for mass spectral matching.
A match factor is calculated from a comparison function outlined in Stein (1994)
as a measure of the overall probability that an obtained spectral
match is correct. Spectral matches are considered perfect if the match factor
(Stein, 1994) is 999, excellent if > 900, good if 800–900, and fair if
700–800. Further, additional work using electron impact (EI) mass spectral
matching to identify components of a complex OA sample found that the
probability of incorrect identification was low (30 and 14 %) for match
factors between 800–900 and > 900, respectively (Worton et al., 2017).
Here we present proposed identities if the spectral match factor is > 800
and the retention index is reasonable with the proposed identity. Retention
index (RI) is helpful for determining the elution order of compounds and
narrowing the possible compound identities for species such as the
sesquiterpenes with similar mass spectra. We calculate the n-alkane-based
retention index for compound i using Eq. (1) below:
RI=100×n+ti-tntn+1-ti,
where n is the number of carbon atoms of the n-alkane that elutes before
species i, and t represents retention time.
Sesquiterpenes with the chemical formula C15H24 were mostly resolved
within single ion chromatograms (Fig. 1) by a characteristic ion, m/z 161,
C12H17+, typically coincident with the molecular ion,
m/z 204. In addition, a few sesquiterpenoids with the chemical formula
C15H22 were resolved at their molecular ion (m/z 202), and
a few diterpenes with the chemical formula C20H32 were resolved
using the characteristic ion, m/z 257. In some cases, peak signal was too low
to provide good mass spectra (MS) matching, so retention index information
was also used to propose identities.
While many sesquiterpenes observed in ambient chromatograms are not available
as authentic standards, sesquiterpene-rich essential oils from Amazonian
trees were injected on SV-TAG for chromatographic separation and to aid in
identification. These included copaiba essential oil and andiroba essential
oil obtained from a local pharmacy in Manacapuru, Brazil and additional
bottles of copaiba essential oil obtained from Young Living Essential Oils, Lehi, UT
and through personal communication (origin Bolivia). Copaiba essential oil
originates from trees of the genus Copaifera, comprising over 70
species (Plowden, 2003), several of which are distributed throughout the
greater Amazon region (Xiloteca Calvino Mainieri et al., 2018) While copaiba trees are commonly
referred to as the “diesel” or “kerosene” tree due to the oil's limited
use as a biofuel, the essential oil is extracted primarily for medicinal
purposes. Andiroba essential oil is derived from Carapa guianensis,
also widely used for medicinal purposes.
While the exact composition, grade, and quality of the essential oils depends
on multiple factors (e.g., extraction method, species, origin), we
hypothesized that the essential oils may serve as a proxy for emissions of
terpenes within the canopy and that the ambient sesquiterpene composition
might reflect a similar composition to the essential oils. Studies have
previously observed the chemical similarity between that of terpene emissions
and within-plant content (Ormeño et al., 2010), specifically for
Copaifera officinalis (F. Chen et al., 2009). Most of the
sesquiterpenes observed in ambient samples (13 February 2014 06:47 UTC,
02:47 LT) coincide with those observed in copaiba essential oil (Fig. 1,
black and red traces, respectively). This similarity between the ambient
sesquiterpene composition and essential oil composition also allowed for
positive identification of some of the peaks in the ambient chromatogram.
Sesquiterpene standards that were commonly available and brought to the field
as standards included (+)-longifolene, β-caryophyllene, and
alloaromadendrene with their chromatographic retention times indicated in
Fig. 1. Further, the essential oils injected on SV-TAG have relatively
similar sesquiterpene composition (Table S3 in the Supplement) and are
comparable to previously analyzed essential oils and tissue from Amazonian
trees (Table S4).
Compound quantification
In-field calibrations on SV-TAG were performed using an auto liquid injection
system (Isaacman et al., 2011) to deliver customized standard solutions. A
calibration point was obtained every 6–7 h, rendering a complete six-point
calibration curve within 48 h. Within a selected ion chromatogram (SIC),
peak signal is integrated at the quantification ion and calibrated based on
the best available authentic standard. The peak-integrated ion signal of both the
analyte and the standard is normalized by the integrated ion signal of an
isotopically labeled internal standard in each sample to account for
differences in recovery by compound functionality in SV-TAG as well as
changing MS detector response over time. For the sesquiterpenes,
n-tetradecane d30 (CAS no. 204244-81-5) was selected as the internal
standard to normalize by. Reported oxidation products are normalized using
2-C13-pentaerythritol. Compounds were quantified using authentic
standards whenever possible, though analytical-grade standards for many of
the observed sesquiterpenes are not commercially available and/or were not
present in the custom standard solution that was used for in-field
calibrations. Longifolene, β-caryophyllene, and alloaromadendrene were
the only sesquiterpenes present in the calibration solution during
deployment, and only β-caryophyllene and alloaromadendrene were
occasionally detected in ambient air. Post-deployment calibrations were
performed in the laboratory with newly acquired sesquiterpene standards using
relative response factors to β-caryophyllene. A range of instrument
responses to sesquiterpene standards was observed. For example, on-column
lower detection limits were 0.14, 0.01, 0.05, and 0.08 ng with precisions of
14, 21, 9.5, and 13 % and accuracies of 12, 7.4, 25, and 17 %, for
β-caryophyllene, longifolene, alloaromadendrene, and α-copaene, respectively. An average response factor for several
sesquiterpenes was used to quantify compounds for which authentic standards
were not available (Chan et al., 2016). All sesquiterpenes are quantified on
m/z= 161; the average sensitivity of most sesquiterpenes to quantification
on this ion is 6.4 ± 6.0 times more sensitive than for β-caryophyllene. The calculated on-column lower detection limit is 0.07 ng with
typical precision of the order of 14 % and accuracy errors within
30 %. Additional details of error analysis for SV-TAG data are presented
in Isaacman et al. (2014).
Standards of several oxidation products from β-caryophyllene
ozonolysis were custom synthesized at Northwestern University. The oxidation
products synthesized were β-caryophyllene aldehyde, β-nocaryophyllone aldehyde, β-caryophyllonic acid, β-nocaryophyllonic acid, β-caryophyllinic acid, and β-nocaryophyllinic acid. Synthesis details and procedures are outlined in
Gray Bé et al. (2017). Each standard was analyzed in the laboratory by
SV-TAG and by two-dimensional gas chromatography with high-resolution
time-of-flight mass spectrometry (GC × GC-HR-ToF-MS) to obtain MS
and retention time information to aid the identification of these compounds
in sample chromatograms. Relative
response factors of the synthesized standards to pinonic acid, a compound in
the regular standard solution for SV-TAG during deployment, were obtained and
used for quantification.
