NMOG measurements were conducted using a high-resolution proton transfer reaction time-of-flight mass spectrometer PTR-ToF-MS, and an iodide-clustering time-of-flight chemical ionization mass spectrometer (I-CIMS, Aerodyne/Tofwerk, AG). The instruments were deployed to measure primary smoke emissions from the stack and room, as well as aged emissions from the mini-chamber. During a burn (and while the mini-chamber was filling with smoke), both instruments sampled from the stack to characterize primary NMOG emissions. At the end of a burn, the sampling lines were switched and the instruments sampled from the mini-chamber through a Teflon inlet that is 10 m in length and has an outer diameter of 3 mm at a total flow rate of ∼3 L min-1. A full description of the primary NMOG measurements is provided elsewhere, and only a brief description of these measurements is provided here .

The PTR-ToF-MS measured at 1 Hz to capture the decay of primary NMOGs and formation of secondary species, respectively. The drift tube was operated with an electric field to number density ratio (E/N) of 120 Td, and the high-resolution mass spectrometer (max resolution ∼4500) scanned ions with m/z 12–500 Th. The mass spectrometer resolves the molecular formulae of isobaric species but cannot distinguish isomers. This presents challenges in reacting systems as secondary NMOGs formed by OH oxidation could have the same molecular formula as primary NMOGs. identified the distribution of primary NMOGs during FIREX-AQ using gas chromatography pre-separation to measure isomer contributions. For most fuel types, over 90 % of the PTR-ToF-MS signal could be assigned. The primary groups detected by PTR-ToF-MS were small oxygenates (∼50 % v/v, dominated by acetic acid, formaldehyde, methanol, and acetaldehyde), aromatics (∼10 %, dominated by catechol, phenol, methoxy phenols, benzene, and toluene), furans (∼10 %, dominated by 5-methylfurfural, 2-furfural, furanone, furan, and methylfurans + dimethylfurans), and a broad range of hydrocarbons (∼15 %, dominated by ethene, propene, butene, and 1,3-butadiene). The same NMOG assignments and sensitivities are applied here to masses observed to be enhanced prior to NMOG oxidation. The temporal profile of some larger primary oxygenates (C>5) were also influenced by the formation of secondary isomers or unidentified primary species (see Sect. ). Calibration factors for primary species are calculated from measured or estimated proton transfer rate constants uncertainty < 50 %,.

Masses detected after the initiation of photochemistry are assigned based on previous literature observations and modeling evidence (see Sect. ). A number of laboratory and field studies employing PTR-ToF-MS and open-path Fourier transform infrared spectrometer (OP-FTIR) have observed formation of formic acid (CH2O2-H+, m/z 47), acetic acid (C2H4O2-H+, m/z 61), maleic anhydride (C4H2O3-H+, m/z 99), and phthalic anhydride (C8H4O4-H+, m/z 149) in aged smoke . Calibration factors for these species are calculated from measured or estimated proton transfer rate constants. In this study, substantial increases in C4H4O3-H+ (m/z 101) and C4H8O2-H+ (m/z 89) are also observed. C4H4O3-H+ could be 5-hydroxy-2(5H)-furanone (or simply hydroxy furanone), its tautomer malealdehydic acid, or succinic anhydride, whereas C4H8O2-H+ is likely a C4 hydroxy carbonyl see Sect.  and). Due to uncertainties in these assignments, calibration factors for these species are calculated using estimated proton transfer rate constants derived from molecular formula relationships uncertainty to within a factor of 2,.

The I-CIMS utilizes a “soft” chemical ionization source that forms iodide clusters with polarizable analyte molecules . The instrument used here was operated in a similar configuration to that described in . To generate reagent ions, 2 SLPM (standard liter per minute) of clean N2 from dewar blow-off was run over a methyl iodide permeation tube and ionized using a Polonium-210 ionizer and into an ion molecule reaction region (IMR). The I-CIMS measured gas-phase signals from the mini-chamber at 1 Hz time resolution. Smoke was diluted with 4 L min-1 of clean, humidified air at a 4:1 ratio to minimize reagent ion depletion. A constant flow of isotopically labeled formic acid was delivered to the instrument to measure consistency of response. Reported here are I-CIMS measurements normalized to 1×106 counts per second of the reagent ion signal at m/z 126.905 (normalized counts per second, ncps). Due to unavailability of standards, I-CIMS data are not reported in mixing ratios. Secondary NMOGs measured by I-CIMS are assigned identifications based on modeling results and previous literature.

NO, NO2, and HONO were measured by an OP-FTIR, as described by . The OP-FTIR was located on the platform and sampled smoke across the diameter of the stack. The OP-FTIR provides fast measurements and avoids potential sampling artifacts due to sample line losses. OP-FTIR measurements were used for most experiments. NO2 and HONO were also measured by the NOAA Airborne Cavity Enhanced Spectrometer (ACES), as described by . ACES was located on the platform and sampled smoke through a 1 m Teflon inlet. These data were used when OP-FTIR data were unavailable, or when NOx emissions were below OP-FTIR detection limits. Supplementary NO measurements were provided by a custom-built chemiluminescence instrument located in a room on the burn chamber floor. A full description of that instrument is provided by .

