Volatile and intermediate-volatility non-methane organic gases (NMOGs) released from biomass burning were measured during
laboratory-simulated wildfires by proton-transfer-reaction time-of-flight ...mass spectrometry (PTR-ToF). We identified NMOG
contributors to more than 150 PTR ion masses using gas chromatography (GC) pre-separation with electron ionization,
H3O+ chemical ionization, and NO+ chemical ionization, an extensive literature review, and
time series correlation, providing higher certainty for ion identifications than has been previously available. Our
interpretation of the PTR-ToF mass spectrum accounts for nearly 90 % of NMOG mass detected by PTR-ToF across all fuel
types. The relative contributions of different NMOGs to individual exact ion masses are mostly similar across many fires
and fuel types. The PTR-ToF measurements are compared to corresponding measurements from open-path Fourier transform
infrared spectroscopy (OP-FTIR), broadband cavity-enhanced spectroscopy (ACES), and iodide ion chemical ionization mass
spectrometry (I− CIMS) where possible. The majority of comparisons have slopes near 1 and values of the linear
correlation coefficient, R2, of > 0.8, including compounds that are not frequently reported by PTR-MS such as
ammonia, hydrogen cyanide (HCN), nitrous acid (HONO), and propene. The exceptions include methylglyoxal and compounds that
are known to be difficult to measure with one or more of the deployed instruments. The fire-integrated emission ratios to
CO and emission factors of NMOGs from 18 fuel types are provided. Finally, we provide an overview of the chemical
characteristics of detected species. Non-aromatic oxygenated compounds are the most abundant. Furans and aromatics, while
less abundant, comprise a large portion of the OH reactivity. The OH reactivity, its major contributors, and the
volatility distribution of emissions can change considerably over the course of a fire.
Biomass burning is a large source of volatile organic compounds
(VOCs) and many other trace species to the atmosphere, which can act as
precursors to secondary pollutants such as ozone and fine ...particles.
Measurements performed with a proton-transfer-reaction time-of-flight mass
spectrometer during the FIREX 2016 laboratory intensive were analyzed with
positive matrix factorization (PMF), in order to understand the
instantaneous variability in VOC emissions from biomass burning, and to
simplify the description of these types of emissions. Despite the complexity
and variability of emissions, we found that a solution including just two
emission profiles, which are mass spectral representations of the relative
abundances of emitted VOCs, explained on average 85 % of the VOC emissions
across various fuels representative of the western US (including various
coniferous and chaparral fuels). In addition, the profiles were remarkably
similar across almost all of the fuel types tested. For example, the
correlation coefficient r2 of each profile between ponderosa pine
(coniferous tree) and manzanita (chaparral) is higher than 0.84. The
compositional differences between the two VOC profiles appear to be related
to differences in pyrolysis processes of fuel biopolymers at high and low
temperatures. These pyrolysis processes are thought to be the main source of
VOC emissions. “High-temperature” and “low-temperature” pyrolysis
processes do not correspond exactly to the commonly used “flaming” and
“smoldering” categories as described by modified combustion efficiency
(MCE). The average atmospheric properties (e.g., OH reactivity, volatility,
etc) of the high- and low-temperature profiles are significantly different.
We also found that the two VOC profiles can describe previously reported VOC
data for laboratory and field burns.
Biomass burning is an important source of aerosol and
trace gases to the atmosphere, but how these emissions change chemically
during their lifetimes is not fully understood. As part of the Fire
...Influence on Regional and Global Environments Experiment (FIREX 2016), we
investigated the effect of photochemical aging on biomass burning organic
aerosol (BBOA) with a focus on fuels from the western United States.
Emissions were sampled into a small (150 L) environmental chamber and
photochemically aged via the addition of ozone and irradiation by 254 nm
light. While some fraction of species undergoes photolysis, the vast
majority of aging occurs via reaction with OH radicals, with total OH
exposures corresponding to the equivalent of up to 10 d of atmospheric
oxidation. For all fuels burned, large and rapid changes are seen in the
ensemble chemical composition of BBOA, as measured by an aerosol mass
spectrometer (AMS). Secondary organic aerosol (SOA) formation is seen for
all aging experiments and continues to grow with increasing OH exposure, but
the magnitude of the SOA formation is highly variable between experiments.
