Introduction
Biomass burning (BB) injects large amounts of primary, fine carbonaceous
particles and trace gases into the global atmosphere and significantly
impacts its physical and chemical properties (Crutzen and Andreae, 1990;
Bond et al., 2004, 2013). While BB emissions are recognized as the second
largest global atmospheric source of gas-phase non-methane organic compounds
(NMOCs) after biogenic emissions, a significant portion of the higher
molecular weight species remains unidentified (Christian et al., 2003;
Warneke et al., 2011; Yokelson et al., 2013). It is widely accepted that the
addition of large amounts of these highly reactive species into the
atmosphere alters chemistry on local to global scales (Andreae and Merlet,
2001; Andreae et al., 2001; Karl et al., 2007). NMOCs particularly impact
smoke evolution by rapid formation of secondary organic aerosols (SOA) and
secondary gases including photochemical ozone (O3) (Reid et al., 1998;
Trentmann et al., 2005; Alvarado and Prinn, 2009; Yokelson et al., 2009;
Vakkari et al., 2014).
The many unknowns and initial gas-phase variability of BB emissions limit
our ability to accurately model the atmospheric impacts of fire at all
scales (Trentmann et al., 2005; Mason et al., 2006; Alvarado and Prinn,
2009; Alvarado et al., 2009; Wiedinmyer et al., 2011). Estimating or
modeling the potential of smoke photochemistry to generate secondary
aerosols or O3 requires realistic estimates of NMOC emissions in fresh
smoke and knowledge of the chemical processing environment. Measurements
capable of identifying and quantifying rarely measured and presently
unidentified emissions of NMOCs, in particular the chemically complex low
volatility fraction, are vital for advancing current understanding of the BB
impact on air quality and climate.
Proton-transfer-reaction time-of-flight mass spectrometry (PTR-TOF-MS) is an
emerging technique that simultaneously detects most NMOCs present in air
samples, including oxygenated organics, aromatics, alkenes, and
nitrogen (N)-containing species at parts per trillion detection limits (pptv)
(Jordan et al., 2009; Graus et al., 2010). The instrument uses H3O+
reagent ions to ionize NMOCs via proton transfer reactions to obtain
high-resolution mass spectra of protonated NMOCs with a low degree of
molecular fragmentation at a mass accuracy sufficient enough to determine
molecular formulas (CwHxNyOz).
Although there are many advantages to PTR-TOF-MS over conventional PTR
quadrupole mass spectrometers (increased mass range, high measurement
frequency, and high mass resolution) there remain several difficulties
involving PTR technology, including (1) detection being limited to molecules
with a proton affinity greater than water, (2) complicated spectra due to
parent ion fragmentation or cluster ion formation, and (3) the inability of
the method to isolate isomers. Despite the limitations of this technology,
PTR-TOF-MS is ideal for studying complex gaseous mixtures such as those
present in BB smoke.
This study was carried out as part of a large-scale experiment to
characterize the initial properties and aging of gas- and particle-phase
emissions in smoke from globally significant fuels. Experiments were
conducted from October to November 2012 during the fourth Fire Lab at
Missoula Experiment (FLAME-4) as detailed by Stockwell et al. (2014). A
major goal of the study focused on the identification and quantification of
highly reactive NMOCs in order to () better characterize the overall
chemical and physical properties of fresh BB emissions; (2) better
understand the distribution of emitted carbon across a range of volatilities
in fresh and aged smoke; and (3) improve the capability of current
photochemical models to simulate the climatic, radiative, chemical, and
ecological impacts of smoke on local to global scales. In a companion paper,
the FLAME-4 emissions were compared extensively to field measurements of
fire emissions and were shown to be representative of “real-world”
BB either as is or after straightforward adjustment procedures
detailed therein (Stockwell et al., 2014). In this work, we describe the first application (to our knowledge) of PTR-TOF-MS technology to laboratory
BB smoke to characterize emissions from a variety of authentic
globally significant fuels. We report on several new or rarely measured
gases and present a large set of useful emission ratios (ERs) and emission
factors (EFs) for major fuel types that can inform/update current
atmospheric models.
Experimental details
Missoula fire sciences laboratory
The US Forest Service Fire Sciences Laboratory (FSL) in Missoula, MT houses
a large indoor combustion room described in detail elsewhere (Christian et
al., 2003; Burling et al., 2010; Stockwell et al., 2014). In short, fuels are
burned on a bed located directly below a 1.6 m diameter exhaust stack. The
room is slightly pressurized by outdoor air that generates a large flow,
entraining the fire emissions up through the stack. Emissions are drawn into
sampling lines fixed in the stack at a platform height 17 m above the fuel
bed. Past studies demonstrated that temperature and mixing ratios are
constant across the width of the stack at the platform height, confirming
well-mixed emissions (Christian et al., 2004).
Burns were conducted using two separate configurations as described in
Stockwell et al. (2014). In this paper we will focus on 125 of the 157
burns. During these fires, well-mixed fresh smoke was sampled directly from
the combustion stack by PTR-TOF-MS roughly 5 s after emission. Results
obtained during the remaining burns that investigate photochemically processed
smoke composition in dual smog chambers with a suite of state-of-the-art
instrumentation are presented elsewhere (Tkacik et al., 2014).
Biomass fuels
Descriptions and ignition methods of each fuel type burned during FLAME-4
are detailed in Stockwell et al. (2014). Authentic globally significant
fuels were collected, including African savanna grasses; US grasses; US and
Asian crop residue; Indonesian, temperate, and boreal peat; temperate and
boreal coniferous canopy fuels; woods in traditional and advanced cooking
stoves; shredded tires; and trash. The range of fuel loading was chosen to
simulate real-world conditions for the investigated fuel types with global
examples of biomass consumption shown in Akagi et al. (2011).
Proton-transfer-reaction time-of-flight mass spectrometer
Real-time analysis of NMOCs was performed using a commercial PTR-TOF-MS 8000
instrument from Ionicon Analytik GmbH (Innsbruck, Austria) that is described
in detail by Jordan et al. (2009). The PTR-TOF-MS sampled continuously at a
frequency of 0.2 Hz through heated PEEK tubing (0.0003 m o.d.,
80 ∘C) positioned facing upward to limit particulate uptake. The instrument was
configured with a mass resolution (m/Δm) in the range of 4000 to
5000 at m/z 21 and a typical mass range from m/z 10 to 600. The drift tube was
operated at 600 V with a pressure of 2.3 mbar at 80 ∘C (E/N∼136 Td, with E as the electric field strength and N as the
concentration of neutral gas; 1 Td =10-17 Vcm2). A dynamic
dilution system was set up to reduce the concentration of sampled smoke and
minimize reagent ion depletion. Mass calibration was performed by permeating
1,3-diiodobenzene (protonated parent mass at m/z 330.85; fragments at m/z 203.94
and 204.94) into a 1 mm section of Teflon tubing used in the inlet flow
system. The high mass accuracy of the data allowed for the determination of
the atomic composition of protonated NMOC signals where peaks were clearly
resolved. The post-acquisition data analysis to retrieve counts per second
based on peak analysis was performed according to procedures described in
detail elsewhere (Müller et al., 2013, 2011, 2010). An initial selection
of ions (∼ 68 masses up to m/z ∼ 143) was chosen
based upon incidence and abundance for post-acquisition analysis. In select
cases (nominally one fire of each fuel type), additional compounds
(∼ 50 masses) were analyzed and are reported separately within
this paper. A reasonable estimation procedure showed that the peaks selected
for analysis accounted for > 99 % of the NMOC mass up to m/z 165
in our PTR-TOF-MS spectra. An earlier BB study (Yokelson et al., 2013) using
mass scans to m/z 214 found that ∼ 1.5 % of NMOC mass was
present at m/z > 165.