The box model described in Sect.  employs NMOG chemistry based on the Master Chemical Mechanism MCM v. 3.3.1,. The MCM explicitly represents the chemistry of biogenic, alkyl, aromatic, and oxygenated aromatic species. The laboratory measurements described in Sect. 3.1 demonstrate that heterocyclic hydrocarbons, such as the furans, could significantly contribute to secondary NMOG formation. The following discussion motivates and describes mechanism development aimed at expanding the MCM representation of biomass burning OH chemistry.

Figure  shows the breakdown in OH reactivity of the primary NMOGs measured by PTR-ToF-MS during the Firelab study . Each bar represents a fraction of the total calculated OH reactivity, as represented by Eq. (). 2fOHR,i=ki×Ci∑inki×Ci, where i is the species of interest, n is the number of species measured by the PTR-ToF-MS, k is the OH rate constant, and C is the average concentration measured during a burn. Shown are the median, average, 25th, and 75th percentiles for all the burns reported by . The color of each bar indicates if a compound is included or missing from the MCM. Species identifications, and the isomer contributions to each detected mass, were determined by for four fuel types (Douglas fir, Engelmann spruce duff, subalpine fir, and sage) using gas chromatography pre-separation (GC-PTR-ToF-MS). Additional evidence for species identification was provided by other measurement techniques (e.g., I-CIMS, gas chromatography electron-impact mass spectrometry, and OP-FTIR). On average, the isomer contribution to a given mass measured by PTR-ToF-MS varied by only 11 %. To be consistent with , it is assumed that the NMOG contribution to each mass detected by PTR-ToF-MS follows the average distribution measured by GC-PTR-ToF-MS.

For most NMOGs measured by PTR-ToF-MS, the contribution to total primary OH reactivity varied by only 25 %. Notably, the contribution from the sum of monoterpenes varied by a factor > 2. Monoterpenes, as well as isoprene and sesquiterpenes, were primarily emitted at the beginning of an experiment, prior to combustion, due to distillation processes associated with fuel heating . This “distillation phase” was most pronounced in fires containing greater amounts of canopy material, or fuel types known to be strong monoterpene emitters (e.g., pines). Other NMOGs were primarily emitted due to pyrolysis processes. For example, found that the proportions of NMOGs emitted during low- and high-temperature pyrolysis did not strongly vary by fuel type. In ambient fires, the contribution of monoterpenes to the total OH reactivity will likely differ from the contributions reported here, owing to the different burning process by which monoterpenes and other NMOGs are emitted. In this study, the primary monoterpene isomers measured by GC-PTR-ToF-MS for Engelmann spruce, Douglas fir, and subalpine fir were camphene, α-pinene, β-pinene, 3-carene, and limonene, followed by smaller amounts of tricyclene and α-terpinene (Fig. S3). No other NMOGs detected by the GC-PTR-ToF-MS produced signals at m/z 137, which is the primary ion used to quantify monoterpene emissions Fig. S3 and.

The monoterpene distribution for Engelmann spruce (F26) was explicitly measured by GC-PTR-ToF-MS, but this was not the case for ponderosa pine (F38). To account for differences in monoterpene reactivity, the sum of monoterpenes for F26 is speciated using the distribution reported in Fig. S3a. For F38, monoterpenes are speciated using the ponderosa pine distribution reported by . Engelmann spruce smoke measured by GC-PTR-ToF-MS had a monoterpene distribution that was 25 % α-pinene, 21 % β-pinene, 25 % 3-carene, 15 % α-terpinene, and ∼5 % each of limonene and camphene. reports that the monoterpenes from ponderosa pine smoke are 10 % α-pinene and 20 % each for β-pinene, 3-carene, myrcene, and limonene. The MCM represents the chemistry of α-pinene, β-pinene, and limonene but does not explicitly describe other important monoterpenes (e.g., 3-carene or camphene). For the mini-chamber model, the fraction attributed to α-pinene, β-pinene, and limonene are explicitly prescribed. The fraction of 3-carene, α-terpinene, and smaller monoterpenes are lumped to α-pinene (endocyclic double bond) and the fractions associated with myrcene + camphene are lumped to β-pinene (exocyclic double bond).

On average, the MCM v. 3.3.1 captures only ∼60 % of the primary OH reactivity measured by the PTR-ToF-MS. The MCM generally lacks information about furan species and substituted aromatics, such as guaiacol and methyl guaiacol. Previous work has shown that furans constitute a significant fraction of the total primary OH reactivity e.g.,; however, no studies have included the known mechanisms of these species when modeling biomass burning smoke chemistry. In this study, 5-methylfurfural and 2,5-dimethylfuran represent the two largest contributors to this “missing reactivity” and account for nearly 10 % of the calculated total reactivity. Up to 75 % of the calculated primary OH reactivity can be accounted for by including the chemistry of furan, 2-methylfuran, 2,5-dimethylfuran, furfural, 5-methylfurfural, and guaiacol, along with the known species represented in the MCM. Much of the remaining reactivity is tied into species whose chemistry has not been extensively studied, including less abundant furans and oxygenated aromatics.

