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To better understand the effects of wildfires on air quality and climate, it is important to assess the occurrence of chromophoric compounds in smoke and characterize their optical properties. This study explores the molecular composition of light-absorbing organic aerosol, or brown carbon (BrC), sampled at the Missoula Fire Sciences laboratory as a part of the FIREX Fall 2016 lab intensive. A total of 12 biomass fuels from different plant types were tested, including gymnosperm (coniferous) and angiosperm (flowering) plants and different ecosystem components such as duff, litter, and canopy. Emitted biomass burning organic aerosol (BBOA) particles were collected onto Teflon filters and analyzed offline using high-performance liquid chromatography coupled to a photodiode array spectrophotometer and a high-resolution mass spectrometer (HPLC–PDA–HRMS). Separated BrC chromophores were classified by their retention times, absorption spectra, integrated absorbance in the near-UV and visible spectral range (300–700 nm), and chemical formulas from the accurate m∕z measurements. BrC chromophores were grouped into the following classes and subclasses: lignin-derived products, which include lignin pyrolysis products; distillation products, which include coumarins and flavonoids; nitroaromatics; and polycyclic aromatic hydrocarbons (PAHs). The observed classes and subclasses were common across most fuel types, although specific BrC chromophores varied based on plant type (gymnosperm or angiosperm) and ecosystem component(s) burned. To study the stability of the observed BrC compounds with respect to photodegradation, BBOA particle samples were irradiated directly on filters with near UV (300–400 nm) radiation, followed by extraction and HPLC–PDA–HRMS analysis. Lifetimes of individual BrC chromophores depended on the fuel type and the corresponding combustion condition. Lignin-derived and flavonoid classes of BrC generally had the longest lifetimes with respect to UV photodegradation. Moreover, lifetimes for the same type of BrC chromophores varied depending on biomass fuel and combustion conditions. While individual BrC chromophores disappeared on a timescale of several days, the overall light absorption by the sample persisted longer, presumably because the condensed-phase photochemical processes converted one set of chromophores into another without complete photobleaching or from undetected BrC chromophores that photobleached more slowly. To model the effect of BrC on climate, it is important to understand the change in the overall absorption coefficient with time. We measured the equivalent atmospheric lifetimes of the overall BrC absorption coefficient, which ranged from 10 to 41 d, with subalpine fir having the shortest lifetime and conifer canopies, i.e., juniper, having the longest lifetime. BrC emitted from biomass fuel loads encompassing multiple ecosystem components (litter, shrub, canopy) had absorption lifetimes on the lower end of the range. These results indicate that photobleaching of BBOA by condensed-phase photochemistry is relatively slow. Competing chemical aging mechanisms, such as heterogeneous oxidation by OH, may be more important for controlling the rate of BrC photobleaching in BBOA.

New techniques have recently been developed and applied to capture reactive nitrogen species, including nitrogen oxides ( NO x = NO + NO 2 ), nitrous acid (HONO), nitric acid ( HNO3 ), and particulate nitrate ( p NO 3 - ), for accurate measurement of their isotopic composition. Here, we report – for the first time – the isotopic composition of HONO from biomass burning (BB) emissions collected during the Fire Influence on Regional to Global Environments Experiment (FIREX, later evolved into FIREX-AQ) at the Missoula Fire Science Laboratory in the fall of 2016. We used our newly developed annular denuder system (ADS), which was verified to completely capture HONO associated with BB in comparison with four other high-time-resolution concentration measurement techniques, including mist chamber–ion chromatography (MC–IC), open-path Fourier transform infrared spectroscopy (OP-FTIR), cavity-enhanced spectroscopy (CES), and proton-transfer-reaction time-of-flight mass spectrometry (PTR-ToF). In 20 “stack” fires (direct emission within ∼5 s of production by the fire) that burned various biomass materials from the western US, δ15N – NOx ranges from −4.3 ‰ to +7.0 ‰, falling near the middle of the range reported in previous work. The first measurements of δ15N –HONO and δ18O –HONO in biomass burning smoke reveal a range of −5.3 ‰ to +5.8 ‰ and +5.2 ‰ to +15.2 ‰, respectively. Both HONO and NOx are sourced from N in the biomass fuel, and δ15N –HONO and δ15N – NOx are strongly correlated ( R2=0.89 , p ), suggesting HONO is directly formed via subsequent chain reactions of NOx emitted from biomass combustion. Only 5 of 20 p NO 3 - samples had a sufficient amount for isotopic analysis and showed δ15N and δ18O of p NO 3 - ranging from −10.6 ‰ to −7.4 ‰ and +11.5 ‰ to +14.8 ‰, respectively. Our δ15N of NOx , HONO, and p NO 3 - ranges can serve as important biomass burning source signatures, useful for constraining emissions of these species in environmental applications. The δ18O of HONO and NO 3 - obtained here verify that our method is capable of determining the oxygen isotopic composition in BB plumes. The δ18O values for both of these species reflect laboratory conditions (i.e., a lack of photochemistry) and would be expected to track with the influence of different oxidation pathways in real environments. The methods used in this study will be further applied in future field studies to quantitatively track reactive nitrogen cycling in fresh and aged western US wildfire plumes.