Calibration of the PTR-TOF-MS was performed every few days at the FSL using
a bottle gas standard (Apel-Riemer Environmental). Calibrations were
performed by adding a known quantity of calibration gas directly to the end
of the PTR-TOF-MS sample inlet. The calibration mixture included
formaldehyde (HCHO), methanol (CH3OH), acetonitrile (CH3CN),
acetaldehyde (CH3CHO), acetone (C3H6O), dimethyl sulfide
(C2H6S), isoprene (C5H8), methyl vinyl ketone
(C4H6O), methyl ethyl ketone (C4H8O), benzene
(C6H6), toluene (C6H5CH3), p-xylene
(C8H10), 1,3,5-trimethylbenzene (C9H12), and α-pinene (C10H16).
The normalized sensitivity of the instrument (ncps ppbv-1) was determined for
calibrated compounds based on the slope of the linear fit of signal
intensities (normalized to the H3O+ signal, ∼ 106 cps) versus a range of volumetric mixing ratios (VMR). Multipoint
calibration curves varied due to instrumental drift and dilution
adjustments accordingly, and average calibration factors (CFs, ncps ppbv-1)
were determined throughout the field campaign as described by Warneke et al. (2011) and were used to calculate concentrations.
(a) The normalized response of calibration factors (“CF,”
ncps ppbv-1) versus mass (calibrated species labeled by name) overlaid with
the linearly fitted mass-dependent transmission curve (black markers and
dotted line). Separate linear approximations of (b) oxygenated (blue) and (c)
hydrocarbon (green) species used to calculate approximate calibration
factors for all observed masses where explicit calibrations were not
available.
Quantification of the remaining species was performed using calculated
mass-dependent calibration factors based on the measured calibration
factors. Figure 1a shows the spread in the normalized response of compounds
versus mass (labeled by compound name) overlaid with the linearly fitted
mass-dependent transmission curve (black markers and dotted line). It is
clear from Fig. 1a that the oxygenated species (blue labels) and the
hydrocarbon species (green labels) exhibit a slightly different mass-dependent
behavior; however, both groups show a linear increase with mass
that is similar to that observed for the transmission efficiency (Fig. 1b
and c). To reduce bias, mass-dependent calibration factors were determined
using a linear approximation for oxygenated and hydrocarbon species
separately (Fig. 1b and c). α-Pinene was not included in the linear
approximation for hydrocarbons as this compound is well known to be
susceptible to substantial fragmentation in the drift tube. Sulfur (S)- and
N-containing compounds were considered collectively, and together they
more closely follow the trend of the oxygenated species. Thus, in cases
where a compound contains a non-oxygen heteroatom (such as methanethiol),
the mass-dependent calibration factor was determined using the relationship
established using the oxygenated species. Calibration factors were then
determined according to the exact mass for all peaks where the chemical
formula has been determined. Our approach does not yet account for the
potential for ions to fragment and/or cluster; however, we expect this
impacts less than 30 % of NMOC and usually to a small degree for any
individual species. These latter issues change the mass distribution of
observed carbon but should not have a large effect on the total observed
carbon.
It is difficult to assess the overall error introduced using this method of
calibration factor approximation, as only a limited number of comparable
measurements of calibration factors are available. The deviation of measured
calibration factors for species contained in the gas standard from the
linear approximation yields a range of errors (21 ± 19 %) with a
maximum of 50 % observed in all cases (excluding α-pinene for
reasons detailed above). While PTR-TOF-MS is typically known as a soft
ionization method, fragmentation is common among higher molecular weight
species and therefore needs to be considered as a limitation of this
technique. For the individual species identified it would be misleading to
give a set error based on this limited analysis; however, in the absence of
any known molecular fragmentation, a maximum error of 50 % is prescribed
although larger errors are possible for compounds with N and S heteroatoms.
Better methods for the calculation of mass-dependent calibration factors by
compound class should be developed in the near future to improve the
accuracy of volatile organic compound (VOC) measurements using PTR-TOF-MS.
OP-FTIR
To enhance application of the MS data, emission ratios to carbon monoxide
(CO) were calculated where possible using measurements from an open-path
Fourier transform infrared (OP-FTIR) spectrometer described elsewhere
(Stockwell et al., 2014). The system includes a Bruker Matrix-M IR cube
spectrometer with an open White cell that was positioned to span the width
of the stack to sample the continuously rising emissions. The spectral
resolution was set to 0.67 cm-1 and spectra were collected every
1.5 s with a duty cycle greater than 95 %. Other gas-phase species
quantified by this method included carbon dioxide (CO2), methane
(CH4), ethyne (C2H2), ethene (C2H4), propylene
(C3H6), formaldehyde (HCHO), formic acid (HCOOH), methanol
(CH3OH), acetic acid (CH3COOH), glycolaldehyde
(C2H4O2), furan (C4H4O), water (H2O), nitric
oxide (NO), nitrogen dioxide (NO2), nitrous acid (HONO), ammonia
(NH3), hydrogen cyanide (HCN), hydrogen chloride (HCl), and sulfur
dioxide (SO2) and were obtained by multi-component fits to selected
regions of the mid-IR transmission spectra with a synthetic calibration
non-linear least-squares method (Griffith, 1996; Yokelson et al., 2007).
The OP-FTIR system had the highest time resolution with no sampling line,
storage, fragmentation, or clustering artifacts; thus, for species in common
with PTR-TOF-MS, the OP-FTIR data was used as the primary data. The results
from the intercomparison (for methanol) of OP-FTIR and PTR-TOF-MS show
excellent agreement using an orthogonal distance regression to determine
slope (0.995 ± 0.008) and the R2 coefficient (0.789). This
result is consistent with the good agreement for several species
measured by both PTR-MS and OP-FTIR observed in numerous past studies of
laboratory BB emissions (Christian et al., 2004; Karl et al.,
2007; Veres et al. 2010; Warneke et al., 2011).
Emission ratio and emission factor determination
Excess mixing ratios (denoted ΔX for each species “X”) were
calculated by applying an interpolated background correction (determined
from the pre- and post-fire concentrations). The molar emission ratio (ER)
for each species “X” relative to CH3OH (ΔX/ΔCH3OH) is the ratio between the integral of ΔX over the entire
fire and the integral of ΔCH3OH over the entire fire.