The mechanisms of select furan and oxygenated species are incorporated to the MCM based on previous work summarized in Figs. S4–S10 . In total, 65 reactions are added. The products resulting from the OH oxidation of furan and 2-methylfuran were first investigated by and later generalized to 3-methylfuran, 2,3-dimethylfuran, and 2,5-dimethylfuran by and . Figure  shows the generalized furan oxidation scheme. Furan oxidation is initiated by an OH addition to the 2, 5, or 3 position. Addition to the 2 or 5 position is most favorable and results in pathways where substituents are either retained or lost (henceforth referred to as the loss and retention pathways). The retention pathway (path a in Fig. ) leads to reactive unsaturated 1,4-dicarbonyls (e.g., 1,4-butenedial from furan oxidation), whereas the loss pathway (paths b1 and b2 in Fig. ) results in the formation of hydroxy furanones and unsaturated carbonyl acids . The loss pathway becomes more dominant with higher number of substituted methyl groups.This study assumes branching ratios of [0.7 (a), 0.3 (b1)] for furan, [0.31 (a), 0.39 (b1), 0.31 (b2)] for 2-methylfuran, and [0.27 (a), 0.73 (b1)] for 2,5-dimethylfuran .

RO2 reactions leading to the formation of carbonyls are implemented based on the mechanisms proposed by and (Figs. S4–S6). It is assumed that RO2 species undergo reactions with HO2, NO, and other RO2 radicals. Other pathways, such as RO2+NO2 and RO2+NO3, are not included; however, these reactions could be important for acyl RO2 species . For RO2+NO reactions, it is assumed that the alkoxy radical quickly decays (either by thermal degradation or reaction with O2) to form carbonyls. did not report species consistent with alkoxy isomerization; thus, these reactions are ignored. Similarly, RO2+RO2 reactions are assumed to only form alkoxy radicals, which may subsequently degrade to form carbonyls. Peroxides are assumed to be the only products of RO2+HO2 reactions. These species are assumed to undergo photolysis to form carbonyls. Peroxides may also react with OH, and the resulting products differ depending on structure. For structures with an alpha hydrogen, it is assumed that OH abstracts at the alpha position, and that the resulting radical quickly decomposes to form a carbonyl + OH (e.g., HYDFURANOOH, Fig. S4). For other structures, it is assumed that the hydrogen of the peroxide group is abstracted to regenerate the RO2 radical.

The generic MCM rate constants are applied for RO2+HO2 and RO2+NO reactions (kRO2NO=2.7×10-12exp⁡(360/T) cm3 molec.-1 s-1, kRO2HO2=2.91×10-13exp⁡(1300/T)[1-exp⁡(-0.245n)] cm3 molec. -1 s-1). The RO2+HO2 rate constant is adjusted for the RO2 carbon number, n, as recommended by . Photolysis frequencies for peroxides are assumed to be the same as for methyl hydroperoxide (J41 in the MCM), and peroxide + OH reactions are assumed to have a rate constant of 4×10-11 cm3 molec.-1 s-1. The assumed rate constants for RO2+RO2 reactions are chosen based on those of structurally similar RO2 radicals reported in the MCM. RO2 H-shift isomerization (autoxidation) is not broadly represented in MCM v. 3.3.1, aside for isoprene oxidation . RO2 isomerization becomes competitive when the bimolecular lifetime of RO2 is on the order of 10 s . Based on the modeled concentrations of HO2, NO, and RO2, the bimolecular lifetime of RO2 radicals from furan oxidation is estimated to be ∼10 s; consequently, RO2 isomerization could play a role for certain species.

Following RO2 reaction, it is assumed that the second-generation products continue through the chemistry prescribed by MCM v. 3.3.1. Unsaturated dicarbonyls, such as 1,4-butenedial, and the tautomers of hydroxy furanones are represented in the MCM (Figs. S4–S6). Here, it is assumed that hydroxy furanones undergo the same reactions as the corresponding tautomer, which ultimately leads to anhydride formation. Maleic anhydride is a multigenerational product in the OH oxidation of furan and is considered to be a significant product of hydroxy furanone oxidation .

No experimental studies have evaluated the OH oxidation mechanism of furfural or 5-methylfurfural. found via theoretical quantum chemistry calculations that OH likely adds to the 2 or 5 position or abstracts a hydrogen from the aldehyde. The resulting reactions follow loss and retention pathways similar to the general mechanism for methyl-substituted furans (Fig. ). When OH adds to the 2 position, the ring most likely opens to form an unsaturated tri-carbonyl (retention, path a). When OH adds to the 5 position, the resulting peroxy radical may react with HO2, NO, or other RO2 species to form a hydroxy furanone / carbonyl acid mixture (loss, path b). Hydrogen abstraction from the aldehyde group is believed to ultimately result in the formation of maleic anhydride (loss, path c). estimate furfural + OH branching ratios of 0.37 for channel (a), 0.6 for channel (b), and 0.03 for channel (c). The same branching ratios are applied here, but a discussion of secondary NMOG sensitivity to the assumed furfural mechanism is provided in the Supplement. The 5-methylfurfural + OH mechanism has not been studied and is assumed to have branching ratios similar to furfural. RO2 reactions are implemented based on the mechanisms proposed by (see Figs. S7–S9).