Laboratory-generated SOA from sesquiterpenes
Filter samples from laboratory oxidation experiments of several
sesquiterpenes were analyzed by GC × GC-HR-ToF-MS to provide mass
spectral information for the identification of potential sesquiterpene-derived
oxidation products in ambient samples. Mass spectra of resolved peaks from
these and previously analyzed filters were added to custom MS libraries and
are listed in Table S1. Sesquiterpenes were oxidized in the U.S. EPA National
Exposure Research Laboratory reactors (Table S2) in the dark via ozonolysis;
some were also oxidized under conditions of OH oxidation in the presence of
NOx according to methods described previously (Jaoui et al.,
2003, 2004, 2013, 2016; Offenberg et al., 2017). The following sesquiterpene
systems were studied: β-caryophyllene, α-cedrene, α-copaene, aromadendrene, β-farnasene, and α-humulene. In
addition, a complex mixture rich in sesquiterpenes (copaiba essential oil,
Amazon origin) was also oxidized under ozonolysis conditions as a
representation of the potential mixture expected in the Amazon atmosphere.
Supporting measurements
Several supporting measurements were made that allow for interpretation of
the chemistry observed. These include gas-phase measurements of BVOCs from a
proton transfer time-of-flight mass spectrometer (PTR-ToF-MS; Ionicon
Analytik) and particle-phase measurements from an Aerodyne high-resolution
aerosol mass spectrometer (HR-ToF-AMS), hereinafter referred to as the AMS
(DeCarlo et al., 2006). Operation and analysis procedures are outlined
elsewhere for the PTR-ToF-MS (Liu et al., 2016) and for the AMS (de Sá et
al., 2017). Positive matrix factorization analysis of AMS data was performed
to resolve the statistical factor, isoprene epoxydiol-SOA (IEPOX-SOA), which
is considered to be a tracer for organic aerosol formed through the particle
uptake of isoprene epoxydiols and has been previously described (de Sá et
al., 2017). Filter-based measurements were also taken using a custom-built
sequential filter sampler. Selected filter samples were analyzed using
various chromatographic, ionization, and mass spectrometric techniques to
provide additional chemical insight. The sequential filter sampler and filter
analysis techniques are described briefly in the following sections. Routine
meteorology data and gas-phase measurements (e.g., O3) were provided
by the Atmospheric Radiation Measurement (ARM) Climate Research Facility, a
U.S. Department of Energy Office of Science user facility sponsored by the
Office of Biological and Environmental Research.
Sequential filter sampler
Aerosol samples were collected on quartz-fiber filters using a custom-built
sequential filter sampler (Aerosol Dynamics, Inc.). Ambient air was sampled
at 120 L min-1 through a 2.4 cm ID
stainless steel tube 4 m above ground level. Sampled air passed through
2.7 m of 2 cm ID copper tubing kept at temperatures below the dew point
of the trailer temperature for trapping excess water with periodic manual
removal. The sample inlet geometry and flow conditions minimized particle
losses (< 5 %) for those between 10 and 1000 nm. We estimate 70 %
removal of intermediate-volatility organic compounds (IVOCs), minimizing
adsorption of gas-phase organics onto the filters. Previous filter
measurements have noted sampling artifacts due to O3 penetration
(Dzepina et al., 2007). We estimate 90 % removal of O3 (estimated
diffusive losses) for the sampler design used in this study, which should
minimize the further reaction of organics collected on filters.
Following the water removal stage was a pair of greaseless cyclones operating
in parallel with aerodynamic diameter cut points of 1 µm. The
cyclones are equivalent to the AIHL cyclone (John and Reischl, 1980)
originally designed for PM2.5 collection at 21.7 L min-1 but
experimentally verified to provide PM1 separation at 60 L min-1.
The sample was then introduced into a 91 cm length of 32 mm ID aluminium
tubing to one of six filter housings (HiQ, ILPH-102) containing a 101.6 mm
diameter quartz-fiber filter (Whatman, QM-A Quartz). Filter housings were
modified from the manufacturer to remove all adhesives to prevent potential
off-gas and contamination of the filter samples. Further modifications to the
housings included replacing filter supports with custom-etched
316L stainless steel support screens and
utilizing an O-ring face seal not in contact with the sample flow or filter.
Before deployment filters were pre-treated by baking at 550 ∘C for
12 h. During IOP1 (wet season), samples were collected at approximately
12 h time resolution from 06:15–18:00 and 18:30–06:15 local time (LT). Filter changes occurred daily from 18:00–18:30 LT. Field blanks were
also collected weekly in each filter holder. Filter samples collected during
IOP2 (dry season) are not presented in this analysis as the wet season
filters were more ideal for targeted isolation and detection of sesquiterpene
oxidation products. Similarly targeted samples from the dry season had
a similar chemical composition in terms of terpene oxidation as that presented
in Sect. 3.3 for the wet season, so this presentation is not repeated, though there
are certainly contributions from additional OA sources (e.g., biomass burning
compounds are more prominent in the dry than wet season). A more complete
analysis of all samples from both seasons will be presented in a separate
forthcoming publication. The goal in the current analysis is to simply
demonstrate the number and chemical complexity of the observed
sesquiterpene-derived compounds and the potential for their significance in
contributing to overall OA mass. Filter samples were kept frozen (or
transported on ice) until analysis. Switching valves were automated to time
sample collection appropriately over each filter throughout the day. Flow
rates were logged in LabVIEW using a TSI mass flow meter (model 4045).
Two-dimensional gas chromatography with
high-resolution time-of-flight mass spectrometry (GC × GC-HR-ToF-MS)
Selected filter
samples and standards were analyzed in the laboratory using
two-dimensional gas chromatography with high-resolution time-of-flight mass
spectrometry. Aliquots of samples (multiples of ovoid filter punches with an
area of 0.4 cm2 each) were introduced into the gas chromatograph using
a thermal-desorption autosampler (Gerstel, TDS-3 and TDSA2) with built-in
derivatization using MSTFA. Compounds were separated first on a nonpolar
column (Restek, Rxi-5Sil-MS,
60 m × 0.25 mm × 250 µm), then transferred to a
Zoex Corporation cryogenic dual-stage thermal modulator comprised of guard
column (Restek, 1.5 m × 0.25 mm, Siltek). The modulation period
was 2.3 s, followed by separation on a secondary column (Restek, Rtx-200MS,
1 m × 0.25 mm × 250 µm) to separate polar
compounds. The GC temperature program ramped from 40 to
320 ∘C at a rate of 3.5 ∘C min-1, holding for 5 min,
and the He carrier gas flow was 2 mL min-1. Following chromatographic
separation, analysis was performed with a Tofwerk high-resolution (mΔm≈4000) mass spectrometer employing electron impact
(70 eV) ionization.
To correct for compound transmission efficiency through the system, total ion
signal for each peak was corrected based on a volatility curve comprising
even-carbon-numbered perdeuterated alkanes as internal standards (from
C12D26 through C36D74). The custom-synthesized
β-caryophyllene oxidation products (described in Sect. 2.2.2) and
several filter samples containing sesquiterpene-derived SOA from laboratory
oxidation experiments (described in Sect. 2.2.3) were analyzed to create
characteristic mass spectra of sesquiterpene-derived oxidation products.