We selected CH3OH as the species in common with the OP-FTIR to serve as
an internal standard for the calculation of the fire-integrated ERs of each
species X to CO (Supplement Table S1). We do this by multiplying the
MS-derived ER (ΔX/ΔCH3OH) by the FTIR-derived ER
(ΔCH3OH/ΔCO), which minimizes error due to occasional
reagent ion depletion or the different sampling frequencies between
instruments that would impact calculating ΔX to ΔCO
directly. Several fires have been excluded from this calculation as data were
either not collected by OP-FTIR and/or PTR-TOF-MS or, alternatively, methanol
data could not be applied for the conversion because (1) the mixing ratios
remained below the detection limit or (2) methanol was used to assist
ignition purposes during a few fires. In the case of the tire fires only, the latter issue with CH3OH was
circumvented by using HCOOH (m/z 47) as a suitable, alternative internal standard. As discussed in Sect. 2.3.,
∼ 50 additional masses were analyzed for selected fires and
the ERs (to CO) for these fires are included in the bottom panels of Table
S1. The combined ERs to CO from the FTIR and PTR-TOF were then used to
calculate emission factors (EFs, g kg-1 dry biomass burned) by the
carbon mass-balance method (CMB) based on the assumption that all of the
burned carbon is volatilized and that all of the major carbon-containing
species have been measured (Ward and Radke, 1993; Yokelson et al., 1996,
1999; Burling et al., 2010). EFs were previously calculated solely from
FLAME-4 OP-FTIR data as described in Stockwell et al. (2014), and a new
larger set of EFs, which includes more carbon-containing species quantified
by PTR-TOF-MS, is now shown in Supplement Table S2. With the additional
carbon compounds quantified by PTR-TOF-MS, the EFs calculated by CMB
decreased ∼ 1–2 % for most major fuels with respect to the
previous EFs reported in Stockwell et al. (2014). In the case of peat and
sugar cane fires, the OP-FTIR-derived EFs are now reduced by a range of
∼ 2–5 % and 3.5–7.5 %, respectively. Along with these
small reductions, this work now provides EFs for many additional species
that were unavailable in Stockwell et al. (2014). Finally, the EFs reported
in Supplement Table S3 were adjusted (when needed) according to procedures
established in Stockwell et al. (2014) to improve laboratory representation
of real-world BB emissions. This table contains the EF we
recommend other workers use and it appears in the Supplement only because of
its large size. In addition to the comparisons considered in Stockwell et al. (2014),
we find that our EFs in Table S3 are consistent (for the limited
number of overlap species) with additional, recent field studies including
Kudo et al. (2014) for Chinese crop residue fires and Geron and Hays (2013)
for North Carolina (NC) peat fires.
Fire emissions are partially dependent on naturally changing combustion
processes. To estimate the relative amount of smoldering and flaming
combustion that occurred over the course of each fire, the modified
combustion efficiency (MCE) is calculated by (Yokelson et al., 1996)
MCE=ΔCO2ΔCO2+ΔCO=11+ΔCOΔCO2.
Though flaming and smoldering combustion often occur simultaneously, a
higher MCE value (approaching 0.99) designates relatively pure flaming
combustion (more complete oxidation), a lower MCE (0.75–0.84) designates
pure smoldering combustion, and thus an MCE of ∼ 0.9
represents roughly equal amounts of flaming and smoldering. Each
fire-integrated MCE is reported in Tables S1–S3.
A typical full mass scan of biomass burning smoke from the
PTR-TOF-MS on a logarithmic (a) and a smaller range linear (b) scale. The
internal standard (1,3-diiodobenzene) accounts for the major peaks
∼ m/z 331 and fragments at peaks near m/z 204 and 205.
Discussion
For all fuel types, there is noticeable variability concerning which
compounds have the most significant emissions. Figure 3 includes both FTIR
and PTR emissions grouped into the following categories: non-methane
hydrocarbons, oxygenates containing only one oxygen, oxygenates containing
two oxygen atoms, and oxygenates containing three oxygen atoms. Within these
categories, the contributions from aromatics, phenolic compounds, and furans are further indicated.
As shown in Fig. 3, oxygenated compounds account for the majority of the emissions for all biomass
or biomass-containing fuels (i.e. tires and plastic bags are excluded). Oxygenated compounds containing only a
single oxygen atom accounted for ∼ 50 % of the total raw
mass signal (> m/z 28, excluding m/z 37) on average and normally had
greater emissions than oxygenated compounds containing two oxygen atoms or
hydrocarbons. Sugar cane has the highest emissions of oxygenated compounds,
as was noted earlier in the FTIR data (Stockwell et al., 2014), and is one of
the few fuels where the emissions of compounds containing two oxygens are
the largest. To facilitate discussion we grouped many of the assigned (or
tentatively assigned) mass peak features into categories including aromatic
hydrocarbons, phenolic compounds, furans, N-containing compounds, and
S-containing compounds. These categories do not account for the majority
of the emitted NMOC mass but do account for most of the rarely measured
species reported in this work. We then also discuss miscellaneous compounds
at increasing m/z.
Aromatic hydrocarbons
Aromatic hydrocarbons contributed most significantly to the emissions for
several major fuel types including ponderosa pine, peat, and black spruce.
The identities of these ringed structures are more confidently assigned due
to the small H to C ratio at high masses. The aromatics confidently
identified in this study include benzene (m/z 79), toluene (m/z 93),
phenylacetylene (m/z 103), styrene (m/z 105), xylenes/ethylbenzene (m/z
107), 1,3,5-trimethylbenzene (m/z 121), and naphthalene (m/z 129), while masses
more tentatively assigned include dihydronaphthalene (m/z 131), p-cymene (m/z
135), and methylnaphthalenes (m/z 143). All masses are likely to have minor
contributions from other hydrocarbon species. The EFs for aromatic species
quantified during all fires are averaged by fuel type and shown in Fig. 4a.
The EF for p-cymene was only calculated for select burns and has been
included in Fig. 4a for comprehensiveness.
Aromatic structures are susceptible to multiple oxidation pathways and
readily drive complex chemical reactions in the atmosphere that are highly
dependent on hydroxyl radical (OH) reactivity (Phousongphouang and Arey,
2002; Ziemann and Atkinson, 2012). Ultimately these gas-phase aromatic
species have high yields for SOA as their physical and chemical evolution
lead to lower volatility species that condense into the particle phase. SOA
yields from these parent aromatic HCs have been shown to strongly vary
depending on environmental parameters including relative humidity,
temperature, aerosol mass concentration, and particularly the level of
nitrogen oxides (NOx) and availability of RO2 radicals, further
adding to the complexity in modeling the behavior and fate of these
compounds (Ng et al., 2007; Song et al., 2007; Henze et al., 2008; Chhabra
et al., 2010, 2011; Im et al., 2014).
(a) The EFs of the aromatics analyzed in all fires averaged
and shown by fuel type. Individual contributions from benzene and other
aromatics are indicated by color. The EFs for p-cymene are only calculated
for select fires and should not be considered a true average. (b) The
correlation plots of selected aromatics with benzene during a black spruce
fire (Fire 74). Similar behavior was observed for all other fuel
types.
Emission ratios to benzene, phenol, and furan for aromatic
hydrocarbons, phenolic compounds, and substituted furans in lumped fuel
categories.