This variability can be explained well by a combination of differences in OH
exposure and the total concentration of non-methane organic gases (NMOGs) in
the chamber before oxidation, as measured by PTR-ToF-MS (r2 values from
0.64 to 0.83). From this relationship, we calculate the fraction of carbon
from biomass burning NMOGs that is converted to SOA as a function of
equivalent atmospheric aging time, with carbon yields ranging from 24±4 % after 6 h to 56±9 % after 4 d.
Decamethylcyclopentasiloxane (D
) is a cyclic volatile methyl siloxane (cVMS) that is widely used in consumer products and commonly observed in urban air. This study quantifies the ambient mixing ...ratios of D
from ground sites in two North American cities (Boulder, CO, USA, and Toronto, ON, CA). From these data, we estimate the diurnal emission profile of D
in Boulder, CO. Ambient mixing ratios were consistent with those measured at other urban locations; however, the diurnal pattern exhibited similarities with those of traffic-related compounds such as benzene. Mobile measurements and vehicle experiments demonstrate that emissions of D
from personal care products are coincident in time and place with emissions of benzene from motor vehicles. During peak commuter times, the D
/benzene ratio (w/w) is in excess of 0.3, suggesting that the mass emission rate of D
from personal care product usage is comparable to that of benzene due to traffic. The diurnal emission pattern of D
is estimated using the measured D
/benzene ratio and inventory estimates of benzene emission rates in Boulder. The hourly D
emission rate is observed to peak between 6:00 and 7:00 AM and subsequently follow an exponential decay with a time constant of 9.2 h. This profile could be used by models to constrain temporal emission patterns of personal care products.
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Reactive nitrogen (Nr, defined as all nitrogen-containing
compounds except for N2 and N2O) is one of the most important
classes of compounds emitted from wildfire, as Nr impacts both
atmospheric ...oxidation processes and particle formation chemistry. In
addition, several Nr compounds can contribute to health impacts from
wildfires. Understanding the impacts of wildfire on the atmosphere requires
a thorough description of Nr emissions. Total reactive nitrogen was
measured by catalytic conversion to NO and detection by NO–O3
chemiluminescence together with individual Nr species during a series
of laboratory fires of fuels characteristic of western US wildfires,
conducted as part of the FIREX Fire Lab 2016 study. Data from 75 stack fires
were analyzed to examine the systematics of nitrogen emissions. The measured
Nr ∕ total-carbon ratios averaged 0.37 % for fuels characteristic of
western North America, and these gas-phase emissions were compared with fuel
and residue N∕C ratios and mass to estimate that a mean (±SD)
of 0.68 (±0.14) of fuel nitrogen was emitted as N2 and N2O.
The Nr detected as speciated individual compounds included the following: nitric
oxide (NO), nitrogen dioxide (NO2), nitrous acid (HONO), isocyanic acid
(HNCO), hydrogen cyanide (HCN), ammonia (NH3), and 44 nitrogen-containing volatile organic compounds (NVOCs). The sum of these
measured individual Nr compounds averaged 84.8 (±9.8) %
relative to the total Nr, and much of the 15.2 % “unaccounted”
Nr is expected to be particle-bound species, not included in this
analysis. A number of key species, e.g., HNCO, HCN, and HONO, were confirmed not to
correlate with only flaming or with only smoldering combustion when using
modified combustion efficiency, MCE=CO2/(CO+CO2), as a
rough indicator. However, the systematic variations in the abundance of
these species relative to other nitrogen-containing species were
successfully modeled using positive matrix factorization (PMF). Three
distinct factors were found for the emissions from combined coniferous
fuels: a combustion factor (Comb-N) (800–1200 ∘C) with emissions
of the inorganic compounds NO, NO2, and HONO, and a minor contribution
from organic nitro compounds (R-NO2); a high-temperature pyrolysis
factor (HT-N) (500–800 ∘C) with emissions of HNCO, HCN, and
nitriles; and a low-temperature pyrolysis factor (LT-N) (<500 ∘C) with mostly ammonia and NVOCs. The temperature ranges
specified are based on known combustion and pyrolysis chemistry
considerations. The mix of emissions in the PMF factors from chaparral fuels
(manzanita and chamise) had a slightly different composition: the Comb-N
factor was also mostly NO, with small amounts of HNCO, HONO, and NH3;
the HT-N factor was dominated by NO2 and had HONO, HCN, and HNCO; and
the LT-N factor was mostly NH3 with a slight amount of NO contributing.