Furfural strongly absorbs at 185 and 254 nm . Smaller amounts may be lost by photolysis at wavelengths > 300 nm . Approximately 15 % of furfural photolysis leads to the formation of furan and CO, while the remaining percentage results in the formation of propyne, CO, and other C3 compounds . The photolysis frequency is calculated based on the cross sections reported by and quantum yield of 0.6 (Vassilis Papadimitriou, personal communication, 2018).

Few studies have evaluated the OH oxidation mechanism of guaiacol. identified products from low-NOx guaiacol oxidation but did not calculate product yields. studied guaiacol oxidation in the presence of NOx and observed substantial SOA formation. The only reported gas-phase species were a suite of nitroguaiacols composed primarily of 3- and 6-nitroguaiacol (6 % yield) and 4-nitroguaiacol (10 % yield). The mechanism proposed by is applied here assuming a 16 % yield of nitroguaiacol species (Fig. S10).

The OH loss of butanol-d9 is also included in the model to validate OH concentrations in the chamber. The OH oxidation of butanol-d9 is assumed to form a single, nonreactive species. The entire mechanism used to model the mini-chamber and ambient biomass burning plume is provided in the Supplement.

Figure  shows the model comparison with PTR-ToF-MS measurements of butanol-d9 and select primary NMOGs for F38. Overall, there is good agreement between the measurements and model output for most NMOGs (exceptions include some primary species and small secondary oxygenates, discussed below). The excellent agreement between the measured and modeled loss of butanol-d9 demonstrates that OH concentrations in the chamber are well-represented by the model. Similar agreement is observed for F26, which had an initial NOx/NMOG ratio that was an order of magnitude lower than that of F38 (Fig. S11). Figure  shows the model output compared to the observed profiles of secondary NMOGs measured during F38. The equivalent for F26 is presented in Fig. S12. Figure a shows measurements of C4H2O3 (maleic anhydride) as measured by PTR-ToF-MS, whereas Fig. b and c show I-CIMS measurements of C5H6O3 and C4H4O3, respectively. The measured secondary NMOGs are compared to model outputs of total C4H2O3 (i.e., the sum of all species with molecular formula C4H2O3), total C5H6O3, total C4H4O3, and individual NMOGs. Figure  also show model runs with the initial conditions of furan, 2-methylfuran, 2,5-dimethylfuran, furfural, 5-methylfurfural, and furanone set to zero. The PTR-ToF-MS measurements of C4H2O3 are reported in units of ppb and can therefore be quantitatively compared to model output. I-CIMS measurements are reported as ncps; consequently, only qualitative comparisons are drawn based on similarities in model and measurement temporal profiles.

C4H2O3, C5H6O3, and C4H4O3 were shown in Sect.  to constitute some of the most abundant photochemical products observed by PTR-ToF-MS and I-CIMS. The model runs in Figs.  and S12 demonstrate that furan chemistry significantly contributes to the modeled formation of these secondary NMOGs. The model also supports the inference that these masses correspond to measurements of maleic anhydride, methyl hydroxy furanone, and hydroxy furanone, respectively. This is most evident by comparing the shape of the temporal profiles between the measurements and modeled output. Maleic anhydride is the only species in the MCM with chemical formula C4H4O3, and the model generally captures the peak C4H4O3 signal after ∼20 min of oxidation to within the uncertainty of the measurement (∼50 %). Several species with chemical formula C5H6O3 are represented in the MCM. Here, the modeling output is dominated by methyl hydroxy furanone and its tautomer, β-acetylacrylic acid. Finally, I-CIMS measurements of C4H4O3 are best captured by the model output of hydroxy furanone and its tautomer, malealdehydic acid.

The temporal profile of C7H7NO4 is also well-described by model output of nitroguaiacol (Fig. S13). Nitroaromatics are formed by the reaction of NO2 with the o-semiquinone radical resulting from OH abstraction of the phenolic hydrogen . NO2 was abundant at the beginning of mini-chamber experiments initialized with NMOGs resulting from flaming emissions (Fig. S2); thus, nitroaromatics are expected to be present in many burns studied here. Note that only the results from F38 are shown. Very little formation of nitroguaiacol was observed in F26, owing to the relatively low amount of NOx emitted from the smoldering combustion of Engelmann spruce duff.

The model provides insights into the formation pathways of important secondary NMOGs, which could be used to place constraints on plume properties. For example, measurements of maleic anhydride, hydroxy furanone, and methyl hydroxy furanone could be used as proxies to estimate plume age since the furanones will likely be enhanced in younger plumes, whereas maleic anhydride will likely be enhanced in aged plumes. In plumes containing high proportions of furans, it could be feasible to evaluate furan chemistry to derive important modeling constraints, such as OH exposures.