These mass spectra were put into custom MS libraries and added to the
available NIST 14 MS libraries within NIST MS Search v.2.2 software to be
searched against when analyzing sample chromatograms. Custom MS libraries
comprising the mass spectra of products from previous laboratory oxidation
experiments of the monoterpenes α-pinene, myrcene, and d-limonene
were also included in MS searches. These custom MS libraries are available
online for the use of the atmospheric chemistry community as indicated in the
“Data availability” section and are the subject of a future publication.
Published MS in the literature was also used to identify previously reported
tracers of isoprene and terpene oxidation as described in Sect. 3.3.2. For
peaks with “good” MS match factors > 800 out of 1000 (Stein, 1994) and
an alkane-based retention index matching within ±10 of that of the
library entry, a tentative match was considered, at least for source
categorization of the peak. Some peaks with < 800 MS match factors were
included after manual review accounting for co-elutions and other factors
affecting MS quality and MS matching.
Ultrahigh-performance liquid chromatography mass spectrometry
(UHPLC-MS)
A subset of filters were extracted and analyzed using ultrahigh-performance
liquid chromatography coupled to quadrupole time-of-flight MS (UHPLC-qToF-MS)
for carboxylic acids and organosulfates. The method was based on Kristensen
and Glasius (2011) and Kristensen et al. (2016). Monoterpene oxidation
products were analyzed using these methods and further optimized for the
analysis of sesquiterpene products. Filters were extracted in 1 : 1
methanol : acetonitrile assisted by sonication, evaporated to dryness using
nitrogen gas, and reconstituted in 200 µL MilliQ water with
10 % acetonitrile and 0.1 % acetic acid. The UHPLC system used an
Acquity T3 column (1.8 µm, 2.1 × 100 mm; Waters) with a
mobile phase of eluent A: 0.1 % acetic acid in MilliQ water and eluent B:
acetonitrile with 0.1 % acetic acid (eluent flow was
0.3 mL min-1). The 18 min gradient was eluent B increased from 3 to
80 % from 1 to 12 min and then increased to 100 % (during 0.5 min)
where it was held for 3 min before returning to initial conditions. The
qToF-MS had an electrospray ionization source and was operated in negative
ionization mode with a nebulizer pressure of 3.0 bar, dry gas flow of
7.0 L min-1, source voltage of 3.0 kV, and transfer time of
50 µs. Oxidation products of β-caryophyllene were identified
by comparison of their mass spectra with previous work (Alfarra et al., 2012;
Chan et al., 2011; van Eijck et al., 2013; Jaoui et al., 2003) and products
obtained from a smog chamber study of the ozonolysis of β-caryophyllene. The quantification of β-caryophyllinic acid was
performed using a synthesized standard provided by
Jevgeni Parshintsev (Helsinki University)
following previously reported synthesis procedures (Parshintsev et al.,
2010), and the quantification of β-nocaryophyllonic acid was performed
using the custom-synthesized standard at Northwestern University described in
Gray Bé et al. (2017).
Wet season timeline of sesquiterpene (SQT) and diterpene species
for those measured with hourly time resolution with derivatization (shaded
colors) (a) and those measured multiple times per day at lower
time resolution without derivatization (b). Legend entries correspond to
compound numbers in Table 1. Total SQTs and
diterpenes quantified during runs without derivatization are overlaid in
black for reference in panel (a). Concurrent speciation shown in (b).
Sesquiterpenes and terpenoids observed in the gas phase during
GoAmazon2014/5 with proposed identification and their alkane-based retention
index, Chemical Abstracts Service (CAS) number, mean concentration during
wet and dry seasons, reaction rate
constant with O3 (kO3), and estimated chemical lifetime
in the presence of 20 ppbvO3. kO3 is
estimated using EPA EPI Suite 4.1 AOPWIN where literature data are
unavailable. Concentrations of each species are estimated by using an average
instrument response factor for several sesquiterpene standards unless
otherwise noted that an authentic standard was used. Other commonly studied
sesquiterpenes typically below detection or unobserved are also included for
comparison of reactive timescales.
No.CompoundRetentionCAS no.Wet season Dry season kO3× 1017τ for [O3]indexmean mean (cm3 molec-1= 20 ppbvconcentration concentration s-1)(min)(ng m-3)(ppqv)(ng m-3)(ppqv)Sesquiterpenes 1α-cubebene135517699-14-82.252501.9021143.078.92unidentified13802.863181.121253cyclosativene138322469-52-92.863171.161297.4458.74α-copaene13873856-25-53.75a4173.92a43516b212.05β-elemene1397515-13-92.082311.461632.61317.26cyperene14202387-78-21.611790.8190NEc7α-cedrene1435469-61-40.25a280.45a492.8b78.98unidentified14434.765296.046709β-gurjunene144717334-55-30.88970.88987.4458.710unidentified145326620-71-30.32350.01211unidentified14621.611791.3815312α-patchoulene1471560-32-70.27301.011137.4458.713unidentified; SQT20214740.29330.434914(–) alloaromadendrenea1476025246-27-91.93a2141.72a1911.22826.415γ-muurolene148630021-74-00.18200.111244.276.716unidentified; SQT20214862.362654.8954817α-amorphene149020085-19-21.231361.0311486.039.418β-selinene150517066-67-00.45502.382642.41413.219α-muurolene150931983-22-91.501661.4516186.039.420unidentified15122.142371.5016721β-bisabolene1513495-61-41.241380.323687.2038.922cuparene152416982-00-61.131270.626923γ-cadinene152639029-41-91.751941.1813144.276.724δ-cadinene1529483-76-10.951050.8190163.020.825cis-calamenene SQT202153472937-55-40.81910.3235NEc26selinene < 7-epi-α>15376813-21-40.14150.2225163.020.827γ-cuprenene15454895-23-20.32350.141550.467.328α-cadinene154924406-05-10.17180.06786.039.429unidentified15530.0110.131430selina-3,7(11)-diene15586813-21-40.0780.1011163.020.8Below detection or unobserved β-caryophyllene142787-44-5n/an/an/an/a1160.02.9trans-α-bergamotene144213474-59-4n/an/an/an/a86.039.4aromadendrene1450489-39-4n/an/an/an/a1.22826.4α-humulene14716753-98-6n/an/an/an/a1170.02.9β-farnesene18794-84-8n/an/an/an/a40.1d84.6α-farnesene1509502-61-4n/an/an/an/a104.032.6valencene15234630-07-3n/an/an/an/a8.6394.7Diterpenes 31rimuene19581686-67-50.21180.22197.6448.132pimaradiene19771686-61-90.11100.14127.6448.133Sandaracopimaradiene19951686-56-20.38330.15137.6448.134kaurene208534424-57-20.97860.69601.12981.7Total sesquiterpenes + diterpenes:41.8461138.84285n/an/a
a Authentic standard used for quantification. b Shu and Atkinson (1994). c NE: no estimate in EPI Suite. d Kourtchev et al. (2009).
