Evolution of Reactive Organic Compounds and Their Potential Health Risk in Wildfire Smoke.

Wildfires are an increasing source of emissions into the air, with health effects modulated by the abundance and toxicity of individual species. In this work, we estimate reactive organic compounds (ROC) in western U.S. wildland forest fire smoke using a combination of observations from the 2019 Fire Influence on Regional to Global Environments and Air Quality (FIREX-AQ) field campaign and predictions from the Community Multiscale Air Quality (CMAQ) model. Standard emission inventory methods capture 40-45% of the estimated ROC mass emitted, with estimates of primary organic aerosol particularly low (5-8×). Downwind, gas-phase species abundances in molar units reflect the production of fragmentation products such as formaldehyde and methanol. Mass-based units emphasize larger compounds, which tend to be unidentified at an individual species level, are less volatile, and are typically not measured in the gas phase. Fire emissions are estimated to total 1250 ± 60 g·C of ROC per kg·C of CO, implying as much carbon is emitted as ROC as is emitted as CO. Particulate ROC has the potential to dominate the cancer and noncancer risk of long-term exposure to inhaled smoke, and better constraining these estimates will require information on the toxicity of particulate ROC from forest fires.

Midlatitude Ozone Depletion and Air Quality Impacts from Industrial Halogen Emissions in the Great Salt Lake Basin.

We report aircraft observations of extreme levels of HCl and the dihalogens Cl2, Br2, and BrCl in an industrial plume near the Great Salt Lake, Utah. Complete depletion of O3 was observed concurrently with halogen enhancements as a direct result of photochemically produced halogen radicals. Observed fluxes for Cl2, HCl, and NOx agreed with facility-reported emissions inventories. Bromine emissions are not required to be reported in the inventory, but are estimated as 173 Mg year-1 Br2 and 949 Mg year-1 BrCl, representing a major uncounted oxidant source. A zero-dimensional photochemical box model reproduced the observed O3 depletions and demonstrated that bromine radical cycling was principally responsible for the rapid O3 depletion. Inclusion of observed halogen emissions in both the box model and a 3D chemical model showed significant increases in oxidants and particulate matter (PM2.5) in the populated regions of the Great Salt Lake Basin, where winter PM2.5 is among the most severe air quality issues in the U.S. The model shows regional PM2.5 increases of 10%-25% attributable to this single industrial halogen source, demonstrating the impact of underreported industrial bromine emissions on oxidation sources and air quality within a major urban area of the western U.S.

Airborne Emission Rate Measurements Validate Remote Sensing Observations and Emission Inventories of Western U.S. Wildfires.

Carbonaceous emissions from wildfires are a dynamic mixture of gases and particles that have important impacts on air quality and climate. Emissions that feed atmospheric models are estimated using burned area and fire radiative power (FRP) methods that rely on satellite products. These approaches show wide variability and have large uncertainties, and their accuracy is challenging to evaluate due to limited aircraft and ground measurements. Here, we present a novel method to estimate fire plume-integrated total carbon and speciated emission rates using a unique combination of lidar remote sensing aerosol extinction profiles and in situ measured carbon constituents. We show strong agreement between these aircraft-derived emission rates of total carbon and a detailed burned area-based inventory that distributes carbon emissions in time using Geostationary Operational Environmental Satellite FRP observations (Fuel2Fire inventory, slope = 1.33 ± 0.04, r2 = 0.93, and RMSE = 0.27). Other more commonly used inventories strongly correlate with aircraft-derived emissions but have wide-ranging over- and under-predictions. A strong correlation is found between carbon monoxide emissions estimated in situ with those derived from the TROPOspheric Monitoring Instrument (TROPOMI) for five wildfires with coincident sampling windows (slope = 0.99 ± 0.18; bias = 28.5%). Smoke emission coefficients (g MJ-1) enable direct estimations of primary gas and aerosol emissions from satellite FRP observations, and we derive these values for many compounds emitted by temperate forest fuels, including several previously unreported species.

Novel Analysis to Quantify Plume Crosswind Heterogeneity Applied to Biomass Burning Smoke.