Despite the success of the model in reproducing a number of observations, several differences exist, as described below. First, the model output exhibits a faster decay of furans than what is observed by PTR-ToF-MS (Fig. ). This likely reflects the uncertainty associated with the mass assignment of furan species. Using GC-PTR-ToF-MS, showed that nearly 50 % of the primary signal at C5H6O-H+ (mass of 2-methylfuran) and C6H8O-H+ (mass of 2,5-dimethylfuran) is associated with unidentified oxygenates. These unidentified species likely have different reactivities towards OH which may lead to model and observation disagreement. Likewise, secondary NMOG isomers could also form at these masses, further confounding model predictions of furans.

The decay of furfural is distinct from other furans because several processes contribute to temporal profile of C5H4O4-H+. Notably, there appears to be a fast decay of C5H4O4-H+, followed by a slower decay after 10s of oxidation. The model generally captures the fast decay of C5H4O4-H+, which is almost entirely due to photolysis of furfural. This degree of photolysis is different from the real atmosphere and results from furfural's exceptionally large cross section at 254 nm >  5×10-17 cm2,. The slower decay appears to result from an interference of another NMOG. This is supported by the I-CIMS, which measured the formation of a mass with formula C5H4O4-I-. The I-CIMS is not sensitive to primary furan species, therefore this species is likely to be a secondary NMOG that is isomeric with furfural. This may explain why PTR-ToF-MS measurements of C5H4O4-H+ do not quickly decay to zero, as suggested by the model. It is notable that the formation of this secondary species is significant (∼50 % of the signal of primary furfural), which indicates that this is likely a secondary product formed from an abundant primary NMOG.

Despite the complications imposed by furfural photolysis, other furans and oxygenated aromatics do not exhibit strong absorption and are expected to be lost mostly by reaction with OH. Other absorbing species, such as methyl ethyl ketone and benzaldehyde, exhibit modeled photolysis losses on the order of 30 %, which is likely a more typical fraction for other photo-active species. More details comparing the chamber results to chemistry of ambient biomass burning plumes are provided in Sect. .

At the beginning of each experiment, PTR-ToF-MS measurements show a sharp increase in C4H2O3 that is not readily captured by the model (Fig. ). This increase could result from fast formation of maleic anhydride, or is possibly another species with molecular formula C4H2O3. The model underpredicts maleic anhydride mixing ratios towards the end of the experiment, which likely points to additional sources of maleic anhydride that are not included in the model. The model reproduces peak maleic anhydride mixing ratios in F38 (Fig. a) but overpredicts peak maleic anhydride mixing ratios by a factor of 1.6 in F26 (Fig. S12a).

After ∼30 min of oxidation, the model sum of C4H4O3 approaches zero, yet the I-CIMS signal remains elevated (Fig. c). This may indicate that a slow-forming product is detected by I-CIMS, or that the OH rate constant of hydroxy furanone is overestimated. As discussed in Sect. , the PTR-ToF-MS detects a slow-forming product that likely corresponds to succinic anhydride (Fig. ). The I-CIMS is sensitive to anhydrides; thus, it is possible that the elevated signal at longer oxidation timescales corresponds to the measurement of succinic anhydride. Succinic anhydride is not represented in the customized mechanism; consequently, no model output is available for comparison.

Figure  shows that small oxygenates are also abundant secondary NMOGs (i.e., acetaldehyde and formaldehyde); however, these species are underpredicted by the model by a factor of 10 or more (Fig. S13). This likely reflects additional chemical precursors or chemical pathways that are unaccounted for within the mini-chamber model. Furthermore, heterogeneous reactions, such as those on aerosol particles or Teflon surfaces e.g., or photolysis at 254 nm may contribute to the formation of these small oxygenates.

It is noted that without calibrated I-CIMS data it is difficult to assess whether the budget of these secondary NMOGs is fully represented by the model. Although it is expected that furans will be a primary precursor of C4H4O3 and C5H6O3 in real biomass burning plumes, the model indicates that other highly reactive species may also contribute to these masses. In F38, furans account for ∼80 % of the modeled production of C4H4O3 and ∼90 % of the modeled production of C5H6O3. In contrast, furans account for ∼60 % of the modeled production of C5H6O3 and ∼85 % of the modeled production of C4H4O3 during F26. The remaining production in the model is attributed to the OH oxidation of oxygenated aromatics – specifically, phenol and cresol. These oxygenated aromatics are more abundant during F26 due to the higher degree of smoldering combustion. These differences highlight the variability of secondary NMOG production and also imply that there could be remaining precursors of C4H4O3 and C5H6O3. These precursors would have to be highly reactive molecules with a carbon number of ≥4. There are a number of furans and oxygenated aromatics that could possibly contribute to the formation of these secondary NMOGs (e.g., hydroxymethylfurfural, Fig. ) but whose chemical mechanism remains unknown.

The high-OH environment in the mini-chamber is similar to those produced in oxidation flow reactors. showed that oxidation flow reactors can be operated under conditions that approximate the chemistry of the atmosphere; however, the use of 254 nm light can lead to non-atmospheric reactions. For example, furfural photolysis is unlikely to play a significant role in the chemistry of real smoke, and NOx cycling is faster under ambient photolysis. Furthermore, high-OH environments may lead to RO2 fates that differ from ambient systems. In the atmosphere, the predominant fate of RO2 is reaction with NO or HO2. In wildfires, RO2 reactions with NO2 are also important in forming PAN and other peroxy nitrates . In low-NOx environments, RO2+RO2 and RO2 isomerization may also play a role . RO2 isomerization was not included in the mechanism described here. The following discussion compares the modeled chemistry in the chamber to that expected for an ambient biomass burning plume.