Biomass burning (BB) is a major source of black carbon (BC), but comparing BC content of different smoke-impacted air masses may be uncertain if different measurement techniques are used to quantify the BC, or if non-BC fractions influence a given measurement. To investigate these potential issues, five instruments reporting BC were compared in well-mixed smoke during the FIREX laboratory campaign in 2016, including two filter-based absorption instruments; one in situ absorption instrument; a laser-induced incandescence instrument; and a thermal-optical instrument. BB aerosols were generated using fuels common to wildfires in the Western US in a relatively controlled environment, with BC concentrations ranging from roughly 10–100 μg m−3 (55 total fires). Applying the Bland-Altman graphical approach, systematic biases and proportional biases were identified between the selected reference instrument (in situ absorption) and the other four instruments. BC emission factors (EFBC) derived from the thermal-optical instrument, laser-induced incandescence instrument, and filter-based absorption instruments were, on average, 83%, 39% and 66%, greater than the in situ absorption instrument, respectively. To understand why these differences exist, principal component analysis combined with a K-means clustering algorithm was implemented to group different fires into three clusters based on several co-dependent fire-related parameters (modified combustion efficiency (MCE), single scattering albedo (SSA) at 870 nm, organic carbon/elemental carbon ratio (OC/EC ratio), and absorption Angstrom exponents (AAE)); clusters are nominally referred to as “Black”, “Mixed”, and “Brown” based on the mean SSA and AAE values for each. The best agreement among all instruments was observed for the “Black” cluster (mean EFBC ratio = 1.89, for the fires with mean SSA = 0.31 and AAE = 1.44); this agreement worsened for the “Mixed” (mean EFBC ratio = 2.94, for the fires with mean SSA = 0.80 and AAE = 1.92) and “Brown” clusters (mean EFBC ratio = 3.12, for the fires with mean SSA = 0.96 and AAE = 2.50), likely due to the increased presence of externally (or internally) mixed aerosols that altered the chemical and optical properties of the aerosols. In general, the discrepancies observed among the BC instruments from this work agree with or slightly exceed the ones from previous ambient and laboratory studies. Care should be taken when interpreting different BC measurements in BB smoke because large artifacts can occur due to co-emitted materials.

In mid-August through mid-September of 2017 a major wildfire smoke and haze episode strongly impacted most of the NW US and SW Canada. During this period our ground-based site in Missoula, Montana, experienced heavy smoke impacts for ∼ 500 h (up to 471 µg m−3 hourly average PM2.5). We measured wildfire trace gases, PM2.5 (particulate matter ≤2.5 µm in diameter), and black carbon and submicron aerosol scattering and absorption at 870 and 401 nm. This may be the most extensive real-time data for these wildfire smoke properties to date. Our range of trace gas ratios for ΔNH3∕ΔCO and ΔC2H4∕ΔCO confirmed that the smoke from mixed, multiple sources varied in age from ∼ 2–3 h to ∼ 1–2 days. Our study-average ΔCH4∕ΔCO ratio (0.166±0.088) indicated a large contribution to the regional burden from inefficient smoldering combustion. Our ΔBC∕ΔCO ratio (0.0012±0.0005) for our ground site was moderately lower than observed in aircraft studies (∼ 0.0015) to date, also consistent with a relatively larger contribution from smoldering combustion. Our ΔBC∕ΔPM2.5 ratio (0.0095±0.0003) was consistent with the overwhelmingly non-BC (black carbon), mostly organic nature of the smoke observed in airborne studies of wildfire smoke to date. Smoldering combustion is usually associated with enhanced PM emissions, but our ΔPM2.5∕ΔCO ratio (0.126±0.002) was about half the ΔPM1.0∕ΔCO measured in fresh wildfire smoke from aircraft (∼ 0.266). Assuming PM2.5 is dominated by PM1, this suggests that aerosol evaporation, at least near the surface, can often reduce PM loading and its atmospheric/air-quality impacts on the timescale of several days. Much of the smoke was emitted late in the day, suggesting that nighttime processing would be important in the early evolution of smoke. The diurnal trends show brown carbon (BrC), PM2.5, and CO peaking in the early morning and BC peaking in the early evening. Over the course of 1 month, the average single scattering albedo for individual smoke peaks at 870 nm increased from ∼ 0.9 to ∼ 0.96. Bscat401∕Bscat870 was used as a proxy for the size and “photochemical age” of the smoke particles, with this interpretation being supported by the simultaneously observed ratios of reactive trace gases to CO. The size and age proxy implied that the Ångström absorption exponent decreased significantly after about 10 h of daytime smoke aging, consistent with the only airborne measurement of the BrC lifetime in an isolated plume. However, our results clearly show that non-BC absorption can be important in “typical” regional haze and moderately aged smoke, with BrC ostensibly accounting for about half the absorption at 401 nm on average for our entire data set.