Nanoelectrospray (nanoESI) ultrahigh-resolution mass
spectrometry (UHRMS)
Selected filter samples from the wet season were extracted and analyzed by
a direct infusion ultrahigh-resolution LTQ Orbitrap Velos mass spectrometer
(Thermo Fisher, Bremen, Germany) equipped with a TriVersa NanoMate robotic
nanoflow chip-based electrospray ionization (nanoESI; Advion Biosciences,
Ithaca NY, USA) source according to methods described previously (Kourtchev
et al., 2013, 2014, 2015, 2016). This analysis provided mass resolution power
of ≥ 100 000 and mass accuracy < 1 ppm to provide molecular
assignments. The direct infusion nanoESI parameters were as follows: the
ionization voltage and back pressure were set at -1.4 kV and 0.8 psi,
respectively. The inlet temperature was 200 ∘C. The sample flow rate
was approximately 200–300 nL min-1. The negative ionization mass
spectra were collected in three replicates over ranges m/z 100–650 and
m/z 150–900 and processed using Xcalibur 2.1 software (Thermo
Scientific). Chemical formulae of the form CcHhNnOoSs
were made according to analysis procedures presented in Kourtchev et
al. (2013, 2015) and Zielinski et al. (2018), and only ions that were observed
in all three replicate extract analyses were kept for evaluation. All sample
intensities were normalized to the aerosol organic carbon loading as well.
Results and discussionChemical characterization of observed sesquiterpenes
A total of 30 sesquiterpene species were observed regularly in the gas phase
in SV-TAG chromatograms during the GoAmazon campaign at T3 (Fig. 1). Compound
names for those compounds positively identified via MS matching and retention
index are labeled accordingly in chromatograms and listed with mean
concentrations observed during the wet and dry seasons in Table 1. Most
sesquiterpene species were observed at mean levels above
100 ppqv, ranging 1–529 ppqv in the wet season and
2–670 ppqv in the dry season. While mean observed
concentrations differed for some species observed in the wet and dry season,
the overall summed mean concentrations of sesquiterpenes were similar in both
seasons (∼ 4–5 ppqv).
Complete timelines of speciated sesquiterpenes are presented in Fig. 2 for
the wet season and Fig. 3 for the dry season. In panel (a), six species are
presented with hourly time resolution under regularly derivatized run
conditions. In panel (b), occasional runs without derivatization allow for
the complete speciation of sesquiterpene and diterpene species and the
calculation of the summed concentration of sesquiterpenes and diterpenes as
overlaid in panel (a). For both seasons, sesquiterpenes exhibit highest
concentrations overnight. Also note that a more dynamic range of summed
sesquiterpene concentrations was observed during the wet season (spanning
across 15 pptv), whereas in the dry season the range is closer
to 8 pptv. Further, the wet season exhibits
greater chemodiversity of
observed sesquiterpenes and terpenes compared to the dry season.
Dry season timeline of sesquiterpene (SQT) and diterpene species
for those measured with hourly time resolution with derivatization (shaded
colors) (a) and those measured multiple times per day at lower
time resolution without derivatization (b). Legend entries correspond to
compound numbers in Table 1 and colors in (b) are the same
used in Fig. 1b. Total SQTs and
diterpenes quantified during runs without derivatization are overlaid in
black for reference in panel (a). Concurrent speciation shown in (b).
Sesquiterpene oxidation
We further explore in this section indicators for sesquiterpene oxidation in
the region. First, by comparing relative concentrations of isoprene,
monoterpenes, and sesquiterpenes at T3 to those made within the
canopy at an upwind Amazon forest site (Alves et al., 2016), we confirm that
the majority of sesquiterpene oxidation must occur within or near the canopy.
Second, differences in chemical composition between ambient samples at our
site away from the canopy and that of sesquiterpene-rich essential oils (used
as a proxy for emission profile within the canopy) reveals that the most
reactive sesquiterpenes in the oils (e.g., β-caryophyllene) are reacted
away to levels below detection by SV-TAG before reaching the sampling
location at T3. Third, we observe known oxidation products of β-caryophyllene in SV-TAG and on filter samples collected at T3.
Sesquiterpene contribution to total O3 reactivity
Sesquiterpenes are observed at concentrations much lower than those of
monoterpenes and isoprene (Figs. 4a and 5a). They generally react relatively
quickly with O3, which dominates their reactive loss, and thus a
strong anticorrelation with O3 concentration is observed (Figs. 4a
and 5a). During daytime, the levels of photochemically produced O3
keep sesquiterpene concentrations low. In addition, the downward transport of
ozone-rich air during convective storms in the Amazon that typically occur
during late morning or early afternoon hours (Gerken et al., 2016) also
contribute to the temporal concentration profile observed with highest
concentrations at night (Fig. 6). This is in contrast with typically observed
daytime maxima for isoprene and monoterpenes (Alves et al., 2016;
Yáñez-Serrano et al., 2015), for which reactive loss is
dominated by reaction with OH and with OH /O3, respectively
(Atkinson, 1997; Kesselmeier et al., 2013; Kesselmeier and Staudt, 1999), and
whose daytime emissions are likely more associated with immediate release
following production as a function of solar radiation input (Alves et al.,
2014; Bracho-Nuñez et al., 2013; Harley et al., 2004; Jardine et al., 2015;
Kuhn, 2002; Kuhn et al., 2004).
To better understand the reactive loss of O3, the estimated cumulative
loss of O3 by reaction with sesquiterpenes compared to that by
reaction with isoprene and monoterpenes is shown for the wet and dry seasons
in Figs. 4b and 5b, respectively. O3 reactivity (s-1) is
defined as the summed product of each terpene concentration
(molec cm-3) and its second-order rate constant
(molec-1 cm3 s-1) for reaction with O3. Of the
approximately 20 regularly observed sesquiterpenes by SV-TAG, only two
species (α-copaene and α-cedrene) have their reaction rate
constants with all three major atmospheric oxidants (O3, OH, and
NO3) measured in the laboratory (Atkinson and Arey, 2003; Shu and
Atkinson, 1994). For the remaining sesquiterpenes, reaction rate constants
were estimated using the Environmental Protection Agency's Estimation Program
Interface Suite (U.S. EPA, 2000). Reaction rate constants
(estimated and measured) are listed in Table 1 for the observed sesquiterpenes.
Additional reaction rate constants for other sesquiterpenes that are commonly
reported but unobserved in this study are provided for comparison. The
estimated contribution to O3 reactivity from sesquiterpenes remains
uncertain, as discrepancies of 1 to 2 orders of magnitude sometimes exist
between measured and estimated reaction rate constants. For example, the
estimated reaction rate constant for β-caryophyllene is
44.2 × 10-17 cm3 molec-1 s-1 and the
experimentally determined rate is
1170 × 10-17 cm3 molec-1 s-1, leading to
calculated lifetimes via ozonolysis at 20 ppbvO3 of
77 and 3 min, respectively. Further, the estimate for monoterpene
contribution to O3 reactivity assumes that all monoterpenes here have
the same rate constant as α-pinene, as monoterpene measurements were
not speciated here and it is one of the more dominant (17 and 45 % by
mass) and longer-lived monoterpenes as observed in upwind forested sites by
Jardine et al. (2015) and Yáñez-Serrano et al. (2018). As observed
concentrations of monoterpenes within canopy are a factor of 3–4 higher than
those observed at T3, it is reasonable to expect that O3 loss due to
reaction with monoterpenes at this measurement site will become increasingly
dominated by reaction with α-pinene.
