Forest fires are major contributors of reactive gas- and particle-phase
organic compounds to the atmosphere. We used offline high-resolution tandem
mass spectrometry to perform a molecular-level speciation of gas- and
particle-phase compounds sampled via aircraft from an evolving boreal forest
fire smoke plume in Saskatchewan, Canada. We observed diverse
multifunctional compounds containing oxygen, nitrogen, and sulfur (CHONS),
whose structures, formation, and impacts are understudied. The
dilution-corrected absolute ion abundance of particle-phase CHONS compounds
increased with plume age by a factor of 6.4 over the first 4 h of
downwind transport, and their relative contribution to the observed
functionalized organic aerosol (OA) mixture increased from 19 % to 40 %.
The dilution-corrected absolute ion abundance of particle-phase compounds
with sulfide functional groups increased by a factor of 13 with plume age,
and their relative contribution to observed OA increased from 4 % to
40 %. Sulfides were present in up to 75 % of CHONS compounds and the
increases in sulfides were accompanied by increases in ring-bound nitrogen;
both increased together with CHONS prevalence. A complex mixture of
intermediate- and semi-volatile gas-phase organic sulfur species was
observed in emissions from the fire and depleted downwind, representing
potential precursors to particle-phase CHONS compounds. These results
demonstrate CHONS formation from nitrogen- and oxygen-containing biomass burning
emissions in the presence of reduced sulfur species. In addition, they
highlight chemical pathways that may also be relevant in situations with
elevated emissions of nitrogen- and sulfur-containing organic compounds from
residential biomass burning and fossil fuel use (e.g., coal), respectively.
Introduction
Forest fires are predicted to become increasingly prevalent and severe with
climate change (Abatzoglou
and Williams, 2016; Barbero et al., 2015; Jolly et al., 2015). These fires
are an important and uncontrolled source of gas- and particle-phase
compounds to the atmosphere, including a complex mixture of gas-phase
reactive organic carbon, primary organic aerosol (POA), carbon monoxide,
carbon dioxide, methane, ammonia, nitrogen oxides, and black carbon (Akagi
et al., 2011; Gilman et al., 2015; Hatch et al., 2015, 2018; Koss et al.,
2018; Vicente et al., 2013; Yokelson et al., 2013). Many of these emitted
compounds are precursors to downwind ozone and secondary organic aerosol
(SOA) production (Ahern et al., 2019; Buysse et al., 2019; Gilman et al., 2015; Hennigan et al., 2011; Lim et al., 2019).
Primary and secondary pollutants from biomass burning have important effects
on air quality locally, regionally, and continentally (Burgos
et al., 2018; Colarco et al., 2004; Cottle et al., 2014; Dreessen et al.,
2016; Forster et al., 2001; Rogers et al., 2020; Val Martín et al.,
2006), and their impacts on human health and climate (e.g., via light-absorbing brown and black carbon) have been well documented (Forrister
et al., 2015; Jiang et al., 2019; Liu et al., 2017; Di Lorenzo et al., 2018;
Reid et al., 2016; Sengupta et al., 2018; Wong et al., 2019). These health
and climate effects are sensitive to the elemental and structural
composition of gas- and particle-phase emissions and transformation products (Hallquist
et al., 2009; Nozière et al., 2015). As a result, past studies have used
online and offline mass spectrometry techniques to characterize the chemical
composition of fresh and aged biomass burning emissions and have revealed a
wide array of emitted hydrocarbons and oxygen-, nitrogen-, or
sulfur-containing functionalized species (Ahern
et al., 2019; Bertrand et al., 2018; Gilman et al., 2015; Hatch et al.,
2015, 2018, 2019; Iinuma et al., 2010; Koss et al., 2018; Laskin et al.,
2009; Yokelson et al., 2013). However, the emissions and chemical
transformations occurring in ambient biomass burning plumes are extremely
complex and despite previous measurements remain poorly understood at the
molecular level.
In this study, we used an aircraft sampling system developed to collect
offline gas- and particle-phase organic compounds above a boreal forest
fire. We examined the molecular-level emissions and evolution of the forest
fire plume via an analysis of these offline samples using gas and liquid
chromatography (GC/LC) with high-resolution mass spectrometry (MS),
including tandem mass spectrometry (MS/MS). This degree of detailed chemical
speciation is important to advance knowledge of in-plume chemical pathways
and reaction products, long-distance transport, and fate of biomass burning
products – all of which will improve modeling capabilities and our
understanding of the health and environmental impacts of biomass burning.
Specifically, the goals of this study were (1) to perform a detailed
speciation of gas- and particle-phase organic compounds derived from the
boreal forest fire in terms of elemental and functional group composition,
to assess changes in composition at the molecular level as the plume aged
and (2) to examine the evolution of oxygen-, nitrogen-, and
sulfur-containing (CHONS) compounds. These CHONS compounds made up 19 %–40 %
of functionalized OA here and have been observed at other ambient sites
(e.g., 9 %–11 %; Ditto et al., 2018), though little is
known about their structures or formation mechanisms. Using our observations
of gas-phase sulfur species, we identified possible precursors and reaction
pathways involved in the formation of these CHONS compounds.