Fuel
Grasses
Coniferous
Chaparral
Peat (6)
Crop
type
(42)
canopy
(8)
residue
(# burns)
(14)
(food, 19)
ER/benzene
MCE
0.968 (0.010)
0.933 (0.032)
0.927 (0.017)
0.767 (0.074)
0.946 (0.022)
Toluene
C7H8
0.44 (0.26)
2.19 (0.84)
0.49 (0.17)
0.53 (0.17)
0.70 (0.22)
Phenylacetylene
C8H6
0.094 (0.022)
0.13
0.067 (0.039)
–
0.65 (0.45)
Styrene
C8H8
0.078 (0.025)
0.11 (0.02)
0.074 (0.020)
0.087 (0.027)
0.10 (0.03)
Xylenes/ethylbenzene
C8H10
0.102 (0.058)
0.21 (0.03)
0.12 (0.03)
0.32 (0.16)
0.20 (0.08)
Trimethylbenzene
C9H12
0.059 (0.045)
0.11 (0.03)
0.043 (0.023)
0.17 (0.08)
0.11 (0.05)
Naphthalene
C10H8
0.18 (0.16)
0.13 (0.05)
0.10 (0.03)
0.15 (0.09)
0.20 (0.17)
Dihydronaphthalene
C10H10
0.040 (0.030)
0.034 (0.016)
0.020 (0.010)
0.050 (0.019)
0.059 (0.028)
p-Cymenea
C10H14
0.018 (0.013)
0.11 (0.01)
0.037
0.15 (0.12)
0.035 (0.019)
Methylnaphthalenes
C11H10
0.032 (0.009)
0.053 (0.005)
0.033 (0.007)
–
0.19 (0.09)
ER/phenol
Cresols (methylphenols)a
C7H8O
0.52 (0.19)
0.55 (0.07)
0.49
0.29 (0.18)
0.57 (0.10)
Catechol (benzenediols)b
C6H6O2
0.73 (0.41)
0.76 (0.29)
1.72 (1.28)
1.58 (1.03)
0.93 (0.45)
Vinylphenol
C8H8O
0.66 (0.19)
0.33 (0.09)
0.30 (0.05)
0.18 (0.05)
0.60 (0.35)
Salicylaldehyde
C7H6O2
0.18 (0.06)
0.17 (0.04)
0.15 (0.04)
0.20 (0.13)
0.18 (0.08)
Xylenol (2,5-dimethyl phenol)
C8H10O
0.25 (0.09)
0.19 (0.06)
0.11 (0.06)
0.31 (0.09)
0.34 (0.07)
Guaiacol (2-methoxyphenol)
C7H8O2
0.40 (0.23)
0.42 (0.12)
0.21 (0.09)
0.71 (0.36)
0.76 (0.33)
Creosol (4-methylguaiacol)a
C8H10O2
0.21 (0.16)
0.21 (0.09)
0.067
0.12 (0.17)
0.19 (0.10)
3-Methoxycatechola
C7H8O3
0.090 (0.072)
0.067 (0.031)
0.028
0.19 (0.04)
0.066 (0.037)
4-Vinylguaiacola
C9H10O2
0.29 (0.19)
0.27 (0.12)
0.052
0.27 (0.04)
0.37 (0.19)
Syringola
C8H10O3
0.13 (0.07)
0.078 (0.029)
0.21 (0.12)
0.22 (0.07)
0.16 (0.10)
ER/furan
2-Methylfuran
C5H6O
0.53 (0.27)
1.02 (0.40)
0.77 (0.30)
0.34 (0.14)
1.50 (0.66)
2-Furanone
C4H4O2
0.93 (0.50)
1.53 (0.80)
0.96 (0.49)
0.44 (0.36)
2.05 (1.09)
2-Furaldehyde (furfural)
C5H4O2
1.61 (0.81)
1.82 (0.85)
1.35 (0.75)
1.34 (0.85)
2.78 (1.21)
2,5-Dimethylfurana
C6H8O
0.27 (0.09)
0.58 (0.20)
0.615573
0.11 (0.01)
0.62 (0.77)
Furfuryl alcohol
C5H6O2
0.77 (0.49)
1.23 (0.57)
0.85 (0.44)
0.25 (0.21)
1.98 (1.21)
Methylfurfuralb
C6H6O2
0.42 (0.24)
1.18 (0.89)
1.95 (1.49)
0.44 (0.35)
0.98 (0.52)
Benzofuran
C8H6O
0.059 (0.028)
0.11 (0.05)
0.10 (0.05)
0.017 (0.010)
0.10 (0.04)
Hydroxymethylfurfural
C6H6O3
0.21 (0.16)
0.64 (0.43)
0.28 (0.19)
0.18 (0.14)
0.49 (0.35)
Methylbenzofuran isomersa
C9H8O
0.67 (0.58)
–
–
–
–
Continued.
Crop
Open three-
Rocket
Gasifier
Trash
Tires
Plastic
residue
stone
cookstoves
cookstove
(2)
(1)
bags
(feed, 9)
cooking (3)
(5)
(1)
(1)
ER/benzene; MCE
0.940 (0.017)
0.968 (0.004)
0.972 (0.015)
0.984
0.973 (0.006)
0.961
0.994
Toluene
1.00 (0.44)
0.095 (0.029)
0.98 (1.39)
0.24
0.41 (0.20)
0.056
0.69
Phenylacetylene
0.14 (0.09)
0.10 (0.05)
–
–
–
0.020
–
Styrene
0.14 (0.05)
0.054 (0.021)
0.076 (0.023)
0.042
0.86 (0.16)
0.064
0.094
Xylenes/ethylbenzene
0.24 (0.11)
0.052 (0.034)
0.10 (0.05)
0.048
0.095 (0.017)
0.043
0.029
Trimethylbenzene
0.11 (0.06)
0.014 (0.007)
0.050 (0.048)
0.026
0.033 (0.016)
0.011
0.047
Naphthalene
0.18 (0.11)
0.21 (0.05)
0.30 (0.17)
0.12
0.10
0.19
0.059
Dihydronaphthalene
0.051 (0.021)
0.019 (0.006)
–
–
–
9.81 × 10-3
–
p-Cymenea
0.11 (0.03)
4.10 × 10-3
–
nm
0.018
nm
nm
Methylnaphthalenes
0.057 (0.037)
–
–
–
–
0.031
–
ER/phenol
Cresols (methylphenols)a
0.61 (0.14)
–
0.34 (0.28)
nm
nm
nm
nm
Catechol (benzenediols)b
0.67 (0.30)
0.74 (0.65)
1.86 (1.29)
0.49
1.12 (0.65)
0.082
0.31
Vinylphenol
0.29 (0.06)
0.18 (0.06)
0.25 (0.18)
0.14
0.34 (0.02)
0.17
0.33
Salicylaldehyde
0.11 (0.04)
0.16 (0.06)
0.27 (0.15)
0.22
0.28 (0.09)
0.17
–
Xylenol (2,5-dimethyl phenol)
0.33 (0.07)
0.18 (0.09)
0.35 (0.11)
0.11
0.23 (0.00)
0.026
–
Guaiacol (2-methoxyphenol)
0.47 (0.16)
0.52 (0.40)
1.30 (0.73)
0.31
0.54 (0.32)
0.019
2.02
Creosol (4-methylguaiacol)a
0.24 (0.07)
0.46
0.62 (0.23)
nm
0.043
nm
nm
3-Methoxycatechola
0.063 (0.035)
0.28
0.44
nm
0.14
nm
nm
4-Vinylguaiacola
0.31 (0.11)
0.34
0.35 (0.22)
nm
0.054
nm
nm
Syringola
0.12 (0.02)
0.94
0.92 (0.53)
nm
–
nm
nm
ER/furan
2-Methylfuran
1.36 (0.38)
0.95 (0.33)
1.66 (1.95)
0.55
0.64 (0.02)
2.10
2.10
2-Furanone
1.16 (0.56)
0.73 (0.21)
2.37 (3.39)
1.28
1.04 (0.49)
3.02
–
2-Furaldehyde (furfural)
1.69 (0.96)
2.47 (1.84)
5.69 (8.46)
1.26
1.03 (0.29)
2.09
0.39
2,5-Dimethylfurana
0.98 (0.14)
–
–
nm
0.2715416
nm
nm
Furfuryl alcohol
1.21 (0.55)
0.86 (0.25)
1.35
0.00
0.78 (0.31)
1.06
1.03
Methylfurfuralb
0.90 (0.42)
0.59 (0.20)
1.06 (1.32)
0.37
0.38 (0.06)
1.33
0.093
Benzofuran
0.11 (0.05)
0.39 (0.57)
0.041 (0.030)
0.069
0.058 (0.018)
2.79
0.056
Hydroxymethylfurfural
0.27 (0.14)
0.20 (0.06)
0.44 (0.52)
0.30
0.39 (0.22)
0.28
–
Methylbenzofuran isomersa
–
–
–
nm
–
nm
nm
Note: “nm” indicates not measured; blank indicates species remained below the detection limits; values in parentheses indicate one standard deviation.a Species were only selected for a few key fires and are not considered the average of each fuel type.b Significant contributions from both methylfurfural and
catechol were reported in pyrolysis reference papers, thus there is no indication
which species is the major contributor at this mass.