Wet season selected timeline of summed gas-phase VOC
concentrations, isoprene (ISOP, red), monoterpenes (MT, purple), and
estimated sesquiterpenes (SQTs, green) and ozone concentration (O3,
black) in panel (a). Average percentage contributions during the wet season for
each group to total VOC concentration are also shown in the top pie chart.
Panel (b) depicts the summed contribution to ozone reactivity from isoprene,
monoterpenes, and sesquiterpenes. Average percentage contributions during
the wet season for each VOC to total ozone reactivity are shown in the bottom pie chart.
Local nighttime hours indicated in grey. Local time is -4 h relative to UTC.
At T3, the observed sesquiterpenes are estimated to have a measurable though
smaller contribution (∼ 10–15 %) to the reactive losses of
O3 compared to isoprene (∼ 40 %) and monoterpenes
(∼ 45 %) for both seasons. The contribution of sesquiterpenes to
total O3 loss varies dramatically in space and time, being highest
right near the sources of emission. Based on average wind speed
(2 m s-1), the transport time from the nearest surrounding trees (1 km)
to the measurement site is of the order of at least 8 min, longer than the
chemical lifetime of some of the more highly reactive sesquiterpenes.
The ozonolysis of sesquiterpenes within the canopy has been estimated to account
for about 50 % of sesquiterpene reactivity during the daytime,
suggesting significant losses of these compounds before escaping the Amazon
forest canopy (Jardine et al., 2011). This is reflected in our measurements
of daytime minima being located far from and outside the canopy (Figs. 4a and
5a). This is also evident for monoterpenes, for which concentrations
and the associated O3 reactivity at the top of the canopy at an upwind
forest site (Jardine et al., 2015) are both approximately 10 times higher
than those at the measurement site in this study. Further, the ratio between
monoterpene and sesquiterpene concentrations typical within canopy sites
upwind during the wet (7.4) and dry (2.4) seasons (Alves et al., 2016) and
that observed at our measurement site for both seasons (∼ 17) indicates
that the majority of sesquiterpenes have reacted away before our measurement.
This is consistent with the fact that lifetimes of all 30 sesquiterpene
species detected in SV-TAG with respect to loss via reaction with O3
tend to be > 20 min, whereas more highly reactive and more commonly
studied sesquiterpenes were below detection (Table 1; unobserved or below
detection). Hence, sesquiterpene contributions to O3 loss in our
study represent a low-end estimate of an undoubtedly important contribution
of sesquiterpenes affecting ozone chemistry in the region.
Dry season selected timeline of summed gas-phase VOC
concentrations, isoprene (ISOP, red), monoterpenes (MT, purple), and
estimated sesquiterpenes (SQTs, green) and ozone concentration (O3,
black) in panel (a). Average percentage contributions during the dry season for
each group to total VOC concentration are also shown in the top pie chart.
Panel (b) depicts the summed contribution to ozone reactivity from isoprene,
monoterpenes, and sesquiterpenes. Average percentage contributions during
the dry season for each VOC to total ozone reactivity are shown in the bottom pie chart.
Local nighttime hours indicated in grey. Local time is -4 h relative to UTC.
Differences in chemical composition between sesquiterpene-rich
essential oils and ambient samples
While it is clear that the sesquiterpenes measured at our site represent only
a subset of what is emitted from regional vegetation, we further explored the
compositional differences between those at our site and those in
sesquiterpene-rich copaiba and andiroba essential oils as proxies for
sesquiterpene composition at the site of emission. A major difference between
the ambient sesquiterpene content (13 February 2014 06:47 UTC, 02:47 LT)
and that of the essential oils (Fig. 1, black and red traces, respectively) is
the general absence of β-caryophyllene (Fig. 1, blue label) in ambient
samples and its presence in copaiba essential oil (Fig. 1 and Table S3),
andiroba oil (Table S3), and previously analyzed essential oils of Amazon
origin (Table S4). The chemical reactivity of β-caryophyllene with
20 ppbv of ozone typical of polluted conditions from Manaus
(Gerken et al., 2016; Trebs et al., 2012) results in a chemical lifetime of
3 min compared to that of α-copaene, which is more than 3 h, so any
emissions are expected to be quickly depleted through reaction near the
emission location. β-Caryophyllene is detected in ambient samples at
our measurement site very rarely, and only when ozone is near zero ppb. This
is also expected to be the case with α-humulene, which also has a
chemical lifetime just under 3 min due to reaction with O3, though
it is not as abundant as β-caryophyllene in copaiba essential oil.
Some sesquiterpenes are routinely observed in ambient air that are not in the
analyzed copaiba essential oil (e.g., cyperene and β-gurjunene), which
is expected given the rich diversity of plant species in the Amazon that
should all have unique terpene contents. The difference in relative
abundances of sesquiterpenes in ambient air and the analyzed essential oils
(Table S3) also reflects many other variables: additional vegetation with
similar sesquiterpene emission profiles, actual emissions relative to tissue
content, and chemical fate of the sesquiterpenes once emitted.
Based on the sesquiterpene profile in various essential oils serving as a
proxy for sesquiterpene composition upon emission, it becomes clear that the
majority of potential O3 reactivity from sesquiterpenes is dominated
by β-caryophyllene (> 80 % even if only comprising
approximately 20 % of total sesquiterpene mass; Table S3). This
demonstrates that the estimate of O3 reactivity via reactive loss
with sesquiterpenes at our measurement site is a significant underestimate
and not representative of near-field O3 chemistry. For example,
taking typical terpenoid concentrations within canopy as reported in Alves et
al. (2016) and assuming monoterpene and sesquiterpene speciation within
canopy to be that of Jardine et al. (2015; for monoterpenes) and that of
copaiba essential oil (for sesquiterpenes), near-field or in-canopy loss of
O3 is dominated (> 50 %) by reaction with sesquiterpenes for
both seasons (Fig. S2). Within the transport time to T3 (> 8 min), the
most reactive sesquiterpenes have been reacted away, and the observed
contribution to O3 reactivity becomes diminished by a factor of
∼ 5 for both seasons. This analysis also highlights the importance of
using speciated terpene measurements for calculating the oxidative loss of
radical species. For example, while the relative contributions to total
terpene VOC concentrations are such that
isoprene > monoterpenes > sesquiterpenes near the canopy (Fig. S2)
and at T3 (Figs. 4a and 5a), the large differences seen in terms of their
relative contribution to O3 reactivity at the two locations result
from the terpene species prevalent at each site (Figs. S2, 4b, and 5b). In
addition, while it is typical practice to take total sesquiterpene
concentration and use kO3+β-caryophyllene for all
sesquiterpenes (Jardine et al., 2011; Khan et al., 2017), this would result
in an overestimate of sesquiterpene reactivity with O3 by an order of
magnitude at T3. Thus, with the majority of sesquiterpene ozonolysis
occurring within or just outside the canopy, we expect to observe fewer
sesquiterpenes at our measurement site but that their oxidation products may
be observed even when the primary sesquiterpenes are not.