Materials and methods
On 25 June 2018, two research flights were conducted by Environment and
Climate Change Canada as part of their Air Pollution research program. These
flights sampled two boreal wildfire smoke plumes originating near Lac La
Loche in northern Saskatchewan, Canada (Fig. S1). The region is dominated
by pine and spruce trees (Canada's National Forest Inventory,
2020). Gas- and particle-phase samples were collected from the National
Research Council of Canada's Convair 580 research aircraft for analysis with
offline high-resolution mass spectrometry, alongside many other measurements
(Sects. S1–S2). The aircraft flew the same straight-line
tracks at multiple altitudes through the smoke plumes (average altitudes
shown in Table S1), which when stacked created a virtual screen intercepting
the plumes at each of five downwind locations (with flight design similar to
those previously reported (Li
et al., 2017; Liggio et al., 2016)). Screen 1 was ∼10 km from
the fire with screens 2–4 following the plumes downwind and screen 5
intercepting the plumes after they had passed over several major surface and
in situ mining oil sands facilities (Fig. S1). The samples discussed here
were collected across both plumes to ensure that enough mass was present to
well surpass the mass spectrometer's detection limits. Based on satellite
information and aircraft measurements at the start of sampling (i.e., screen 1), the fire was a low-intensity surface fire with smoldering conditions;
aircraft measurements indicated a modified combustion efficiency of
0.90±0.01 for both plumes.
Combined gas- and particle-phase samples were collected onto custom
adsorbent tubes packed with high-purity quartz wool, glass beads, Tenax TA,
and Carbopack X (Sheu et al.,
2018). Samples were collected along screens 1–4 in Fig. S1 (no adsorbent
tubes collected at screen 5) via an external pod mounted under the wing of
the aircraft, which included remote switching between adsorbent tubes at
various transect altitudes and online measurements of temperature, pressure,
and flow (Sect. S1, Fig. S2).
All adsorbent tubes were analyzed using a Gerstel Thermal Desorber TD 3.5+
with gas chromatography (Agilent 7890B GC), atmospheric pressure chemical
ionization (APCI), and quadrupole time-of-flight mass spectrometry (Agilent
6550 Q-TOF), similar to past work (Khare et al.,
2019). For adsorbent tubes, the APCI was operated in positive ionization
mode and the Q-TOF was operated in MS mode (i.e., TOF data collection only,
hereafter “GC-APCI-MS”). Adsorbent tube data were processed primarily via
a targeted approach for CxHy, CxHyO1,
CxHyS1, and CxHyN1 compounds using custom Igor
Pro code (Sects. S2–S3).
In order to reduce losses of lower-volatility gases onto upstream surfaces,
particles were not explicitly filtered out at the inlet of the wing-pod
sampler used for adsorbent tube collection. For several reasons, it was
concluded that the CxHy hydrocarbons smaller than C22–23 (and
CxHyS1, CxHyO1, and CxHyN1 compounds
of similar volatility) measured in the adsorbent tubes were predominantly in
the gas phase. This was based on (1) significant undersampling for particles
at the wing pod inlet since the adsorbent tube sampling flow rate was a
factor of ∼4 lower than its corresponding isokinetic flow
rate, resulting in a significant divergence of particles away from the inlet
during sampling, and (2) partitioning theory, using average in-plume organic
aerosol (OA) concentrations of 18–22 µg/m3 across adsorbent tube
sampling periods for screens 1–4, concurrently measured by an aerosol mass
spectrometer (AMS) on board the aircraft (see Sects. S1, S3,
Table S2). Thus, the C22 and smaller CxHy compounds (and
other compound classes of similar volatility) should have primarily been in
the gas phase at equilibrium. As such, we limited the following adsorbent
tube data analysis to compounds in the C10–C25 range to focus on
intermediate-volatility and semivolatile compounds (I/SVOCs) present in the
gas phase. In our analyses and interpretation, the compounds included in
each of these volatility ranges are defined based on fixed values of
saturation mass concentrations (e.g., Donahue et
al., 2011; Murphy et al., 2014) at the observed 18–22 µg/m3 OA loadings
present during adsorbent tube sampling times.
Dedicated particle-phase samples were collected on 47 mm PTFE filters (2.0 µm pore; Pall Corporation) from a sampling manifold in the aircraft
cabin containing six independent anodized aluminum filter holders. The
filters were sampled behind an isokinetic inlet with a size cutoff of
approximately 2.5 µm. One filter sample was collected per screen for
screens 1–5 shown in Fig. S1.