In the atmosphere, primary NMOGs are mostly consumed by reaction with OH or via photolysis during the daytime, and reaction with O3 or NO3 at night. Table S4 shows the estimated contribution of each process to the primary NMOGs measured during F26 and F38. Also shown are the NMOG losses calculated from simulations of the ambient biomass burning plume described by ; see Sect.  for details of this modeling. For most species, the predominant loss pathway is reaction with OH. Ozonolysis is negligible, except for a small fraction of the monoterpenes (∼1 %). On the other hand, significant NMOG losses occur by photolysis, and to a lesser extent, reaction by NO3. Aside from furfural, loss by photolysis was dominant for acetone and 2,3-butanedione (> 50 %), significant for methyl ethyl ketone and benzaldehyde (∼30 %), and moderate for hydroxyacetone, glyoxal, methyl glyoxal, formaldehyde, and acetaldehyde (< 10 %). In general, photolysis losses were dependent on the relative ratio between the OH rate constant and photolysis frequency at 254 nm (Table S3). Photolysis losses were greatest for conjugated aldehydes and species with low OH rate constants and high-absorption cross sections. (e.g., acetone, Table S3). Conjugated aldehydes such as furfural are highly reactive towards OH; consequently, losses due to photolysis are notable since these processes represent unintended sinks of potentially important SOA and ozone precursors. In contrast, photolysis losses of other species, such as acetone, are likely less important since these species are less reactive towards OH. It is noted that other species reported here are likely to photolyze, but their absorption cross sections at 254 nm have not been measured e.g., 5-methylfurfural,.

Because numerous species absorb across a wide wavelength spectrum e.g., acetaldehyde and hydroxyacetone, there is some agreement between the photolysis losses estimated in the mini-chamber with those expected under ambient conditions. For hydroxyacetone, 2,3-butanedione, and acetaldehyde, reaction by photolysis was comparable to what was estimated for the ambient biomass burning plume described by . In contrast, conjugated aldehydes, such as furfural and benzaldehyde, are characterized with absorption cross sections that favor shorter wavelengths; consequently, photolysis losses in the mini-chamber greatly exceed those expected in ambient plumes. These results highlight the challenges associated with studying multiday oxidation of biomass burning smoke in environmental chambers. Biomass burning emissions contain a myriad of functionalized NMOGs that readily photolyze at wavelengths required to generate high-OH environments. Experimental setups employing UVC lights must weigh the options of operating at high OH exposures (and thus progressing through chemistry quickly) or operating at gradual OH exposures that allow for longer sampling but higher exposures to UVC light. Similar considerations are made for oxidation flow reactors, although nearly all experiments are conducted at high-OH exposures since sampling is conducted at pseudo-steady-state . To avoid high photolysis exposures, future experiments employing 254 nm light may consider operating with higher ozone mixing ratios to increase the losses due to reaction by OH. Alternatively, chamber experiments operated at high relative humidity may employ 185 nm light to generate high-OH environments . In both cases, the chemistry will progress quickly, which may be undesirable for chamber experiments using low time resolution instrumentation. Another approach may be to use UVB or UVA lights and photolyze HONO to generate OH. This approach will reduce photolysis exposures but may only access 1–2 d of atmospheric-equivalent oxidation.

Unlike primary emissions that are dependent on the balance between photolysis and oxidant concentrations, the formation of secondary NMOGs largely depends on the fate of the RO2 radical. Figure a and b show a breakdown of the modeled RO2 pathways that contributed to the chemistry of the mini-chamber. The bars show the fraction RO2 radicals, separated by carbon number, that reacted through RO2+NO, RO2+HO2, RO2+NO2, and RO2+RO2 pathways. Figure c shows the breakdown of RO2 pathways for simulations of the ambient biomass burning plume.

RO2 radicals in F38 largely reacted through two pathways: RO2+HO2 and RO2+NO2. The RO2+NO2 pathway primarily influenced the fate of smaller RO2 radicals (C≤3), leading to the formation of peroxy nitrates (specifically, PAN). For larger RO2 radicals (C≥4), the dominant pathway was RO2+HO2. On the other hand, the model suggests that the RO2+RO2 pathway played a significant role in the chemistry of F26. This results, in part, from the higher initial NMOG loading for F26 (∼280 ppb) compared to that of F38 (∼180 ppb), which enhanced the rate of RO2 production. The model suggests that the relative contribution of RO2 cross-reactions was lower for higher-carbon species; however, it is possible that these reactions produced accretion products unlikely to be found under ambient conditions. As discussed in Sect.  and shown in Fig. S2, F26 was not representative of most fires studied here. For the majority of fires presented in Figs.  and , the secondary NMOGs were likely formed through the pathways consistent with F38.