Observation of β-caryophyllene oxidation products
β-Caryophyllene was infrequently observed in ambient samples despite
its prevalence in copaiba essential oil, consistent with its rapid reaction
with ozone during transport from the canopy to the measurement site. However,
with SV-TAG we regularly observed β-caryophyllene oxidation products
(specifically β-caryophyllene aldehyde and β-caryophyllonic
acid) in both the gas and particle phases. These products have maxima at
local nighttime hours (Fig. 7) of at most a few ng m-3 (Fig. S3) and
exist predominantly in the gas phase despite previous observations of these
products in filter-based measurements (Chan et al., 2011; van Eijck et al.,
2013; Jaoui et al., 2003; Li et al., 2011; Winterhalter et al., 2009). Based
on saturation vapor pressure estimates for several β-caryophyllene
oxidation products presented in Li et al. (2011), the gas-phase-only
observation of β-caryophyllene aldehyde (therein referred to as P-236,
C*= 4.0 × 103µg m-3) is
consistent with the typical organic loadings of sampled air at T3
(∼ 1 µg m-3 during wet season), though β-caryophyllonic acid (therein referred to as P-252-5,
C*= 8.7 × 10-1µg m-3) might be
expected to have some contributions to the particle phase.
Average diel profiles of selected sesquiterpenes and one diterpene
(rimuene) for the wet season (light green) and dry season (black) in gas phase.
A solid line is drawn through median values, and bars indicate a range from
25th
to 75th percentiles. Local nighttime hours are indicated in grey shading.
The speciation of additional oxidation products β-nocaryophyllonic acid
and β-caryophyllinic acid (typically observed at sub ng m-3
levels, Fig. S3) was obtained from UHPLC-qToF-MS analysis of selected
filters. These products were observed to have daily maxima during local
daytime hours (Fig. 7). The differing diel profiles between the four observed
β-caryophyllene oxidation products likely reflects differences in
the chemical lifetimes of each product and multiple possible reaction pathways of
formation (i.e., β-caryophyllene initiated oxidation by O3
followed by continued ozonolysis or continued OH oxidation during daytime).
Further, both of these acids are estimated to have atmospheric lifetimes of
2–10 days (Nozière et al., 2015), consistent with the flatter diel
profile observed. In addition, because the filter-based measurements have
approximately 12 h time resolution and only a selected period representing
significant influence from the Manaus plume was analyzed, the diurnal
dynamics of these concentration profiles may not be fully captured.
Contribution of sesquiterpene oxidation to secondary organic
aerosolIdentification and quantification of common terpenoid SOA
tracers
Diel profiles during the wet season of total sesquiterpenes (SQTs) and
four β-caryophyllene oxidation products in gas and particle phases.
For each series, data are normalized by the maximum observed concentration
within the series and shown as concentration relative to max. Local nighttime hours indicated in grey.
Estimated contributions to total organic aerosol from
particle-phase tracers and statistical factors attributed to the oxidation of
isoprene (red; IEPOX-SOA, 2-MTs +C5-alktriols + 2-MG), monoterpene
(purple; MT filter tracers), and sesquiterpene (green; SQT filter
tracers). 2-MTs: 2-methyl tetrols, 2-MG: 2-methylglyceric acid,
C5-alktriols: C5-alkene triols. Local nighttime hours indicated in grey.
Here, we roughly examine the contribution of isoprene and terpene oxidation
to total submicron organic aerosol (OA). Figure 8 shows a selected timeline
from the wet season of the total observed OA from the AMS (averaged over the
same time frame as our filter samples) and the summed contributions of
positively identified molecular tracers from isoprene, monoterpene, and
sesquiterpene oxidation. This analysis combines observed tracers from both
SV-TAG (particle-phase measurements averaged to 12 h time resolution to
match the filter sampling times) and the UHPLC filter-based measurements of monoterpene and sesquiterpene-derived
tracers, as listed in Table 2. Estimated contributions to total OA from the
oxidation of isoprene have a lower limit of SOA formed through the uptake of
isoprene epoxydiol (IEPOX) based on positive matrix factorization (PMF)
analysis of AMS data (de Sá et al., 2017). This statistical factor, known
as the IEPOX-SOA factor, represents OA mass formed from isoprene under
sufficiently low NOx conditions such that IEPOX forms in the
gas phase (Paulot et al., 2009), followed by uptake to the particle phase to
form SOA (Lin et al., 2012; Liu et al., 2015; Nguyen et al., 2014); it is
estimated to account for approximately half of total isoprene-derived SOA in
environments with low levels of NOx (Liu et al., 2015).
Tracers associated with the IEPOX channel of SOA formation include
2-methyltetrols (2-MTs) observed previously in the Amazon by Claeys et
al. (2004) and C5-alkene triols (Surratt et al., 2006b,
2010), both of which are measured by SV-TAG (Isaacman-VanWertz et al., 2016)
and correlate well with OA mass attributed to the AMS IEPOX-SOA factor (de
Sá et al., 2017). For this reason they are presented in Fig. 8 as
separate from the remaining IEPOX-SOA factor mass (i.e., IEPOX-SOA factor
subtracting SV-TAG measured 2-methyltetrols and C5-alkene triols).
In the presence of NOx, there is also production of
2-methylglyceric acid, another tracer of isoprene chemistry (Surratt et al.,
2006a) measured at the molecular level by SV-TAG and also presented in
Fig. 8. Lower-limit estimates to total OA from the oxidation of monoterpenes
(MTs) comprise filter tracers (DTAA, Iinuma et al., 2009; MBTCA, Szmigielski
et al., 2007; pinic acid, Yu et al., 1999; pinonic acid, Yu et al., 1999; and
terpenylic acid, Claeys et al., 2009). Finally, lower-limit estimates to
total OA from the oxidation of sesquiterpenes (SQTs) include the two
filter-quantified β-caryophyllene oxidation products (β-nocaryophyllonic acid and β-caryophyllinic acid). Only approximately
20 % of all OA mass is composed of these identified tracer species and
statistical factors, almost all of which (18 %) is represented by known
IEPOX products. However, the yields of known tracer compounds typically
represent only a minor fraction of total terpene SOA in laboratory studies:
MT acid tracers used here contribute < 10 % of monoterpene SOA
(Kristensen et al., 2016), and the SQT acid tracers contribute < 5 %
of sesquiterpene SOA (van Eijck et al., 2013; Jaoui et al., 2003).