Filter samples were extracted in methanol (Ditto et
al., 2018). Samples were analyzed via liquid chromatography (Agilent 1260
LC) with electrospray ionization (ESI) and the same Q-TOF. For filters, the
ESI source was operated in both positive and negative ionization modes, and
the Q-TOF was operated in both MS mode (i.e., TOF data collection,
“LC-ESI-MS”) and MS/MS mode (i.e., tandem mass spectral data collection,
“LC-ESI-MS/MS”) (Ditto et
al., 2018, 2020). Filter extracts were also analyzed using GC-APCI-MS in
positive ionization mode. Filter data from LC-ESI-MS, LC-ESI-MS/MS, and
GC-APCI-MS were analyzed with a non-targeted approach, using Agilent Mass
Hunter, SIRIUS with CSI:FingerID and custom R code (Sects. S2–S3) (Ditto et al., 2018,
2020). All peaks that passed strict QA/QC (Sect. S3) were assigned
molecular formulas, with candidate formulas limited to 20 oxygen, 3
nitrogen, and/or 1 sulfur atom(s). Hereafter, LC-ESI compound classes are
discussed here without subscripts.
The particle-phase compounds observed via LC-ESI vs. GC-APCI techniques
varied significantly in their oxygen, nitrogen, and sulfur content since
these two chromatographic and ionization approaches are sensitive towards
different types of compounds (see Sect. S5). As the forest fire plume
aged, the complex mixture of emissions and secondary products became
increasingly functionalized and thus less GC-amenable without
derivatization. Therefore, we focused on LC-ESI-MS data to study these
functionalized particle-phase compounds. We acknowledge that this method
excludes fully reduced hydrocarbons and fully reduced sulfur-containing
particle-phase compounds (i.e., CH and CHS; Ditto et
al., 2018) and thus these compound classes are not accounted for in the
relative particle-phase distributions shown here.
For particle-phase analyses, we estimated saturation mass concentration
based on the Li et al. parameterization (Li et al.,
2016) and then grouped compounds based on fixed volatility bins (Donahue et
al., 2011; Murphy et al., 2014). Particle-phase compounds were observed
across the intermediate-volatility organic compound (IVOC), semivolatile organic compound (SVOC), low-volatility organic compound (LVOC), and extremely low-volatility organic compound (ELVOC) ranges. Particle-phase IVOCs have
been observed in the past and despite their higher volatility may exist in
the particle phase due to their polarity and water solubility. Additional
details on these methods, including a discussion of total mass analyzed from
filters/adsorbent tubes and QA/QC, are discussed in Sects. S1–S. A method summary is shown in Fig. S3.
ResultsEvolution of organic aerosol elemental composition and
functionality with plume age
Our analysis of functionalized OA showed several compositional trends in the
evolving boreal forest fire smoke plume (screens 1–4) and exhibited marked
changes after emissions from the oil sands facilities were mixed with the
forest fire plume (screen 5). Here, we focused on the forest fire plume in
screens 1–4. We observed a diverse elemental composition in functionalized
OA across oxygen-, nitrogen-, and/or sulfur-containing compound classes
(Figs. 1a–b, S5). This included oxygenates (CHO), such as common
biomass burning tracers and their isomers (e.g., levoglucosan, Sect. S2); as well as oxygen- and nitrogen-containing (CHON) compounds; oxygen- and sulfur-containing (CHOS) compounds; reduced
nitrogen-containing (CHN) compounds; reduced nitrogen- and sulfur-containing (CHNS)
compounds; and compounds containing oxygen, nitrogen, and
sulfur (CHONS).
(a) The compound class distribution of functionalized
OA (from non-targeted LC-ESI-MS) weighted by ion abundance, shown as percent
contribution of each compound class to the total compound abundance measured
by LC-ESI-MS. (b) Percent contribution of CHO, CHON, and CHONS compound
classes in functionalized OA as a function of plume age. CHON compounds in
(b) are summed across all O/N ratios. (c) Functional groups and
structural features present in measured functionalized OA (from non-targeted
LC-ESI-MS/MS). The sulfide functional group is shown here for emphasis and
will be the subject of subsequent analyses. The inset in (c) shows the
absolute CHONS ion abundance normalized by CO mass (orange trace, left
y axis) and the absolute sulfide ion abundance normalized by CO mass (black
trace, right y axis). Full CO-adjusted compound class and functional group
abundance data are shown in Figs. S5–S6. For panels (a–c), results tabulated
by occurrence are also shown in Figs. S5–S6.
There was a continual decrease in the relative abundance of particle-phase
CHO compounds in the observed functionalized OA across screens 1–4,
accompanied by a consistent relative increase in CHON and CHONS compounds
(Fig. 1b). Notably, the relative abundance of CHONS compounds increased
from 19 % to 40 % of measured functionalized OA from screens 1 to 4.
Trends in absolute ion abundances were also similar (note: carbon monoxide
mixing ratio was used to account for dilution). This is shown in Fig. 1c
(inset) and Fig. S5c; CHO generally decreased from screens 1 to 4, while
CHONS and CHON generally increased, suggesting that CHONS compounds were
possibly formed from CHO, CHN, and/or CHON precursors in the gas and/or
particle phases. Specifically, the dilution-corrected abundance of CHONS
species increased by a factor of 6.4 from screens 1 to 4 (Fig. 1c, inset).