Domestic biofuel burning and open BB together comprise the largest global atmospheric
source of benzene (Andreae and Merlet, 2001; Henze et al., 2008); thus, not
surprisingly, benzene is a significant aromatic in our data set. The ERs
relative to benzene for the aromatics listed above are shown in Table 2 and
are positively correlated with benzene as demonstrated by Fig. 4b. Henze et al. (2008) outline how ERs to CO of major aromatics (benzene, xylene, and
toluene) can be implemented as a part of a model to predict SOA formation.
An identical or similar approach that incorporates the additional aromatics
detected by PTR-TOF-MS in this work may be useful to predict the
contribution of aromatics from BB to global SOA by various reaction
pathways.
Toluene, another major emission, often serves as a model compound to study
the formation of SOA from other small ringed volatile organic compounds
(Hildebrandt et al., 2009). Black spruce yielded the greatest toluene ER (to
benzene) during FLAME-4 (3.24 ± 0.42) and has been linked to
significant OA enhancement during chamber photo-oxidation aging experiments
investigating open BB emissions during FLAME-3, though toluene was not
significant enough to account for all of the observed SOA (Hennigan et al.,
2011).
Naphthalene is the simplest species in a class of carcinogenic and
neurotoxic compounds known as polycyclic aromatic hydrocarbons (PAH) and was
detected from all fuels. The rapid rate of photo-oxidation of these
smaller ringed gas-phase PAHs (including naphthalene and methylnaphthalenes)
can have important impacts on the amount and properties of SOA formed and
yields significantly more SOA over shorter time spans in comparison to
lighter aromatics (Chan et al., 2009). Under low-NOx conditions (BB
events generate NOx, though at lower ratios to NMOC and/or CO than those
present in urban environments) the SOA yield for benzene, toluene, and
m-xylene was ∼ 30 % (Ng et al., 2007), while naphthalene
yielded enhancements as great as 73 % (Chan et al., 2009).
In summary, many of the species identified and detected during FLAME-4 are
associated with aerosol formation under diverse ambient conditions (Fisseha
et al., 2004; Na et al., 2006; Ng et al., 2007; Chan et al., 2009). We
present here initial emissions for a variety of aromatics from major global
fuels. A more focused study to probe the extent and significance of SOA
formation in BB plumes by these aromatic precursors was performed by chamber
oxidation during the FLAME-4 campaign and will be presented in Tkacik et al. (2014).
(a) The distribution in average fuel EF for several phenolic
compounds, where compound-specific contributions are indicated by color. The
EFs for compounds additionally analyzed a single time for select fires are
included but are not a true average. (b) The linear correlation of select
phenolic compounds with phenol during an organic hay burn (Fire 119).
Phenolic compounds
Phenol is detected at m/z 95. Earlier studies burning a variety of biomass
fuels found that OP-FTIR measurements of phenol accounted for the observed
PTR-MS signal at this mass even at unit mass resolution, though small
contributions from other species such as vinyl furan were possible but not
detected (Christian et al., 2004). 2D-GC grab samples in FLAME-4 found that other species
with the same formula (only vinyl furan) were present at levels less than 2 % of phenol (Hatch et al., 2014). Thus, we
assumed that within experimental uncertainty, m/z 95 was a phenol measurement in
this study and found that phenol was one of the most abundant oxygenated
aromatic compounds detected. Several substituted phenols were speciated for
every fire and included catechol (m/z 111), vinylphenol (m/z 121), salicylaldehyde
(m/z 123), xylenol (m/z 123), and guaiacol (m/z 125) (Fig. 5a). Several additional
species were quantified for selected fires and included cresol (m/z 109),
creosol (m/z 139), 3-methoxycatechol (m/z 141), 4-vinylguaiacol (m/z 151), and
syringol (m/z 155). The EFs for these additional phenolic compounds were
calculated for select burns and are included in Fig. 5a with the regularly
analyzed compounds. Significant emissions of these compounds are reported in
Table 2 relative to phenol, and the selected compounds shown in Fig. 5b
demonstrate the tight correlation between these derivatives and phenol.
Phenol, methoxyphenols (guaiacols), dimethoxyphenols (syringol), and their
derivatives are formed during the pyrolysis of lignin (Simoneit et al.,
1993) and can readily react with OH radicals leading to SOA formation
(Coeur-Tourneur et al, 2010; Lauraguais et al., 2014). Hawthorne et al. (1989, 1992)
found that phenols and guaiacols accounted for 21 and 45 %
of aerosol mass from wood smoke, while Yee et al. (2013) noted large SOA
yields for phenol (24–44 %), guaiacol (44–50 %), and syringol
(25–37 %) by photo-oxidation chamber experiments under low-NOx
conditions (<10 ppb).
Softwoods are considered lignin-rich and are associated predominately with
guaiacyl units (Shafizadeh, 1982). Thus not surprisingly, guaiacol emissions
were significant for ponderosa pine. Peat, an accumulation of decomposing
vegetation (moss, herbaceous, woody materials), has varying degrees of
lignin content depending on the extent of decomposition, sampling depth,
water table levels, etc. (Williams et al., 2003). The peat burns all emitted
significant amounts of phenolic compounds, with noticeable compound-specific
variability between regions (Indonesia, Canada, and North Carolina). It is
also noteworthy that sugar cane, which also produced highly oxygenated
emissions based on FTIR and PTR-TOF-MS results, had the greatest total
emissions of phenolic compounds.
The photochemical formation of nitrophenols and nitroguaiacols by
atmospheric oxidation of phenols and substituted phenols via OH radicals in
the presence of NOx is a potential reaction pathway for these compounds
(Atkinson et al., 1992; Olariu et al., 2002; Harrison et al., 2005;
Lauraguais et al., 2014). Nitration of phenol in either the gas or aerosol
phase is anticipated to account for a large portion of nitrophenols in the
environment. Higher nitrophenol levels are correlated with increased plant
damage (Hinkel et al., 1989; Natangelo et al., 1999) and consequently are
linked to forest decline in central Europe and North America (Rippen et al.,
1987). Nitrophenols are also important components of brown carbon and can
contribute to SOA formation in BB plumes (Kitanovski et al.,
2012; Desyaterik et al., 2013; Mohr et al., 2013; Zhang et al., 2013).