Evaluating our understanding of smoke from wild and prescribed fires can benefit from downwind measurements that include both inert tracers to test production and transport and reactive species to ...

The chemical composition of aerosol particles is a key aspect in determining their impact on the environment. For example, nitrogen-containing particles impact atmospheric chemistry, air quality, and ecological N deposition. Instruments that measure total reactive nitrogen (Nr = all nitrogen compounds except for N2 and N2O) focus on gas-phase nitrogen and very few studies directly discuss the instrument capacity to measure the mass of Nr-containing particles. Here, we investigate the mass quantification of particle-bound nitrogen using a custom Nr system that involves total conversion to nitric oxide (NO) across platinum and molybdenum catalysts followed by NO−O3 chemiluminescence detection. We evaluate the particle conversion of the Nr instrument by comparing to mass-derived concentrations of size-selected and counted ammonium sulfate ((NH4)2SO4), ammonium nitrate (NH4NO3), ammonium chloride (NH4Cl), sodium nitrate (NaNO3), and ammonium oxalate ((NH4)2C2O4) particles determined using instruments that measure particle number and size. These measurements demonstrate Nr-particle conversion across the Nr catalysts that is independent of particle size with 98 ± 10 % efficiency for 100–600 nm particle diameters. We also show efficient conversion of particle-phase organic carbon species to CO2 across the instrument's platinum catalyst followed by a nondispersive infrared (NDIR) CO2 detector. However, the application of this method to the atmosphere presents a challenge due to the small signal above background at high ambient levels of common gas-phase carbon compounds (e.g., CO2). We show the Nr system is an accurate particle mass measurement method and demonstrate its ability to calibrate particle mass measurement instrumentation using single-component, laboratory-generated, Nr-containing particles below 2.5 µm in size. In addition we show agreement with mass measurements of an independently calibrated online particle-into-liquid sampler directly coupled to the electrospray ionization source of a quadrupole mass spectrometer (PILS–ESI/MS) sampling in the negative-ion mode. We obtain excellent correlations (R2 = 0.99) of particle mass measured as Nr with PILS–ESI/MS measurements converted to the corresponding particle anion mass (e.g., nitrate, sulfate, and chloride). The Nr and PILS–ESI/MS are shown to agree to within ∼ 6 % for particle mass loadings of up to 120 µg m−3. Consideration of all the sources of error in the PILS–ESI/MS technique yields an overall uncertainty of ±20 % for these single-component particle streams. These results demonstrate the Nr system is a reliable direct particle mass measurement technique that differs from other particle instrument calibration techniques that rely on knowledge of particle size, shape, density, and refractive index.