In Fig. 1, we primarily presented data as relative contributions to
functionalized OA to examine changes in the evolution of the complex mixture
as a whole and the relationships between different compound classes and
functional groups with plume age. However, dilution-corrected abundances are
also important for understanding absolute formation (or depletion) and are
shown for all species in Figs. S5–S6.
Based on MS/MS analyses, these evolving CHO, CHN, CHON, and CHONS compounds
were often comprised of variable combinations of hydroxyl and ether
functional groups (e.g., primary emissions from forest fires like
methoxyphenols and similar structures), as well as amine, imine, and sulfide
groups, along with cyclic nitrogen structural features (consistent with past
laboratory observations of biomass burning emissions; Laskin
et al., 2009; Lin et al., 2018; Liu et al., 2015; Updyke et al., 2012).
Detailed speciation of CHONS compounds in functionalized OA
While some individual CHONS species contained grouped oxygen, nitrogen, and
sulfur atom moieties (e.g., sulfonamides), the majority of CHONS compounds
had a combination of multiple separate oxygen-, nitrogen-, and/or
sulfur-containing functional groups (Fig. 2a–b). Sulfide groups were
important contributors to CHONS compounds (Fig. 2b) and showed a notable
increase in relative contribution to the overall functional group
distribution with plume age (Fig. 1c). They increased from 4 % to 40 %
of measured compound abundance across screens 1 to 4 (Fig. 1c), which
corresponded to an increase by a factor of 13 in terms of their
dilution-corrected abundance (Fig. 1c, inset). Their increasing relative
contribution to CHONS compounds with plume evolution was even more
pronounced – by screen 4, the sulfide functional group was present in 75 %
of detected CHONS compound abundance (Fig. S7a). Here, we focused on the
presence of sulfides in CHONS compounds because most of the observed
particle-phase sulfides occurred as part of CHONS species (71 %), while a
smaller fraction occurred in CHOS (21 %) or CHNS (8 %) compounds (Fig. 3a).
(a) The distribution of
nitrogen-containing functional groups to particle-phase CHONS compounds
(organonitrates are excluded here due to challenges with their
identification using SIRIUS with CSI:FingerID, but contributed minimally to
CHONS, Sect. S3). (b) The distribution of sulfur-containing
groups to particle-phase CHONS compounds.
For panels (a–b), data are averaged across screens 1–4, with individual
screens shown in Fig. S7a. (c) Volatility distribution of particle-phase
CHONS species. These volatility data were averaged across screens 1–4, and
individual screens are shown in Fig. S8. Volatility was estimated with the
parameterization in Li et al. (2016) and
grouped according to volatility bins in Donahue et al. (2011).
(a) Compound class distribution of sulfide groups:
71 % of sulfide functional groups observed (weighted by ion abundance)
were present in CHONS compounds. (b) Co-occurrence of sulfides and
nitrogen-containing groups. Data shown in (a–b) are cumulative across
compounds in screens 1–4. (c) The relative contribution of sulfides and
cyclic nitrogen groups to all functionalized OA increased together with the
increasing contribution of CHONS compounds. The other functional groups in
(b) showed no relationship with the increase in CHONS (Fig. S7b).
Structures represent examples of commonly observed sulfide and cyclic
nitrogen substructures from SIRIUS and CSI:FingerID (Sect. S3), where ring structures associated with nitrogen heteroatoms were free
standing, adjacent to other rings, and/or contained additional attached
functional groups.
To explore possible precursors and formation pathways for these
particle-phase sulfide-containing CHONS species, we used MS/MS to identify
nitrogen-containing functional groups that co-occurred with sulfides. In
CHONS compounds, most sulfides co-occurred with cyclic nitrogen (36 % of
sulfide-containing species), amine (32 %), or imine features (43 %)
(Fig. 3b). The prevalence of sulfide and cyclic nitrogen features in the
measured functionalized OA increased together screen to screen and
increased together with the rising proportion of CHONS compounds (Fig. 3c). While sulfides often co-occurred with amines or imines and while amines
and imines were prevalent in all four screens (Fig. 1c), there was no
relationship between these functional groups and the increasing contribution
of CHONS compounds to measured functionalized OA (Fig. S7b).
The sulfide substructures observed via MS/MS often contained linear carbon
chains or phenyl groups bonded to the sulfur atom (Fig. 3c inset). Thus,
we hypothesize that precursors with similar reduced sulfur-containing
structures reacted with cyclic nitrogen-containing species to form the
observed particle-phase sulfide-containing CHONS compounds (see further
discussion of possible precursors in Sect. 3.3 and potential chemical
pathways in Sect. 4.2).
CHONS compounds were predominantly SVOCs in screens 1–4 (i.e., 89 % of
CHONS ion abundance, Figs. 2c, S8), suggesting that these compounds
were formed from higher-volatility gas-phase species. In contrast, with the
added influence of the oil sands facilities in screen 5, 68 % of CHONS
compounds were extremely low-volatility organic compounds (ELVOCs), though
CHONS made up only 2 % of functionalized OA at screen 5.