In both cases, the Comb-N factor correlated best with CO2
emission, while the HT-N factors from coniferous fuels correlated closely
with the high-temperature VOC factors recently reported by Sekimoto et al. (2018), and the LT-N had some correspondence to the LT-VOC factors. As a
consequence, CO2 is recommended as a marker for combustion Nr
emissions, HCN is recommended as a marker for HT-N emissions, and the family
NH3 ∕ particle ammonium is recommended as a marker for LT-N emissions.
Hydroxyl radical (OH) oxidation of toluene produces ring-retaining products: cresol and benzaldehyde, and ring-opening products: bicyclic intermediate compounds and epoxides. Here, first- and ...later-generation OH oxidation products from cresol and benzaldehyde are identified in laboratory chamber experiments. For benzaldehyde, first-generation ring-retaining products are identified, but later-generation products are not detected. For cresol, low-volatility (saturation mass concentration, C* ∼ 3.5 × 104 − 7.7 × 10−3 µg m−3), first- and later-generation ring-retaining products are identified. Subsequent OH addition to the aromatic ring of o-cresol leads to compounds such as hydroxy, dihydroxy, and trihydroxy methyl benzoquinones and dihydroxy, trihydroxy, tetrahydroxy, and pentahydroxy toluenes. These products are detected in the gas phase by chemical ionization mass spectrometry (CIMS) and in the particle phase using offline direct analysis in real-time mass spectrometry (DART-MS). Our data suggest that the yield of trihydroxy toluene from dihydroxy toluene is substantial. While an exact yield cannot be reported as authentic standards are unavailable, we find that a yield for trihydroxy toluene from dihydroxy toluene of ∼ 0.7 (equal to the reported yield of dihydroxy toluene from o-cresol; Olariu et al., 2002) is consistent with experimental results for o-cresol oxidation under low-NO conditions. These results suggest that even though the cresol pathway accounts for only ∼ 20 % of the oxidation products of toluene, it is the source of a significant fraction (∼ 20–40 %) of toluene secondary organic aerosol (SOA) due to the formation of low-volatility products.
Biomass burning is the largest combustion-related source of volatile organic compounds (VOCs) to the atmosphere. We describe the development of a state-of-the-science model to simulate the ...photochemical formation of secondary organic aerosol (SOA) from biomass-burning emissions observed in dry (RH <20%) environmental chamber experiments. The modeling is supported by (i) new oxidation chamber measurements, (ii) detailed concurrent measurements of SOA precursors in biomass-burning emissions, and (iii) development of SOA parameters for heterocyclic and oxygenated aromatic compounds based on historical chamber experiments. We find that oxygenated aromatic compounds, including phenols and methoxyphenols, account for slightly less than 60% of the SOA formed and help our model explain the variability in the organic aerosol mass (R 2 = 0.68) and O/C (R 2 = 0.69) enhancement ratios observed across 11 chamber experiments. Despite abundant emissions, heterocyclic compounds that included furans contribute to ∼20% of the total SOA. The use of pyrolysis-temperature-based or averaged emission profiles to represent SOA precursors, rather than those specific to each fire, provide similar results to within 20%. Our findings demonstrate the necessity of accounting for oxygenated aromatics from biomass-burning emissions and their SOA formation in chemical mechanisms.
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The emissions of volatile organic compounds (VOCs) from
volatile chemical products (VCPs) – specifically personal care products,
cleaning agents, coatings, adhesives, and pesticides – are emerging as ...the
largest source of petroleum-derived organic carbon in US cities. Previous
work has shown that the ambient concentration of markers for most VCP
categories correlates strongly with population density, except for VOCs
predominantly originating from solvent- and water-borne coatings (e.g.,
parachlorobenzotrifluoride (PCBTF) and Texanol®, respectively). Instead, these enhancements were dominated by distinct emission events likely driven by industrial usage patterns, such as
construction activity. In this work, the headspace of a variety of coating
products was analyzed using a proton-transfer-reaction time-of-flight mass
spectrometer (PTR-ToF-MS) and a gas chromatography (GC) preseparation
front end to identify composition differences for various coating types
(e.g., paints, primers, sealers, and stains). Evaporation experiments of
several products showed high initial VOC emission rates, and for the length
of these experiments, the majority of the VOC mass was emitted during the
first few hours following application. The percentage of mass emitted as
measured VOCs (<1 % to 83 %) mirrored the VOC content reported by
the manufacturer (<5 to 550 g L−1). Ambient and laboratory
measurements, usage trends, and ingredients compiled from architectural
coatings surveys show that both PCBTF and Texanol account for ∼10 % of the total VOC ingredient sales and, therefore, can be useful tracers for solvent- and water-borne coatings.