Significant work therefore remains for the scientific community to resolve
and identify a large fraction of terpene products to understand sources and
formation processes. However, these laboratory studies provide an opportunity
to estimate the total contribution of monoterpene and sesquiterpene oxidation
products to OA using an approximate tracer-based scaling method.
Oxidation products from β-caryophyllene observed in central
Amazonia in gas and/or particle phases listed by analysis method. These
products are attributed to oxidation from the sesquiterpene (SQTOX) source
category. Synthesized standards for tracers 1–6 were used to confirm
identification and for quantification.
Following the approach of Kleindienst et al. (2007), we take the average
summed concentration of β-nocaryophyllonic acid and β-caryophyllinic acid (0.15 ng m-3) and scale by their summed product
yields (0.045) from the laboratory ozonolysis of β-caryophyllene (Jaoui et
al., 2003). This suggests that the contribution of β-caryophyllene to
typical wet season OA concentrations of 1000 ng m-3 is approximately
0.3 %. However, β-caryophyllene represents only a minor fraction
(median: 22 %, range 6–83 %) of the total sesquiterpene content of
analyzed essential oil and plant tissues (Table S3). This suggests that the
sesquiterpene SOA contribution should be approximately 1 % (range
0.4–5 %). This is only a rough low-end estimate because (1) it derives
from scaling two molecular tracers of β-caryophyllene oxidation; the
actual SOA yield of β-caryophyllene and other sesquiterpene oxidation
is not known under conditions relevant to the Amazon, and yields likely vary
as a function of air pollution and other environmental variables. Further,
vapor-phase wall losses of the more semi-volatile and lower-volatility
oxidation products such as these in environmental chambers may result in
non-atmospherically relevant phase partitioning and underestimated product
and SOA yields (Krechmer et al., 2016; La et al., 2016; Loza et al., 2010;
Matsunaga and Ziemann, 2010; McMurry and Grosjean, 1985; McVay et al., 2014;
Ye et al., 2016; Yeh and Ziemann, 2014; Zhang et al., 2014, 2015). (2) This
estimate derives from the available analysis of three essential oil types
derived from Amazonian tree species: copaiba essential oil (this study;
F. Chen et al., 2009; Soares et al., 2013), rosewood essential oil (Fidelis
et al., 2012), and andiroba essential oil (this study). While these
essential oils typically comprise ∼ 30 sesquiterpenes,
Courtois et al. (2009) report 169 sesquiterpenes emitted by 55 species of
tropical trees. Thus, the impact of additional sesquiterpenes reported to be
emitted from these plants but not found in the essential oils presented here
is unaccounted for. Nevertheless, this estimate demonstrates that
sesquiterpene oxidation contributes measurably to SOA based on scaling from
specific identified tracers. In addition, previous analysis for the region
utilizing PMF during the AMAZE-08 campaign suggests that 50 % of freshly
produced secondary organic material may derive from isoprene, 30 % from
monoterpenes, and 20 % from sesquiterpenes (Chen et al., 2015). This
suggests that a considerable fraction of OA from sesquiterpene oxidation
remains to be accounted for through speciated measurements. Considering the
chemical diversity of the sesquiterpenes observed here, it would be most
ideal to have additional representative tracers and authentic standards to
perform a more accurate scaling estimate. While this remains a challenge due
to the enormous range of sesquiterpenes and their oxidation products, we
provide new tracers and data below.
GC × GC chromatogram of nighttime filter sample during the wet season.
Peaks are assigned source categories of isoprene oxidation products (ISOPOXs)
in red, monoterpene oxidation products (MTOXs) in purple, terpene oxidation
products (TERPOXs) in yellow, and sesquiterpene oxidation products (SQTOXs) in
green. Other unidentified OA in grey. The percentage of total ion chromatogram
is made up by each source category shown in the pie chart.
Additional identification of terpene oxidation products by mass
spectral matching
It is clear that the majority of oxidation products and SOA mass from β-caryophyllene and other sesquiterpenes (i.e., beyond β-nocaryophyllonic acid and β-caryophyllinic acid) were not
identified, and thus a vast array of additional oxidation products must be
present. To explore the contributions of additional sesquiterpene oxidation
products to OA, non-targeted chemical characterization was performed on
selected filters from the wet season that were collected during times when
sesquiterpenes were prevalent. The compounds observed in these analyses were
uniquely identified by their first- and second-dimension retention times and
their mass spectra. The observed compounds in ambient air samples were
compared to the laboratory-generated SOA compounds from sesquiterpene
oxidation and a subset of known products from isoprene and
monoterpene oxidation.
Figure 9 is a GC × GC chromatogram of a nighttime filter sample taken
12 February 2014 18:30–06:15 LT (22:30–10:15 UTC) when sesquiterpenes
were relatively abundant. Approximately 460 sample peaks were separated in
the chromatogram and their mass spectra and retention indices searched using
NIST MS Search via GC Image (Zoex, LLC). A table of the peaks in this sample
that could be attributed as BVOC oxidation products, their compound names (if
positively identified), and their assigned source category is available in
Table S5. The source categories and number of peaks, n, assigned to each category
in the present analysis include the following: isoprene-derived oxidation
products (ISOPOXs, n=6), monoterpene-derived oxidation products (MTOXs, n=13), sesquiterpene-derived oxidation products (SQTOXs, n=41),
terpene-derived oxidation products (TERPOXs, n=10), and other organic
aerosol constituents (some not easily categorized) as BVOC oxidation products
(other OA, n=385). The TERPOX category represents compounds that are
suspected to derive from monoterpene and/or sesquiterpene oxidation based on
MS similarity. That is, good MS matches include oxidation products observed
in samples of laboratory monoterpene and sesquiterpene oxidation experiments
(some may be overlapping products), but there was no positive delineation
between MTOX or SQTOX. Categorized peaks accounted for 45 % of the total
signal for this sample, and the rest is labeled other OA.
Oxidation products from isoprene (ISOPOX) and monoterpenes (MTOXs)
in central Amazonia in gas and/or particle phases. Only particle-phase
measurement is presented in this study. Products marked not applicable (n/a)
are not detected in the UHRMS analysis method.
The pie chart inset in Fig. 9 shows the percentage of the sample signal
(total ion chromatogram, TIC, corrected for compound transmission efficiency)
associated with each source category. The peaks associated with SQTOX account
for 9 % of the corrected TIC, and the peaks associated with TERPOX
account for another 5 %, at least part of which are likely from
sesquiterpene oxidation. This sample highlights the chemical complexity still
to be elucidated and quantified in forthcoming analyses of these filter
samples to constrain source contributions to total SOA, while demonstrating
that numerous unidentified oxidation products derived from sesquiterpenes in
this region are present during local nighttime hours. This is consistent with
sesquiterpenes dominating ozone reactivity during nighttime hours (Figs. 4b
and 5b).