Overall, dilution-corrected abundances of functionalized OA in each
particle-phase volatility bin increased with plume age, but the relative
contribution of SVOCs increased from 37 % to 58 % while the relative
contribution of IVOCs dropped from 38 % to 20 %, potentially due to
oxidation reactions that formed SVOCs and/or due to evaporation (Fig. S9).
These particle-phase IVOCs consisted predominantly of CHO, CHN, and CHON
(O/N< 3) compounds, which could include possible non-sulfur-containing precursors to the observed CHONS species. Fragmentation of
particle-phase L/ELVOC compounds also could have contributed to some of the
observed SVOC mass, but the overall increasing total abundance across all
volatility bins with plume age supports the idea that these compounds were
predominantly formed from more volatile precursors (e.g., I/SVOCs).
Targeted search for CHONS precursors in the gas phase
To investigate the precursors and chemistry that could have formed the
sulfide-containing CHONS species observed in the particle-phase samples, we
performed a targeted search for possible gas-phase I/SVOC species in each
adsorbent tube, across all C10–C25 CxHyS1 species
with the equivalent of 0–15 double bonds and/or rings (i.e.,
CxH2x+2S1–CxH2x-28S1, Figs. 4a, S10).
We observed a distribution of CxHyS1 compounds and their
isomers; based on the high-mass-resolution and high-mass-accuracy molecular
formulas from targeted GC-APCI-MS analysis, 27 % of CxHyS1 compounds were fully saturated (i.e., CxH2x+2S1) and 25 %
contained the equivalent of four to six double bonds and/or rings (i.e.,
CxH2x–6S1–CxH2x–10S1), which included single-ring aromatics. We focused on these sulfur-containing gases as candidate
precursors to the observed particle-phase sulfide-containing CHONS compounds
as they contained sulfur substructures with linear carbon chains or phenyl
groups, similar to those observed on particle-phase CHONS compounds via
MS/MS analysis (Figs. 3c inset, S10–S11). However, we also observed
contributions from other sulfur-containing structures (e.g., with the
equivalent of one to three double bonds and/or rings, Fig. 4a), which could also
have been precursor species.
(a) The average CxHyS1 distribution
from targeted GC-APCI-MS across all gas-phase adsorbent tube samples from
screens 1–4 (see Fig. S10 for individual CxHyS1 screens
and Fig. S13 for CxHy). (b) Concentrations of gas-phase
CxHy, CxHyO1, and CxHyS1 from targeted
GC-APCI-MS analysis of adsorbent tubes, shown with total OA from AMS, all
corrected for dilution using carbon monoxide measurements (see Fig. S12a
for non-normalized concentrations). Data in (b) are averaged over low-
and high-altitude adsorbent tube samples.
The observed gas-phase C10–C25 CxHyS1 compounds
were present in gas-phase emissions from the fire and likely also evaporated
from primary OA during early plume dilution. Gas-phase C10–C25 CxHyS1 concentrations increased relative to carbon monoxide
from screens 1 to 2 and then steadily decreased with plume age (Figs. 4b,
S12a). This suggests the gas-phase emission and/or evaporation of
CxHyS1 compounds from OA between screens 1 and 2, and
subsequent participation in plume chemistry from screens 2 to 4.
Similar OA evaporation with plume dilution has been observed in many past
studies (Ahern
et al., 2019; Garofalo et al., 2019; Hennigan et al., 2011; Lim et al.,
2019). To better understand the dynamics of these sulfur-containing
compounds and their possible particle-phase origin, we compared their
concentrations to those of C10–C25 aliphatic and aromatic
CxHy and CxHyO1 species from a similar targeted
search of adsorbent tube gas-phase compounds. Overall, CxHy and
CxHyO1 compound classes dominated the observed
C10–C25 compounds (Figs. 4b, S12b), with 61 %
CxHy, 36 % CxHyO1, and just 3 %
CxHyS1 on average. CxHy and CxHyO1 concentrations generally increased with plume age (Fig. 4b) and included
many known compound types (e.g., monoterpenes, aromatics, hydroxyls,
carbonyls; Akagi
et al., 2011; Andreae, 2019; Gilman et al., 2015; Hatch et al., 2015, 2019;
Koss et al., 2018). This suggests the direct emission of these gas-phase
compound classes from the fire (observed at screen 1) along with the
evaporation of semivolatile particle-phase emissions as the plume evolved,
and formation of CxHyO1 via CxHy oxidation. AMS
measurements of total OA concentrations provided supporting evidence of OA
evaporation; the ratio of AMS OA concentration to CO decreased by 7 % from
screens 1 to 2 (corresponding to an AMS OA/CO ratio of -0.0044 or a decrease
in OA concentrations of -2.3µg/m3), while the ratio of total
gas-phase CxHy, CxHyO1, and
CxHyS1 concentration to CO increased by 55 % (corresponding
to a total gas-phase concentration/CO ratio of 0.022 or an increase in
gas-phase concentration of 7.0 µg/m3, with changes summarized in
Table S3). While not the focus of the analytical approaches applied in this
study, to further substantiate the observation of OA evaporation, we
performed the same targeted analysis of C10–C25 CxHy,
CxHyO1, and CxHyS1 compounds in the
particle-phase filter sample extracts analyzed via GC-APCI-MS. We observed a
similar decrease in concentration from screens 1 to 2. However, these filter
measurements (even with APCI ionization) were not geared towards
CxHy and CxHyS1 speciation due to possible
solubility limitations in the extraction solvent (Sect. S4). Direct thermal desorption of quartz filters with GC-APCI analysis would
be better suited for these CxHy and CxHyS1
measurements (as performed in this study with adsorbent tubes).