Decades of air quality improvements have substantially reduced the motor vehicle emissions of volatile organic compounds (VOCs). Today, volatile chemical products (VCPs) are responsible for half of ...the petrochemical VOCs emitted in major urban areas. We show that VCP emissions are ubiquitous in US and European cities and scale with population density. We report significant VCP emissions for New York City (NYC), including a monoterpene flux of 14.7 to 24.4 kg ⋅ d−1 ⋅ km−2 from fragranced VCPs and other anthropogenic sources, which is comparable to that of a summertime forest. Photochemical modeling of an extreme heat event, with ozone well in excess of US standards, illustrates the significant impact of VCPs on air quality. In the most populated regions of NYC, ozone was sensitive to anthropogenic VOCs (AVOCs), even in the presence of biogenic sources. Within this VOC-sensitive regime, AVOCs contributed upwards of ∼20 ppb to maximum 8-h average ozone. VCPs accounted for more than 50% of this total AVOC contribution. Emissions from fragranced VCPs, including personal care and cleaning products, account for at least 50% of the ozone attributed to VCPs. We show that model simulations of ozone depend foremost on the magnitude of VCP emissions and that the addition of oxygenated VCP chemistry impacts simulations of key atmospheric oxidation products. NYC is a case study for developed megacities, and the impacts of VCPs on local ozone are likely similar for other major urban regions across North America or Europe.
We use a large laboratory, modeling, and field dataset to investigate the isoprene + O3 reaction, with the goal of better understanding the fates of the C1 and C4 Criegee intermediates in the ...atmosphere. Although ozonolysis can produce several distinct Criegee intermediates, the C1 stabilized Criegee (CH2OO, 61 ± 9%) is the only one observed to react bimolecularly. We suggest that the C4 Criegees have a low stabilization fraction and propose pathways for their decomposition. Both prompt and non-prompt reactions are important in the production of OH (28% ± 5%) and formaldehyde (81% ± 16%). The yields of unimolecular products (OH, formaldehyde, methacrolein (42 ± 6%) and methyl vinyl ketone (18 ± 6%)) are fairly insensitive to water, i.e., changes in yields in response to water vapor (≤4% absolute) are within the error of the analysis. We propose a comprehensive reaction mechanism that can be incorporated into atmospheric models, which reproduces laboratory data over a wide range of relative humidities. The mechanism proposes that CH2OO + H2O (k(H2O)∼ 1 × 10(-15) cm(3) molec(-1) s(-1)) yields 73% hydroxymethyl hydroperoxide (HMHP), 6% formaldehyde + H2O2, and 21% formic acid + H2O; and CH2OO + (H2O)2 (k(H2O)2∼ 1 × 10(-12) cm(3) molec(-1) s(-1)) yields 40% HMHP, 6% formaldehyde + H2O2, and 54% formic acid + H2O. Competitive rate determinations (kSO2/k(H2O)n=1,2∼ 2.2 (±0.3) × 10(4)) and field observations suggest that water vapor is a sink for greater than 98% of CH2OO in a Southeastern US forest, even during pollution episodes (SO2 ∼ 10 ppb). The importance of the CH2OO + (H2O)n reaction is demonstrated by high HMHP mixing ratios observed over the forest canopy. We find that CH2OO does not substantially affect the lifetime of SO2 or HCOOH in the Southeast US, e.g., CH2OO + SO2 reaction is a minor contribution (<6%) to sulfate formation. Extrapolating, these results imply that sulfate production by stabilized Criegees is likely unimportant in regions dominated by the reactivity of ozone with isoprene. In contrast, hydroperoxide, organic acid, and formaldehyde formation from isoprene ozonolysis in those areas may be significant.
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