In general, most of the higher-carbon species in F38 followed atmospherically relevant pathways. For some species, RO2+RO2 reactions were also observed. While initial NMOG loadings may explain part of the enhanced RO2+RO2 rate, some fraction may also be attributed to the limited degree of NOx cycling in the mini-chamber. Under ambient conditions, NO2 is photolyzed to NO, which then reacts with RO2 (and HO2) radicals. Consequently, the RO2 + NO pathway may act to lower the fraction of radicals that follow the RO2+RO2 pathway. This is evidenced by the biomass burning plume described by , which shows that the RO2+NO pathway is the dominant fate for most RO2 species under ambient photolysis. In the mini-chamber, NO2 does not strongly absorb at 254 nm and is quickly lost to PAN or HNO3; consequently, NOx does not play a significant role in the chemistry of higher-carbon RO2 radicals.

The primary focus of this study is to understand the formation of major secondary NMOGs measured by I-CIMS; thus, it is instructive to identify the modeled radical pathways that contribute to these formation rates. Figure S14 shows the pathways for the radicals that lead to the formation of hydroxy furanone and methyl hydroxy furanone. For both experiments, the hydroxy furanone radicals predominantly react through RO2+HO2 pathways. For F26, ∼15 % of the RO2 radicals react through the RO2+RO2 pathways, whereas ∼8 % follow this pathway for F38. These results suggest that the RO2+RO2 pathway played some role in both experiments but that the formation of hydroxy furanones in F38 predominantly followed atmospherically relevant pathways. It is noted that, as with other RO2 radicals, these species are expected to mostly react with NO under ambient conditions (Fig. S14). Despite this difference, the RO2+HO2 pathway and the RO2+NO pathway are both expected to lead to hydroxy furanone formation (Figs. S4 and S5).

Section  shows that the radical pathways in the mini-chamber exhibited similarities, as well as differences, to those expected under atmospheric photolysis. In order to evaluate the impact of furan chemistry under ambient conditions, this work builds upon the model described by to evaluate the customized MCM mechanism with measurements from a real biomass burning plume.

During the 2013 DISCOVER-AQ aircraft campaign, the NASA P-3B conducted several plume intercepts downwind of a controlled burn conducted in a mixed-forested ecosystem. NMOGs were monitored by a PTR-ToF-MS, meteorological parameters (temperature, pressure, and relative humidity) were monitored by a suite of aircraft instrumentation, and NOx and O3 were monitored by chemiluminescence. modeled the chemical evolution of NMOGs using a semi-Lagrangian box model with a modified MCM mechanism that included a basic oxidation scheme for furan and furfural. The authors successfully modeled the loss of primary NMOGs, including furan and furfural, and captured trends in ozone, NOx, and peroxyacetyl nitrate (PAN). Downwind of the fire, the authors observed the formation of NMOGs, such as maleic anhydride, which could not be explained by the model.

The authors initialized the chemistry with measurements of NMOGs, NOx, and O3 sampled in close proximity of the fire. Plume dilution was constrained based on the temporal evolution of CO. Background NMOGs, NOx, and O3 concentrations were prescribed based on aircraft measurements conducted outside of the plume. The plume was simulated for 1 h, and meteorological parameters were constrained based on measurements conducted at each plume crossing. Photolysis was prescribed based on observed NO2 photolysis frequencies.

The analysis described is recreated here but with the full mechanisms of furan, furfural, 2,-methylfuran, 2,5-dimethylfuran, and 5-methylfurfural incorporated into the MCM. Other mechanisms that were not previously analyzed by are also considered, including phenol, cresol, and catechol. These species have important contributions to the primary OH reactivity of biomass burning smoke and are explicitly represented in MCM v. 3.3.1 (Fig. ). The initial conditions of furan and furfural are prescribed based on the observed mixing ratios reported by . The mixing ratios of 2-methylfuran, phenol, and cresol are constrained based on the signals measured at m/z 83.05 (C5H6O-H+), m/z 95.045 (C6H6O-H+), and m/z 109.066 (C7H8O-H+), respectively. The signal attributable to 2,5-dimethylfuran is isobaric with furfural and was not fully resolved by PTR-ToF-MS due to the overwhelming signal of furfural (∼10 times greater than the signal of other furan species). The mixing ratio of 2,5-dimethylfuran is expected to significantly contribute to total NMOG mixing ratios and OH reactivity (Fig. ); therefore, the initial mixing ratio of 2,5-dimethylfuran is constrained based on the dimethylfuran / methylfuran ratio reported elsewhere ∼0.5,. Finally, methylfurfural and catechol are included based on the signal at m/z 111.049 (C6H6O2-H+), and it is assumed that 50 % of the signal can be attributed to each compound as recommended by . The initial mixing ratios of methylfurfural + catechol and cresol are adjusted to best match the decay of C6H6O2-H+ and C7H8O-H+, respectively.

Figure  compares the model output to the dilution-corrected mixing ratios of furans, oxygenated aromatics, maleic anhydride, and ozone. The model output of hydroxy furanone is also shown but not compared to measurements since an I-CIMS was not onboard the P-3B. Red lines show the model output with furan chemistry included in the model. Dotted blue lines show model output with initial furan mixing ratios set to zero.