As discussed in the Materials and methods section, the observed compounds with volatilities below that of
∼ C22–C23 hydrocarbons existed primarily in the
gas phase (Table S2), while larger compounds favored the particle phase
(with a smaller fraction in the gas phase at equilibrium). The presence of
gas-phase compounds across this I/SVOC range (e.g., Figs. S10, S13) further
corroborates the possibility of contributions from both direct gas-phase
emissions and from evaporation of particle-phase emissions (Sect. S3). In contrast to CxHy concentrations,
CxHyS1 concentrations dropped markedly after screen 2 despite
similarities in the volatility distribution of CxHy and
CxHyS1 I/SVOC mixtures (Figs. 4a, S10 and S13).
This difference across screens shows that the observed CxHyS1
I/SVOCs were removed (e.g., via chemical reactions) more quickly than
CxHy (Fig. 4b), thus supporting their potential contribution to
CHONS formation.
In addition to the C10–C25 CxHyS1 compounds
measured in the adsorbent tubes, smaller sulfur-containing compounds could
have also acted as CHONS precursors, like those identified by the onboard
proton-transfer-reaction mass spectrometer (PTR-ToF-MS). While dimethyl
sulfide (DMS, previously observed in biomass burning smoke (Andreae, 2019)) was often below the instrument's
limit of detection, both dimethyl and diethyl sulfide showed good
correlation with acetonitrile (a well-known biomass burning product; Andreae, 2019) in the smoke plume during screen 1
(r∼0.95, Fig. S12c). This suggests that these compounds
were co-emitted by the fire.
DiscussionInvestigating possible origins of gas-phase sulfur compounds
The gas-phase sulfur-containing compounds observed in the plume were emitted
from the smoldering fire. However, their origins are uncertain, since the
broader range of sulfur species found here (Figs. 4a, S10–S11) has
not been previously reported; many of the compounds in the complex mixture
of sulfur-containing compounds discussed in this work were outside the
detection range of previously employed methods (Hatch
et al., 2015; Khare et al., 2019; Koss et al., 2018; Sekimoto et al., 2018).
Here, we explore two potential origins of these gas-phase sulfur-containing
precursors to the observed particle-phase CHONS compounds: the biomass fuel
itself and the deposition of sulfur species from anthropogenic/industrial
operations.
Fuel. In past studies, emissions of sulfur-containing organic compounds were
typically minor compared to oxygen- or nitrogen-containing compounds,
and the relative balance of oxygen-, nitrogen-, or sulfur-containing
compound emissions was typically proportional to fuel content (Hatch et al., 2015;
Ward, 1990). The estimated N:S ratio for boreal forest fuel near the fire
was ∼10:1 (Huang and Schoenau,
1996), which was similar to the average N:S ratio from a non-targeted search
for nitrogen- and sulfur-containing I/SVOCs from the adsorbent tube samples
in this study of (8.1±4.8):1. Sulfur is an essential nutrient in
plants and can be taken up from soil (as sulfate) or from the atmosphere
via deposition (as SO2 and sulfate) (Aas
et al., 2019; Gahan and Schmalenberger, 2014; Leustek, 2002). Both SO2 and sulfate are metabolized in plants to yield a variety of compounds
critical to plant functions including cysteine and a range of other sulfur-containing
(as well as oxygen- and nitrogen-containing) compounds (Leustek, 2002). In addition,
disulfide bonds contribute to plant protein structure, and these bonds can
cleave and form thiols (Gahan
and Schmalenberger, 2014; Leustek, 2002; Onda, 2013). Sulfur-containing
compounds like these may have been emitted during the fire, along with other
known sulfur products from boreal fuels (e.g., DMS, thiophenes; Akagi
et al., 2011; Hatch et al., 2015; Koss et al., 2018; Landis et al., 2018).
Deposition. While sulfur can be naturally occurring, it is also associated with
anthropogenic activities (e.g., transportation, power generation, industry). A portion of the sulfur in the forest fire emissions could have
originated from sulfur deposited via such anthropogenic activities. The
closest large anthropogenic sulfur source to the fire location was the oil
sands mining region north of Fort McMurray, Alberta, which was approximately
150 km away and which contains known SO2 emitters (Liggio et al.,
2017; McLinden et al., 2016). Regional concentrations of SO2 or other
sulfur species from these nearby industrial activities could have led to
accumulated deposition of inorganic and/or organic sulfur compounds over
time, though it is uncertain how much of this deposited sulfur would have
been taken up and transformed by vegetation due to sulfur uptake and
assimilation regulatory pathways in plants (Davidian and
Kopriva, 2010). This possible accumulated deposition may have acted as a
reservoir of sulfur to be emitted during fires via the re-volatilization of
deposited compounds, in addition to the evaporation of typical sulfur
metabolites or the formation of sulfur-containing combustion by-products.