Western wildfires have a major impact on air quality in the US. In the fall of 2016, 107 test fires were burned in the large-scale combustion facility at the US Forest Service Missoula Fire Sciences Laboratory as part of the Fire Influence on Regional and Global Environments Experiment (FIREX). Canopy, litter, duff, dead wood, and other fuel components were burned in combinations that represented realistic fuel complexes for several important western US coniferous and chaparral ecosystems including ponderosa pine, Douglas fir, Engelmann spruce, lodgepole pine, subalpine fir, chamise, and manzanita. In addition, dung, Indonesian peat, and individual coniferous ecosystem fuel components were burned alone to investigate the effects of individual components (e.g., duff) and fuel chemistry on emissions. The smoke emissions were characterized by a large suite of state-of-the-art instruments. In this study we report emission factor (EF, grams of compound emitted per kilogram of fuel burned) measurements in fresh smoke of a diverse suite of critically important trace gases measured using open-path Fourier transform infrared spectroscopy (OP-FTIR). We also report aerosol optical properties (absorption EF; single-scattering albedo, SSA; and Ångström absorption exponent, AAE) as well as black carbon (BC) EF measured by photoacoustic extinctiometers (PAXs) at 870 and 401 nm. The average trace gas emissions were similar across the coniferous ecosystems tested and most of the variability observed in emissions could be attributed to differences in the consumption of components such as duff and litter, rather than the dominant tree species. Chaparral fuels produced lower EFs than mixed coniferous fuels for most trace gases except for NOx and acetylene. A careful comparison with available field measurements of wildfires confirms that several methods can be used to extract data representative of real wildfires from the FIREX laboratory fire data. This is especially valuable for species rarely or not yet measured in the field. For instance, the OP-FTIR data alone show that ammonia (1.62 g kg−1), acetic acid (2.41 g kg−1), nitrous acid (HONO, 0.61 g kg−1), and other trace gases such as glycolaldehyde (0.90 g kg−1) and formic acid (0.36 g kg−1) are significant emissions that were poorly characterized or not characterized for US wildfires in previous work. The PAX measurements show that the ratio of brown carbon (BrC) absorption to BC absorption is strongly dependent on modified combustion efficiency (MCE) and that BrC absorption is most dominant for combustion of duff (AAE 7.13) and rotten wood (AAE 4.60): fuels that are consumed in greater amounts during wildfires than prescribed fires. Coupling our laboratory data with field data suggests that fresh wildfire smoke typically has an EF for BC near 0.2 g kg−1, an SSA of ∼ 0.91, and an AAE of ∼ 3.50, with the latter implying that about 86 % of the aerosol absorption at 401 nm is due to BrC.

Particle morphology is an important parameter affecting aerosol optical properties that are relevant to climate and air quality, yet it is poorly constrained due to sparse in situ measurements. Biomass burning is a large source of aerosol that generates particles with different morphologies. Quantifying the optical contributions of non-spherical aerosol populations is critical for accurate radiative transfer models, and for correctly interpreting remote sensing data. We deployed a laser imaging nephelometer at the Missoula Fire Sciences Laboratory to sample biomass burning aerosol from controlled fires during the FIREX intensive laboratory study. The laser imaging nephelometer measures the unpolarized scattering phase function of an aerosol ensemble using diode lasers at 375 and 405 nm. Scattered light from the bulk aerosol in the instrument is imaged onto a charge-coupled device (CCD) using a wide-angle field-of-view lens, which allows for measurements at 4–175∘ scattering angle with ∼ 0.5∘ angular resolution. Along with a suite of other instruments, the laser imaging nephelometer sampled fresh smoke emissions both directly and after removal of volatile components with a thermodenuder at 250 ∘C. The total integrated aerosol scattering signal agreed with both a cavity ring-down photoacoustic spectrometer system and a traditional integrating nephelometer within instrumental uncertainties. We compare the measured scattering phase functions at 405 nm to theoretical models for spherical (Mie) and fractal (Rayleigh–Debye–Gans) particle morphologies based on the size distribution reported by an optical particle counter. Results from representative fires demonstrate that particle morphology can vary dramatically for different fuel types. In some cases, the measured phase function cannot be described using Mie theory. This study demonstrates the capabilities of the laser imaging nephelometer instrument to provide realtime, in situ information about dominant particle morphology, which is vital for understanding remote sensing data and accurately describing the aerosol population in radiative transfer calculations.

The light-absorbing organic aerosol (OA) commonly referred to as “brown carbon” (BrC) has attracted considerable attention in recent years because of its potential to affect atmospheric radiation balance, especially in the ultraviolet region and thus impact photochemical processes. A growing amount of data has indicated that BrC is prevalent in the atmosphere, which has motivated numerous laboratory and field studies; however, our understanding of the relationship between the chemical composition and optical properties of BrC remains limited. We conducted chamber experiments to investigate the effect of various volatile organic carbon (VOC) precursors, NOx concentrations, photolysis time, and relative humidity (RH) on the light absorption of selected secondary organic aerosols (SOA). Light absorption of chamber-generated SOA samples, especially aromatic SOA, was found to increase with NOx concentration, at moderate RH, and for the shortest photolysis aging times. The highest mass absorption coefficient (MAC) value is observed from toluene SOA products formed under high-NOx conditions at moderate RH, in which nitro-aromatics were previously identified as the major light-absorbing compounds. BrC light absorption is observed to decrease with photolysis time, correlated with a decline of the organic nitrate fraction of SOA. SOA formed from mixtures of aromatics and isoprene absorb less visible (Vis) and ultraviolet (UV) light than SOA formed from aromatic precursors alone on a mass basis. However, the mixed SOA absorption was underestimated when optical properties were predicted using a two-product SOA formation model, as done in many current climate models. Further investigation, including analysis on detailed mechanisms, are required to explain the discrepancy.