The model satisfactorily reproduces the temporal profiles of furans, oxygenated aromatic species, maleic anhydride, and ozone. The model also predicts significant formation of hydroxy furanone. Similar to the mini-chamber observations (Fig. ), maleic anhydride exhibits a temporal profile that is consistent with a slow-forming product. The model output of hydroxy furanone exhibits a fast-forming temporal profile as expected from the mini-chamber experiments (Fig. ).

The production of maleic anhydride and hydroxy furanone is negligible when the initial concentrations of furan species are set to zero. For both species, furfural oxidation accounts for more than 50 % of the total production. As discussed in Sect. , the furfural mechanism is based on theoretical calculations and the exact branching ratios may differ from those estimated by . The assumed branching ratios weakly impact the formation of maleic anhydride, whereas hydroxy furanone is most impacted by the assumed branching ratio of the ring-retaining pathway (channel b, Fig. S16). I-CIMS measurements of hydroxy furanone may provide better constraints on the relative importance of each pathway.

Good agreement between ozone measurements and model output was also observed by . Most of the ozone production results from reactions involving HO2 (formed primarily from OH + formaldehyde, CO, and furfural reactions), CH3O2 radicals (formed primarily from reactions involving acetaldehyde, 2,3-butanedione, and methylglyoxal), and NO. When furans are removed from the model, predicted ozone mixing ratios decrease by 12 %. It is estimated that ∼5 ppb of ozone was produced from furan chemistry after 60 min of oxidation. Notably, ozone formation was not sensitive to the assumed furfural branching ratios (Fig. S16). It is noted that the contribution of ozone from furan chemistry will vary depending on NOx conditions and that this estimate is not equivalent to a generalized ozone formation potential.

The products of furan chemistry are also reactive (e.g., hydroxy furanone, 1,4-butenedial, methyl hydroxy furanone, Figs. S4–S9), and 1 h of oxidation is too short to capture the total potential ozone produced from the oxidation of furan precursors. Figure  extrapolates the model forward to evaluate multigenerational oxidation processes of ozone formation. The model is extrapolated assuming that the dilution rate continues to follow an exponential decay (calculated based on the measured CO loss). The solar zenith angle and jNO2 are calculated based on time of day. Relative humidity, temperature, and pressure are assumed to remain constant following the last measured plume intercept.

Figure  shows the extrapolated modeling results of ozone, hydroxy furanone, and maleic anhydride until 17:30 local time when the solar zenith angle approaches 70∘ (∼1.5 h before sunset). Hydroxy furanone production maximizes after 1 h of oxidation, and subsequently decays due to OH oxidation. In contrast, maleic anhydride continues to increase. Figure a shows the estimated ozone produced from furan chemistry (calculated as the difference between model runs initialized with and without furan species). Ozone production from furan oxidation continues to rise after 1 h of aging, in part because of the oxidation of reactive secondary NMOGs such as hydroxy furanone. After 4.5 h of oxidation, the total ozone produced from furan chemistry is ∼8 ppb.

Figure  demonstrates that furan chemistry contributed to the evolution of ozone within 4 h of emission. After 2 h of aging, most furans have reacted (< 20 % remain), and their contribution to ozone production via reactions of secondary NMOGs diminishes. It is important to note that ozone production will vary depending on NOx availability, NOx/NMOG ratios, the chemical composition of the NMOG mixture, and meteorological conditions. Despite these factors, furan chemistry will likely play a role in ozone production for many biomass burning plumes due to the ubiquitous presence of furans in smoke .

The plume described above is relatively young; however, observations from the mini-chamber (Fig. ) and the continued formation of maleic anhydride in the extrapolated model suggests that this compound could be present in highly aged plumes. During the NOAA Shale Oil and Natural Gas Nexus (SONGNEX https://www.esrl.noaa.gov/csd/projects/songnex/, last access: 4 January 2018) field campaign, the NOAA WP-3D aircraft intercepted a large biomass burning plume in the free troposphere above MT, US, on 21 April 2015 . The plume had been transported at least 4 d from wildfires in Siberia and affected large portions of the western US. The PTR-ToF-MS described in this study was also deployed on the WP-3B and the resulting measurements are presented in Fig. . The aged plume (indicated by elevated mixing ratios of acetonitrile and acetic acid) exhibited clear enhancements in maleic anhydride and C4H4O3, which is attributed to succinic anhydride. In contrast, furan mixing ratios did not increase above background levels, indicating that these species completely reacted before sampling by the P-3.

The lifetimes of maleic and succinic anhydride are long (> 5 d at OH concentrations of 1.5×106 molec. cm-3); consequently, these species may have formed from the OH oxidation of furans and oxygenated aromatics shortly downwind of the fire and survived transport to the western US. These species may have also formed during transit from the oxidation of slow-reacting aromatics, such as benzene. The detection of anhydrides in highly aged plumes is consistent with the behavior of the mini-chamber (Fig. ) and demonstrates the relevance of furans and aromatic oxidation on plume chemistry far downwind of fire sources.