This hypothesis is consistent with recent deposition measurement and
modeling results for the region, which indicated that sulfur deposition from
the oil sands operations potentially impacted areas downwind, including the
region where this fire occurred (Makar et al., 2018).
Interestingly, lichen and spruce trees, which are prominent in the region of
the fire discussed here, have been reported to accumulate sulfur from
SO2 in regions near large industrial SO2 sources (Meng et al.,
1995; Nyborg et al., 1991). Also, past studies have reported enhancements in
sulfate (as well as nitrate/ammonium) aerosols coming from biomass burning
in areas with urban influence (Fenn
et al., 2015; Hecobian et al., 2011; Hegg et al., 1987). Inorganic aerosol
components from both urban (e.g., Edmonton, Alberta) and industrial (e.g., oil
sands) sources could deposit in the area surrounding the emissions source
along with an organic phase, which we postulate could contain a range of
sulfur-containing species including the CxHyS1 compounds
shown here. These deposited inorganic and organic species may have
re-volatilized during the fire. However, further work is needed to
disentangle the contribution of natural vs. anthropogenic sulfur to biomass
burning emissions in the region. Also, we note that the fire was
∼1 h old at the time of sampling, so the recent
application of fire suppressants was unlikely to contribute to the species
observed.
Potential reaction pathways leading to sulfides in CHONS from
sulfur precursors
A number of potential reactions involving sulfur-containing precursors,
often thiols (R-SH), may have contributed to the formation of the observed
sulfide functional groups in particle-phase CHONS compounds (Fig. S14). On
average, our gas-phase measurements were comprised of 27 % fully saturated
sulfur-containing hydrocarbons (i.e., CxH2x+2S1, Figs. S10–S11). It is likely that some fraction of the sulfur compounds
observed in the gas-phase adsorbent tube measurements (e.g., the compounds
identified as CxH2x+2S1) and in PTR-ToF-MS measurements (e.g.,
dimethyl sulfide, diethyl sulfide) were thiols, but the distinction between
sulfide vs. thiol isomers was challenging with these methods without
specific internal standards.
In some of the following possible reactions, a thiol interacts with a
non-sulfur precursor to yield a sulfide-containing compound. The non-sulfur
precursor (in the gas or particle phase) may have contained O and N atoms,
thus yielding a CHONS compound immediately after participating in one of the
proposed reactions. Alternatively, the newly formed sulfide-containing
compound may have undergone subsequent separate reactions with oxygen-
and/or nitrogen-containing species (in the gas or particle phase) to form
the observed sulfide-containing CHONS species. Here, we focused on possible
reactions that could have contributed the sulfide group to these oxygen-
and/or nitrogen-containing compounds (known emissions from forest fires, as
discussed above). Earlier, we postulated that because most of the observed
particle-phase CHONS compounds were SVOCs, these compounds were
predominantly formed by reactions with more volatile gas-phase compounds.
However, it is uncertain whether these sulfide-forming and CHONS-forming
reactions all occurred in the gas phase with subsequent partitioning to the
particle phase, heterogeneously, or in a combination of separate gas- and
particle-phase chemistry. We suggest some possible sulfide-forming reactions
here, yet we note that these proposed reactions are likely not
comprehensive. Further work to elucidate the chemistry driving this sulfide
and CHONS formation is needed.
Some possible reactions include (1) thiol-ene reactions, where a thiol
reacts with an alkene (or alkyne), which can form carbon–sulfur bonds
(Lowe, 2010). Alkenes are known to be prominent in emissions
from boreal fires (Gilman
et al., 2015; Hatch et al., 2015), and we observed similar structures in our
gas-phase samples that likely included alkenes, cyclic alkanes, and/or
monoterpenes (Fig. S13). (2) Thiol reactions with carbonyls, which can
form hemithioacetals that subsequently dehydrate in the atmosphere to yield
sulfides, are also possible (Jencks and Lienhard, 1966). This
reaction is similar to the formation of enamines from carbonyls and dimethyl
amine via the formation and subsequent dehydration of a carbinolamine, which
has been shown to occur in ambient conditions (Duporté et al.,
2016, 2017). (3) Thiol reactions with alcohols, which can form sulfides, are possible;
these reaction rates are low in the absence of catalysts and require
relatively high temperature to occur (i.e., 200–450 ∘C
(Mashkina, 1991), temperatures that are relevant very
close to the fire but unlikely in the cooled plume). (4) Another possibility
is that a radical intermediate product formed during atmospheric oxidation
of DMS (e.g., the methylthiomethyl radical (CH3SCH2⚫)
from OH⚫-driven hydrogen abstraction of DMS; Barnes et al., 2006) interacted with CHN and CHON precursors
to yield the observed sulfide-containing CHONS products. However, the
concentrations of the methylthiomethyl radical and similar radicals from
other small sulfide precursors would likely be lower than those of other
major drivers of in-plume radical chemistry (e.g., O2, NOx),
thus making this reaction pathway less likely to contribute.