The chemical composition of aerosol particles is a key aspect in determining their impact on the environment. For example, nitrogen (N)-containing particles impact atmospheric chemistry, air quality, and ecological N-deposition. Instruments that measure total reactive nitrogen (N r = all nitrogen compounds except for N 2 and N 2 O) focus on gas-phase nitrogen and very few studies directly discuss the instrument capacity to measure the mass of N r –containing particles. Here, we investigate the mass quantification of particle-bound nitrogen using a custom N r system that involves total conversion to nitric oxide (NO) across platinum and molybdenum catalysts followed by NO-O 3 chemiluminescence detection. We evaluate the particle conversion of the N r instrument by comparing to mass derived concentrations of size-selected and counted ammonium sulfate ((NH 4 ) 2 SO 4 ), ammonium nitrate (NH 4 NO 3 ), ammonium chloride (NH 4 Cl), sodium nitrate (NaNO 3 ), and ammonium oxalate ((NH 4 ) 2 C 2 O 4 ) particles determined using instruments that measure particle number and size. These measurements demonstrate N r -particle conversion across the N r catalysts that is independent of particle size with 98 ± 10 % efficiency for 100–600 nm particle diameters. We also show conversion of particle-phase organic carbon species to CO 2 across the instrument’s platinum catalyst followed by a non-dispersive infrared (NDIR) CO 2 detector. We show the N r system is an accurate particle mass measurement method and demonstrate its ability to calibrate particle mass measurement instrumentation using single component, laboratory generated, N r -containing particles below 2.5 µm in size. In addition we show agreement with mass measurements of an independently calibrated on-line particle-into-liquid sampler directly coupled to the electrospray ionization source of a quadrupole mass spectrometer (PILS-ESI/MS) sampling in the negative ion mode. We obtain excellent correlations (R 2 = 0.99) of particle mass measured as N r with PILS-ESI/MS measurements converted to the corresponding particle anion mass (e.g. nitrate, sulfate, and chloride). The N r and PILS-ESI/MS are shown to agree to within ~ 6 % for particle mass loadings up to 120 µg m −3 . Consideration of all the sources of error in the PILS-ESI/MS technique yields an overall uncertainty of ±20 % for these single component particle streams. These results demonstrate the N r system is a reliable direct particle mass measurement technique that differs from other particle instrument calibration techniques that rely on knowledge of particle size, shape, density, and refractive index.

Wildfires emit significant amounts of pollutants that degrade air quality. Plumes from three wildfires in the western U.S. were measured from aircraft during the Studies of Emissions and Atmospheric Composition, Clouds and Climate Coupling by Regional Surveys (SEAC4RS) and the Biomass Burning Observation Project (BBOP), both in summer 2013. This study reports an extensive set of emission factors (EFs) for over 80 gases and 5 components of submicron particulate matter (PM1) from these temperate wildfires. These include rarely, or never before, measured oxygenated volatile organic compounds and multifunctional organic nitrates. The observed EFs are compared with previous measurements of temperate wildfires, boreal forest fires, and temperate prescribed fires. The wildfires emitted high amounts of PM1 (with organic aerosol (OA) dominating the mass) with an average EF that is more than 2 times the EFs for prescribed fires. The measured EFs were used to estimate the annual wildfire emissions of carbon monoxide, nitrogen oxides, total nonmethane organic compounds, and PM1 from 11 western U.S. states. The estimated gas emissions are generally comparable with the 2011 National Emissions Inventory (NEI). However, our PM1 emission estimate (1530 ± 570 Gg yr−1) is over 3 times that of the NEI PM2.5 estimate and is also higher than the PM2.5 emitted from all other sources in these states in the NEI. This study indicates that the source of OA from biomass burning in the western states is significantly underestimated. In addition, our results indicate that prescribed burning may be an effective method to reduce fine particle emissions. ©2017. American Geophysical Union. All Rights Reserved.