Nitrated phenols including nitroguaiacols and methyl-nitrocatechols are
suggested as suitable BB molecular tracers for secondary BB aerosol
considering their reactivity with atmospheric oxidants is limited (Iinuma et
al., 2010; Kitanovski et al., 2012; Lauraguais et al., 2014). The oxidation
products from the phenolic compounds detected in fresh smoke here have not
been directly examined and would require a more focused study beyond the
scope of this paper.
As with the aromatic compounds, the ERs provided in Table 2 can be used to
estimate initial BB emissions of phenolic species, both rarely measured or
previously unmeasured, from a variety of fuels in order to improve
atmospheric modeling of SOA and nitrophenol formation.
(a) The distribution in average fuel EF for furan and
substituted furans, where individual contributions are indicated by color.
The EFs for substituted furans additionally analyzed a single time are not
true averages (b) The linear correlation of furan with select substituted
furans for an African grass fire (Fire 49).
Furans
Other significant oxygenated compounds include furan and substituted furans
which arise from the pyrolysis of cellulose and hemicellulose. The
substituted furans regularly quantified included 2-methylfuran (m/z 83),
2-furanone (m/z 85), furfural (m/z 97), furfuryl alcohol (m/z 99), methylfurfural
(m/z 111), benzofuran (m/z 119), and hydroxymethylfurfural (m/z 127), while
2,5-dimethylfuran (m/z 97) and methylbenzofurans (m/z 133) were occasionally
quantified. The ERs to furan for these compounds are summarized in Table 2,
and Fig. 6a shows the average EF for the regularly quantified masses and the
individual fire EFs for the occasionally quantified compounds.
Furan and substituted furans are oxidized in the atmosphere primarily by OH
(Bierbach et al., 1995), but also by NO3 (Berndt et al., 1997) or Cl
atoms (Cabañas et al., 2005; Villanueva et al., 2007). Photo-oxidation
of furan, 2-methylfuran, and 3-methylfuran produces butenedial,
4-oxo-2-pentenal, and 2-methylbutenedial (Bierbach et al 1994, 1995). These
products are highly reactive and can lead to free radical (Wagner et al.,
2003), SOA, or O3 formation. In fact, aerosol formation from
photo-oxidation chamber experiments has been observed for furans and their
reactive intermediates listed above (Gomez Alvarez et al., 2009; Strollo and
Ziemann, 2013). Even less is known concerning SOA yields from furans with
oxygenated functional groups, which comprise the majority of the furan
emissions in this study. Alvarado and Prinn (2009) added reaction rates for
furans based on 2-methylfuran and butenedial values (Bierbach et al., 1994,
1995) to model O3 formation in an aging savanna smoke plume. Although a
slight increase in O3 was observed after 60 min, it was not large
enough to account for the observed O3 concentrations in the plume. The
furan and substituted furan ERs compiled here may help explain a portion of
the SOA and O3 produced from fires that cannot be accounted for based
upon previously implemented precursors (Grieshop et al., 2009).
Furfural was generally the dominant emission in this grouping, consistent
with concurrent 2D-GC measurements (Hatch et al., 2014), while emissions from
2-furanone and furan also contributed significantly. Friedli et al. (2001)
observed that ERs of alkyl furans linearly correlated with furan and
concluded that these alkylated compounds likely break down to furan. Our
expanded substituted furan list includes a variety of functionality ranging
from oxygenated substituents to those fused with benzene rings for diverse
fuel types. Similar to the behavior observed for alkylated furans, the
emissions of our substituted furans linearly correlate with furan as shown
in Fig. 6b. As noted for phenolic compounds, sugar cane produced the largest
emissions of furans excluding Canadian peat, supporting sugar cane as an
important emitter of oxygenated compounds. The emissions from furan, phenol,
and their derivatives reflect variability in cellulose and lignin
composition of different fuel types. Cellulose and hemicellulose compose
∼ 75 % of wood while lignin only accounts for
∼ 25 % on average (Sjöström, 1993). Accordingly, the
Σ furans/Σ phenols for initially analyzed compounds indicate
that furans are dominant in nearly every fuel type.
Nitrogen-containing compounds
Many N-containing peaks were not originally selected for
post-acquisition analysis in every fire. However, the additional analysis of
selected fires included a suite of N-containing organic compounds to
investigate their potential contribution to the N budget and new particle
formation (NPF). Even at our mass resolution of ∼ 5000, the
mass peak from N compounds can sometimes be overlapped by broadened 13C
“isotope” peaks of major carbon-containing emissions. This interference
was not significant for the following species that we were able to quantify
in the standard or added analysis: C2H3N (acetonitrile,
calibrated), C2H7N (dimethylamine; ethylamine), C2H5NO
(acetamide), C3H9N (trimethylamine), C4H9NO (assorted
amides), C4H11NO (assorted amines), and C7H5N
(benzonitrile). As illustrated by the multiple possibilities for some
formulas, several quantified N-containing species were observed but
explicit single identities or relative contributions could not be confirmed.
The logical candidates we propose are based upon atmospheric observations
and include classes of amines and amides shown in Table S4 (Lobert et al.,
1991; Schade and Crutzen, 1995; Ma and Hays et al., 2008; Barnes et al.,
2010; Ge et al., 2011). Additional N-containing compounds were clearly
observed in the mass spectra such as acrylonitrile, propanenitrile, pyrrole,
and pyridine, but they were often overlapped with isotopic peaks of major
carbon compounds; thus a time-intensive analysis would be necessary to
provide quantitative data. For the species in this category, quantification
was possible for select fires by 2D-GC-MS and they are reported by Hatch et al. (2014) for the FLAME-4 campaign.
We present in Supplement Table S5 the abundance of each N-containing gas
quantified by PTR-TOF-MS and FTIR relative to NH3 for selected fires.
The additional N-containing organic gases detected by PTR-TOF-MS for
these 29 fires summed to roughly 22 ± 23 % of NH3 on average
and accounted for a range of 0.1–8.7 % of the fuel N. These
compounds contributed most significantly to fuel N for peat and this varied
by sampling location. This is not surprising since environmental conditions
and field sampling depths varied considerably. Stockwell et al. (2014)
reported large differences for N-containing compounds quantified by FTIR
between FLAME-4 and earlier laboratory studies of emissions from peat burns.
In any case, the additional NMOCs (including N-containing compounds)
speciated by PTR-TOF-MS substantially increases the amount of information
currently available on peat emissions.
The relevance of the N-containing organics to climate and the N cycle is
briefly summarized next. Aerosol particles acting as cloud condensation
nuclei (CCN) critically impact climate by production and modification of
clouds and precipitation (Novakov and Penner, 1993). NPF, the formation of
new stable nuclei, is suspected to be a major contributor to the amount of
CCN in the atmosphere (Kerminen et al., 2005; Laaksonen et al., 2005;
Sotiropoulou et al., 2006). Numerous studies have suggested that organic
compounds containing N can play an important role in the formation
and growth of new particles (Smith et al., 2008; Kirkby et al., 2011; Yu and
Luo, 2014). The primary pathways to new particle formation include (1) the
reaction of organic compounds with each other or atmospheric oxidants to
form higher molecular weight, lower volatility compounds that subsequently
partition into the aerosol phase or (2) rapid acid/base reactions forming
organic salts. The observation of significant emissions of N-containing
organic gases in FLAME-4 could improve understanding of the compounds,
properties, and source strengths contributing to new particle formation and
enhance model predictions on local to global scales. The identities and
amounts of these additional N-containing emissions produced by peat
and other BB fuels are also important in rigorous analysis of the
atmospheric N budget.