Identification of terpene oxidation products by chemical
formulae
Five filter samples from IOP1 (wet season) during the period
9 February 18:30 LT to 11 February 18:30 LT were analyzed according to
procedures described in Sect. 2.3.4 to provide additional insight into the
chemical identity (by chemical formula) of sample compounds. The presence of
ions with chemical formulae consistent with oxidation products identified in
the chromatography–MS methods (SV-TAG, GC × GC-HR-ToF-MS, and UHPLC-MS) provides
additional support for their prevalence throughout the wet season and is
presented in Table 2 for β-caryophyllene oxidation products and
Table 3 for monoterpene oxidation products. Note that this ESI-UHRMS method
is not sensitive to the isoprene oxidation products specified in Table 3,
and thus entries of not applicable (n/a) for the observation of their
chemical formulae are used. An average percentage of signal intensity for
each UHRMS m/z is also presented for reference, but these should not be
directly interpreted as the percentage of total OA. Rather, these are the
percentage of sample that this technique is sensitive to, as these samples
have complex matrices of organic species that also affect ionization
efficiencies.
Conclusion
We have provided speciated time-resolved measurement of 30 sesquiterpenes
and 4 diterpenes and have observed a broad array of sesquiterpene oxidation
products in Amazonia, demonstrating that the emitted sesquiterpenes and their
oxidation products in this region are both relatively abundant and highly
chemically diverse. Most of the observed oxidation products have yet to be
fully chemically characterized and quantified, which will be the subject of
forthcoming publications. Our observations provide a low-end estimate of the
sesquiterpenes in the atmosphere closer to the forest and suggest that
sesquiterpene oxidation via ozonolysis is likely the primary reactive fate of
these compounds in the region. They exhibit nighttime maxima anticorrelated
with ozone and contribute at least 10 % to the reactive loss of O3
compared to that from reaction with isoprene and monoterpenes at our
measurement site and > 50 % within or near the canopy. As ozone levels in
this region are directly influenced by the outflow of the Manaus plume, we
would expect contributions of sesquiterpene-derived SOA to also depend highly
on anthropogenic activities. In addition, since ozone enhancements at
ground level can result from downdrafts of convective storms, sesquiterpenes
are part of an intricate aerosol–oxidant–cloud life cycle. Based on the
observation of two β-caryophyllene oxidation products in aerosols, we
estimate that sesquiterpene oxidation contributes at least 0.4–5 %
(median 1 %) of observed submicron organic aerosol. As the within- or near-canopy reactive loss of sesquiterpenes to O3 is significant, the
measurements at T3 cannot account for the most reactive sesquiterpene
species, and their contributions to SOA formation in the region likely remain
underestimated by the estimates reported here.
Because several of the observed sesquiterpenes have not been studied in terms
of their reaction kinetics with various oxidants (i.e., O3, OH,
NO3) or their oxidative pathways leading to SOA, it is still
challenging to fully constrain their role in the atmospheric chemistry of the
region. Part of this challenge is stymied by the lack of available authentic
standards, so further work in the separation of complex mixtures (e.g., essential
oils) to isolate pure sesquiterpenes and chemical synthesis of oxidation
products would prove beneficial for furthering our knowledge of sesquiterpene
chemistry. Future fieldwork should focus on performing speciated
sesquiterpene measurements within the canopy and connecting the chemical fate
and transport from emissions to regional atmospheric chemistry. Further,
laboratory oxidation experiments of newly observed sesquiterpenes or relevant
mixtures could be used to improve estimates of reactive oxidant loss and
contributions to SOA.
The datasets used in this publication are available at the
ARM Climate Research Facility database for the GoAmazon2014/5 campaign
(https://www.arm.gov/research/campaigns/amf2014goamazon, last
access: 1 January 2018). Mass spectra of
observed terpene oxidation products are available at
https://nature.berkeley.edu/ahg/data/MSLibrary/ within the file
GoAmazon2014_terpox_v1.msp (last access: 23 June 2018).
The Supplement related to this article is available online at https://doi.org/10.5194/acp-18-10433-2018-supplement.
AG, LM, RdS, AM, PA, JJ, and SM designed, coordinated,
and supervised the GoAmazon field campaign and LY, GI, RW, NK, SV, SdS, LA,
BB, WH, PC, DD, YL, KM, JV, MO, and KL carried out the measurements and model
simulations. LY, GI, MM, VR, RW, SdS, BB, and YL performed data analysis. MB,
MG, IK, and MK performed additional analysis of collected filter samples. MA,
AB, RT, and FG synthesized and provided chemical standards. JO and ML
provided supplementary filter samples from laboratory oxidation experiments
to aid data interpretation. LY prepared the paper with contributions
from all coauthors.
The authors declare that they have no conflict of
interest.
The EPA has not reviewed this paper, and thus no endorsement should be
inferred.
This article is part of the special issue “Observations and
Modeling of the Green Ocean Amazon (GoAmazon2014/5) (ACP/AMT/GI/GMD
inter-journal SI)”. It is not associated with a conference.
Acknowledgements
The UC Berkeley team was supported for the GoAmazon2014/15 field campaign by
NSF ACP grant no. 1332998 and for further analysis of the dataset by DOE ASR
grant no. DE-SC0014040. The Northwestern University team was supported by the
National Science Foundation (NSF) under grant no. CHE-1607640. The instrument
as deployed was developed through support from the U.S. Department of Energy
(DOE) SBIR grant DE-SC0004698. We gratefully acknowledge support from the
Central Office of the Large-Scale Biosphere–Atmosphere Experiment in Amazonia
(LBA), the Instituto Nacional de Pesquisas da Amazonia (INPA), the
Universidade do Estado do Amazonia (UEA), and the local foundation FAPEAM.
The work was conducted under 001030/2012-4 of the Brazilian National Council
for Scientific and Technological Development (CNPq). We acknowledge
logistical support from the Atmospheric Radiation Measurement (ARM) Climate
Research Facility, a U.S. Department of Energy Office of Science user
facility sponsored by the Office of Biological and Environmental Research.
ARM-collected data, including ozone and meteorology, were obtained from MAOS.
Lindsay D. Yee acknowledges support from a University of California Berkeley
Chancellor's Postdoctoral Fellowship. Gabriel Isaacman-VanWertz acknowledges
support from an NSF Graduate Research Fellowship (no. DGE 1106400).
Brett B. Palm acknowledges support from a U.S. EPA STAR Graduate Fellowship
(FP-91761701-0). The University of Colorado group acknowledges support from
DOE (BER/ASR) DE-SC0016559 and EPA-STAR 83587701-0. Ariana Gray Bé and
Mary Alice Upshur gratefully acknowledge support from NSF Graduate Research
Fellowships. Mary Alice Upshur also acknowledges an NSF GROW award, National
Aeronautics and Space Administration Earth and Space (NASA ESS) Fellowship,
and a P. E. O. Scholar Award. Franz M. Geiger gratefully acknowledges support
from the Alexander von Humboldt Foundation.
Edited by: Harald Saathoff Reviewed by: two anonymous referees
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