Based on our observations of these sulfide-containing products across flight
screens, the overall timescale for these sulfide-forming reactions was
likely approximately 1 h (or less). For the literature reactions
referenced above, reaction timescales ranged from minutes to hours in
laboratory experiments, but extrapolation to rates in an ambient wildfire
plume is uncertain. Specifically, it is challenging to compare to predicted
timescales for the proposed reactions without knowing the exact
structure/identities of the reactants or the possible role of other key
modifying factors in the plume (e.g., aerosol pH, presence of water).
Implications and conclusions
In this work, we performed the first high-resolution tandem mass
spectrometry analysis of an evolving plume from a smoldering boreal forest
fire. The results show clear evidence of gas-phase sulfur-containing
emissions from the fire, and an increasing contribution from particle-phase
CHONS compounds with sulfide functional groups as the plume evolved.
Together, these results suggest the emission of gas-phase sulfur-containing
compounds from the fire and subsequent gas- and/or particle-phase chemistry
that produced multifunctional sulfide-containing CHONS compounds.
Sulfide functional groups in ambient air have been reported at a range of
US locations from urban inland (1 %–7 % sulfides), urban coastal (5 %–12 %
sulfides), and remote forested (7 % sulfides), and on average, sulfides
comprised 28 % of sulfur-containing functional groups at these sites
(Ditto et al., 2020). However, in past
work, 53 % of these sulfides were present in CHOS compounds, while 34 %
were CHONS, and 13 % were CHNS (in contrast to 21 %, 71 %, and 8 %
in this study, respectively). Notably, at a northeastern US coastal site
where there were several pollution events linked to long-distance transport
of biomass burning smoke during field sampling (Rogers et al., 2020), 70 %–90 % of
sulfides were present in CHONS compounds (Ditto et al., 2020), similar to the
compound class distribution of sulfides discussed here (Fig. 3a).
These results, along with past observations, highlight that this type of
chemistry and these types of reaction products may be relevant to other
regions where concentrations of nitrogen and sulfur-containing precursors
are high, such as in developing regions, emerging economies, or megacities
where residential biomass burning is common and coincident with extensive
use of sulfur-containing fossil fuels (e.g., coal). CHONS compounds have been
reported in similar regions in past studies (Lin
et al., 2012; Pan et al., 2013; Song et al., 2019; X. Wang et al., 2017, 2016; X. K. Wang et al., 2017). Their formation is potentially important since the presence of
sulfur, oxygen, and nitrogen atoms in organic compounds can affect particle
phase state (e.g., solid, semi-solid, liquid), and mixing state (e.g.,
well-mixed, phase-separated) (DeRieux
et al., 2018; Ditto et al., 2019; Van Krevelen and Te Nijenhuis, 2009).
These physical properties may influence particles' chemical reactivity and
overall persistence in the atmosphere, all of which contribute to the health
and environmental impacts that communities and ecosystems experience from OA
exposure. Future work to identify prominent functional groups in CHONS
species in regions with high CHONS concentrations will help elucidate the
formation chemistry of these functionalized compounds and understand and
mitigate their associated impacts.
Code and data availability
Code and data are available upon request.
Author contributions
JCD ran samples, processed filter data, and compiled and interpreted results.
MH processed adsorbent tube data. THM contributed to MS/MS analysis.
SGM, KH, JL, and DRG collaborated on data interpretation. KH
collected and processed AMS data. AL collected and processed PTR-ToF-MS
data. SML designed the aircraft adsorbent tube collection system. PL
and JJBW implemented the wing pod design. PL prepared the wing pods
for collection and JJBW and JL collected the adsorbent tube samples.
MJW and JL designed the filter collection system. MJW and KH
collected filter samples. SML, KH, and JL designed the aircraft
sampling study. JCD and DRG wrote the manuscript, with input from all
co-authors.
Competing interests
The authors declare that they have no conflict of interest.
Acknowledgements
The authors acknowledge GERSTEL for their collaboration with the TDU 3.5+
used to run the GC-APCI adsorbent tubes and filters discussed in this study.
We thank the Environment and Climate Change Canada and National Research Council
technical teams for their help in the construction and maintenance of
cartridge sampling systems, specifically Tak Chan (Environment and Climate
Change Canada) for help collecting samples. We also thank Jo Machesky (Yale)
for help running adsorbent tube samples, Joe Lybik (Yale) for help packing
adsorbent tubes, and Daniel Thompson (Natural Resources Canada) for
informative discussion. The flights discussed in this study were embedded within an intensive
2018 oil sands monitoring campaign, and the oil sands monitoring
program is acknowledged for enabling the flights.
Financial support
This research was supported by the National Science Foundation (grant no. AWD0001666) and Environment and Climate Change Canada.
Review statement
This paper was edited by James Allan and reviewed by two anonymous referees.
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