Sulfur, phosphorous, and chlorine-containing compounds
S emissions are important for their contribution to acid deposition and
climate effects due to aerosol formation. Several S-containing gases have
been detected in BB emissions including SO2, carbonyl sulfide (OCS),
dimethyl sulfide (DMS), and dimethyl disulfide (DMDS); DMS is one of
the most significant organosulfur compounds emitted by BB and is quantified
by PTR-TOF-MS in our primary data set (Friedli et al., 2001; Meinardi et al.,
2003; Akagi et al., 2011; Simpson et al., 2011). The signal at m/z 49 had a
significant mass defect and is attributed to methanethiol (methyl mercaptan,
CH3SH), which to our knowledge has not been previously reported in real-world BB smoke, though it has been observed in cigarette smoke (Dong et
al., 2010) and in emissions from pulp and paper plants (Toda et al., 2010).
Like DMS, the photochemical oxidation of CH3SH leads to SO2
formation (Shon and Kim, 2006), which can be further oxidized to sulfate or
sulfuric acid and contribute to the aerosol phase. The emissions of
CH3SH are dependent on the fuel S content and are negatively correlated
with MCE. The greatest EF (CH3SH) in our additional analyses arose from
organic alfalfa, which had the highest S content of the selected fuels and
also produced significant emissions of SO2 detected by FTIR.
Other organic gases containing chlorine and phosphorous were expected to be
readily detectable because of their large, unique mass defects and possible
enhancement by pesticides and fertilizers in crop residue fuels. However,
they were not detected in significant amounts by our full mass scans. Fuel P
and Cl may have been emitted primarily as aerosol, ash, low proton affinity
gases, or as a suite of gases that were evidently below our detection limit.
Miscellaneous (order of increasing m/z)
m/z 41:
The assignment of propyne is reinforced by previous observations in BB
fires, and it is of some interest as a BB marker even though it has a
relatively short lifetime of ∼ 2 days (Simpson et al., 2011;
Akagi et al., 2013; Yokelson et al., 2013). Considering that propyne was not
detected in every fuel type, a level of uncertainty is added to any use of
this compound as a BB tracer, and in general the use of multiple tracers is
preferred when possible.
m/z 43:
The high-resolution capabilities of the PTR-TOF-MS allowed propylene to be
distinguished from ketene fragments at m/z 43. The propylene concentrations are
superseded in our present data set by FTIR measurements; however, the two
techniques agree well.
m/z 45:
PTR technology has already been reported as a reliable way to measure
acetaldehyde in BB smoke (Holzinger et al., 1999; Christian et al., 2004).
Photolysis of acetaldehyde can play an important role in radical formation
and is the main precursor of peroxy acetyl nitrate (PAN) (Trentmann et al.,
2003). A wide range in EF (acetaldehyde) (0.13–4.3 g kg-1) is observed
during FLAME-4 and reflects variability in fuel type. The detailed emissions
from a range of fuels in this data set can aid in modeling and interpretation
of PAN formation in aging BB plumes of various regions (Alvarado et al.,
2010, 2013). Crop-residue fuels regularly had the greatest emissions of
acetaldehyde, which is important considering many crop-residue fires evade
detection and are considered both regionally and globally underestimated.
Sugar cane burning had the largest acetaldehyde EF (4.3 ± 1.4 g kg-1) and had significant emissions of oxygenated and N-containing
compounds; consequently it is likely to form a significant amount of PAN.
m/z 57:
The signal at m/z 57 using unit-mass resolution GC-PTR-MS was observed to be
primarily acrolein with minor contributions from alkenes (Karl et al.,
2007). In the PTR-TOF-MS, the two peaks at m/z 57 (C3H5O+ and
C4H9+) are clearly distinguished and acrolein is often the
dominant peak during the fire with the highest emissions from ponderosa pine
and sugar cane.
m/z 69:
The high resolution of the PTR-TOF-MS allowed three peaks to be
distinguished at m/z 69, identities attributed to carbon suboxide
(C3O2), furan (C4H4O), and mostly isoprene
(C5H8) (Fig. 7). Distinguishing between isoprene and furan is an
important capability of the PTR-TOF-MS. The atmospheric abundance and
relevance of carbon suboxide is fairly uncertain and with an atmospheric
lifetime of ∼ 10 days (Kessel et al., 2013), the reactivity and
transport of C3O2 emitted by fires could have critical regional
impacts. The emissions of C3O2 by BB will be interpreted in detail
at a later date (S. Kessel, personal communication, 2014).
m/z 75:
Hydroxyacetone emissions have been reported from both field and laboratory
fires (Christian et al., 2003; Akagi et al., 2011; Yokelson et al., 2013;
St. Clair et al., 2014). Christian et al. (2003) first reported BB
emissions of hydroxyacetone and noted very large quantities from burning
rice straw. The EF (C3H6O2) for rice straw was noticeably high
(1.1 g kg-1) in the FLAME-4 data set and only sugar cane had greater
emissions.
m/z 85, 87:
The largest peak at m/z 85 was assigned as pentenone as it
was monitored/confirmed by PIT-MS/GC-MS in an earlier BB study (Yokelson et al., 2013).
Pentenone was a substantial emission from several fuels with ponderosa pine having the
greatest EF. By similar evidence the minor peak at m/z 87 was assigned to pentanone but
was only detected in a few of the fires in the second set of analyses with the most
significant emissions arising from Indonesian peat.
m/z 107:
Benzaldehyde has the same unit mass as xylenes, but is clearly separated by
the TOF-MS. Greenberg et al. (2006) observed benzaldehyde during
low-temperature pyrolysis experiments with the greatest emissions from ponderosa
needles (ponderosa pine produced the greatest EF in our data set, with a range
of 0.1–0.28 g kg-1). Benzaldehyde emissions were additionally quantified
by GC-MS during a laboratory BB campaign and produced comparable EF to that
of xylenes (Yokelson et al., 2013). During FLAME-4 the EF (benzaldehyde) was
comparable to EF (xylenes calibrated as p-xylene) as seen earlier, except for
peat burns where xylenes were significantly higher.
m/z 137:
At unit mass resolution, the peak at m/z 137 is commonly recognized as
monoterpenes, which can further be speciated by GC-MS. However, as shown in
Fig. 8 there can be up to three additional peaks at this mass that presently
remain unidentified oxygenated compounds. As anticipated, the hydrocarbon
monoterpene peak is significant for coniferous fuels such as ponderosa pine
but much smaller for grasses. In this work we calibrated for α-pinene, which has been reported as a major monoterpene emission from fresh
smoke (Simpson et al., 2011; Akagi et al., 2013).
Expanded view of the PTR-TOF-MS spectrum at m/z 69
demonstrating the advantage over unit mass resolution instruments of
distinguishing multiple peaks, in this instance separating carbon suboxide
(C3O2), furan (C4H4O), and mostly isoprene
(C5H8) in ponderosa pine smoke (fire 70).
Expanded view of the PTR-TOF-MS spectrum of NC peat (fire
61) at m/z 137 showing multiple peaks.
Cookstoves
Trace gas emissions were measured for four cookstoves including a
traditional three-stone cooking fire, the most widely used stove design
worldwide; two “rocket” type designs (Envirofit G3300 and Ezy stove); and
a “gasifier” stove (Philips HD4012). Several studies focus on fuel efficiency of cookstove technology (Jetter et al., 2012), while the detailed
emissions of many rarely measured and previously unmeasured gases are
reported here and in Stockwell et al. (2014) for FLAME-4 burns. For cooking
fires, ∼ 3–6 % of the NMOC mass remained unidentified, with
the Envirofit rocket stove design generating the smallest percentage in the
study. To improve the representativeness of our laboratory open cooking
emissions, the EFs of smoldering compounds reported for three-stone cooking
fires were adjusted by multiplying the mass ratio of each species “X” to
CH4 by the literature-average field EF (CH4) for open cooking in
Akagi et al. (2011). Flaming compounds were adjusted by a similar procedure
based on their ratios to CO2. The preferred values are reported in
Table S3. With these adjustments, the emissions of aromatic hydrocarbons
(Fig. 9a), phenolic compounds (Fig. 9b), and furans (Fig. 9c) distinctively
increased with the primitiveness of design; thus, three-stone cooking fires
produced the greatest emissions. The advancement in emissions
characterization for these sources will be used to upgrade models of
exposure to household air pollution and the ERs/EFs should be factored in to
chemical-transport models to assess atmospheric impacts.
Emission factors (g kg-1) of aromatic hydrocarbons (a),
phenolic compounds (b), and furans (c) for traditional and advanced
cookstoves. The EFs for traditional stoves were adjusted from original lab
data (Sect. 4.7).
BB is an important source of reactive N in the atmosphere, producing
significant emissions of NOx and NH3 while non-reactive HCN and
CH3CN are commonly used as BB marker compounds (Yokelson et al., 1996,
2007; Goode et al., 1999; de Gouw et al., 2003). The FTIR used in FLAME-4
provided the first detection of HCN emissions from cooking fires and the
HCN/CO ER was about a factor of 5 lower than most other BB fuels burned
(Stockwell et al., 2014). Similarly, acetonitrile emissions were measured
for the first time for cooking fires by PTR-TOF-MS in this study, and the
CH3CN/CO ERs from cooking fires are much lower (on average a factor of
∼ 15) than those from other fuels. This should be considered
when using CH3CN/CO ERs to drive source apportionment in areas with
substantial emissions from biofuel cooking sources.
Conclusions
We investigated the primary BB NMOC emissions from laboratory simulated
burns of globally significant fuels using a PTR-TOF-MS instrument. In this
first PTR-TOF-MS deployment dedicated to fires, we encountered some specific
challenges. The fast change in concentration necessitated a fast acquisition
rate, which decreased the signal to noise for the emissions above
background. The large dynamic concentration range necessitated dilution to
minimize reagent ion depletion at peak emissions and the dilution further
reduced the signal to noise ratio. Positive identification of some species
by co-deployed grab sampling techniques will be explored further in a
separate paper, but is challenged by the difficulty of transmitting some
important fire emissions through GC columns (Hatch et al., 2014). We
attempted to enhance compound identification by switching reagent ions
(O2+ and NO+); however, this approach with two broadly
sensitive ions in a complex mixture resulted in complex spectra
for which comparative analysis is beyond the scope of the present effort. Future
experiments might consider instead using a less broadly sensitive reagent
ion such as NH3+ as the alternate reagent ion. We were limited to
our pre-chosen calibration mixture based primarily on gases previously
observed in smoke. For future experiments we suggest adding more standards
to generate more accurate calibration factors, specifically including major
species such as furan and phenol and more compounds with S and N
heteroatoms. In addition, measuring the fragmentation, if any, of more of
the species identified in this work would be of great value. Despite these
practical limitations, the experiment produced a great deal of useful new
information.
The PTR-TOF-MS obtains full mass scans of NMOCs with high enough resolution
to distinguish multiple peaks at the same nominal mass and high enough
accuracy to assign chemical formulas from the “exact” masses. This aided
in compound identification and more than 100 species were categorized as a
confirmed identity, a tentative (most likely) assignment, or unidentified
but with a chemical formula. Chemical identification was aided by
observations of compounds reported in smoke emissions, pyrolysis
experiments, and those species at relevant concentrations in the atmosphere.
This allowed the identification of more masses up to m/z 165 than in earlier work at unit mass resolution, although an estimated range of 12–37 % of the
total mass still remains unidentified and tentatively identified. The
analysis provides a new set of emission factors for ∼ 68
compounds in all fires plus ∼ 50 more in select fires, in
addition to species previously quantified by FTIR (Stockwell et al., 2014)
and other techniques during FLAME-4 (Hatch et al., 2014). While significant
variability was observed between fuels, oxygenated compounds collectively
accounted for the majority of emissions in all fuels, with sugar cane
producing the highest EF of oxygenated species on average, possibly due to
its high sugar content.
We also report emission ratios to benzene, phenol, or furan for the aromatic
hydrocarbons, phenolic compounds, and substituted furans, respectively.
Reporting emissions of previously unmeasured or rarely measured compounds
relative to these more regularly measured compounds facilitates adding
several new compounds to fire emissions models. To our knowledge this is the
first on-line, real-time characterization of several compounds within these
“families” for BB. Observed emissions varied
considerably between fuel types. Several example compounds within each class
(toluene, guaiacol, methylfuran, etc.) have been shown, by chamber
experiments, to be highly reactive with atmospheric oxidants and contribute
significantly to SOA formation. The ERs and EFs characterized by PTR-TOF-MS
of fresh BB smoke are presented in Tables S1–S3 and (especially the
recommended values in Table S3) should aid model predictions of O3 and
SOA formation in BB smoke and the subsequent effects on air quality and
climate on local–global scales.
A large number of organic N-containing species were detected with
several identities speculated as amines or amides. These N-containing
organic gases may play an important role in new particle formation by
physical, chemical, and photochemical processes, though a more focused study
is necessary to measure NPF yields from these compounds and processes. The
additional N-containing gases detected here account for a range of 1–87 %
of NH3 dependent on fuel type with the most significant contribution of
additional N species to fuel N arising from peat burns. The ERs of
acetonitrile to CO for cooking fires were significantly lower than other
fuels and should be factored into source apportionment models in regions
where biofuel use is prevalent if CH3CN is used as a tracer.
The S-containing compounds detected by PTR-TOF-MS included dimethyl sulfide
and methanethiol, where methanethiol was detected for the first time in BB
smoke to our knowledge. These compounds may play a role in acid deposition
and aerosol formation, though to what extent has yet to be extensively
studied. Phosphorous- and chlorine-containing organic gases were not readily observed in
our data set, which may indicate that these species were below our detection
limit.
Using full mass scans from a high-resolution PTR-TOF-MS to characterize
fresh smoke has aided in identifying several compounds and provided
the chemical formula of other organic trace gases. The additional NMOCs
identified in this work are important for understanding fresh BB emissions
and will improve our understanding of BB atmospheric impacts. The subsequent
oxidation products of these gases are the focus of a companion paper probing
BB aging. Taken together, this work should improve BB representation in
atmospheric models, particularly the formation of ozone and secondary
organic aerosol at multiple scales.