We present a novel method, the Gaussian observational model for edge to center heterogeneity (GOMECH), to quantify the horizontal chemical structure of plumes. GOMECH fits observations of short-lived emissions or products against a long-lived tracer (e.g., CO) to provide relative metrics for the plume width (wi/wCO) and center (bi/wCO). To validate GOMECH, we investigate OH and NO3 oxidation processes in smoke plumes sampled during FIREX-AQ (Fire Influence on Regional to Global Environments and Air Quality, a 2019 wildfire smoke study). An analysis of 430 crosswind transects demonstrates that nitrous acid (HONO), a primary source of OH, is narrower than CO (wHONO/wCO = 0.73-0.84 ± 0.01) and maleic anhydride (an OH oxidation product) is enhanced on plume edges (wmaleicanhydride/wCO = 1.06-1.12 ± 0.01). By contrast, NO3 production [P(NO3)] occurs mainly at the plume center (wP(NO3)/wCO = 0.91-1.00 ± 0.01). Phenolic emissions, highly reactive to OH and NO3, are narrower than CO (wphenol/wCO = 0.96 ± 0.03, wcatechol/wCO = 0.91 ± 0.01, and wmethylcatechol/wCO = 0.84 ± 0.01), suggesting that plume edge phenolic losses are the greatest. Yet, nitrophenolic aerosol, their oxidation product, is the greatest at the plume center (wnitrophenolicaerosol/wCO = 0.95 ± 0.02). In a large plume case study, GOMECH suggests that nitrocatechol aerosol is most associated with P(NO3). Last, we corroborate GOMECH with a large eddy simulation model which suggests most (55%) of nitrocatechol is produced through NO3 in our case study.

Coupled Air Quality and Boundary-Layer Meteorology in Western U.S. Basins during Winter: Design and Rationale for a Comprehensive Study.

Wintertime episodes of high aerosol concentrations occur frequently in urban and agricultural basins and valleys worldwide. These episodes often arise following development of persistent cold-air pools (PCAPs) that limit mixing and modify chemistry. While field campaigns targeting either basin meteorology or wintertime pollution chemistry have been conducted, coupling between interconnected chemical and meteorological processes remains an insufficiently studied research area. Gaps in understanding the coupled chemical-meteorological interactions that drive high pollution events make identification of the most effective air-basin specific emission control strategies challenging. To address this, a September 2019 workshop occurred with the goal of planning a future research campaign to investigate air quality in Western U.S. basins. Approximately 120 people participated, representing 50 institutions and 5 countries. Workshop participants outlined the rationale and design for a comprehensive wintertime study that would couple atmospheric chemistry and boundary-layer and complex-terrain meteorology within western U.S. basins. Participants concluded the study should focus on two regions with contrasting aerosol chemistry: three populated valleys within Utah (Salt Lake, Utah, and Cache Valleys) and the San Joaquin Valley in California. This paper describes the scientific rationale for a campaign that will acquire chemical and meteorological datasets using airborne platforms with extensive range, coupled to surface-based measurements focusing on sampling within the near-surface boundary layer, and transport and mixing processes within this layer, with high vertical resolution at a number of representative sites. No prior wintertime basin-focused campaign has provided the breadth of observations necessary to characterize the meteorological-chemical linkages outlined here, nor to validate complex processes within coupled atmosphere-chemistry models.

Oxygen-18 Isotopic Studies of HOOO and DOOO.

Owing to questions that still persist regarding the length of the O-H and central O-O bond, and large-amplitude torsional motion of trans hydridotrioxygen HOOO, a weakly bound complex between OH and O2, new 18O isotopic measurements of HOOO and DOOO were undertaken using Fourier transform microwave and microwave-millimeter-wave double resonance techniques. Rotational lines from three new 18O species of DOOO (D18OOO, DO18O18O, and D18O18O18O) were detected, along with the two singly substituted 18O isotopic species of HOOO (HO18OO and HOO18O) that were not measured in the previous isotopic investigation. From a least-squares fit, spectroscopic constants, including the three rotational constants, were precisely determined for all five species. The inertial defect of DOOO and its 18O species is uniformly negative: of order -0.04 amu Å2, regardless of the number or location of the 18O atoms, in contrast to that found for HOOO or its 18O isotopic species. A reanalysis of the molecular structure was performed using either normal HOOO and its four singly substituted isotopic species, the new DOOO data, or all the isotopic species (10 in total). The differences between the purely experimental (r0) structures are generally quite small, of order ±0.01 Å for the bond lengths and ±1° for the bond angle. The length of the O-H bond remains unrealistically short compared to free OH, and the central O-O bond length is consistently very close to 1.68 Å. On the basis of the effective O-H bond length derived from the experimental structure, the average displacement of the large amplitude torsional motion from planarity is estimated to be ∼22°.

Spontaneous and Selective Formation of HSNO, a Crucial Intermediate Linking H2S and Nitroso Chemistries.

Thionitrous acid (HSNO), a potential key intermediate in biological signaling pathways, has been proposed to link NO and H2S biochemistries, but its existence and stability in vivo remain controversial. We establish that HSNO is spontaneously formed in high concentration when NO and H2S gases are mixed at room temperature in the presence of metallic surfaces. Our measurements reveal that HSNO is formed by the reaction H2S + N2O3 → HSNO + HNO2, where N2O3 is a product of NO disproportionation. These studies also suggest that further reaction of HSNO with H2S may form HNO and HSSH. The length of the S-N bond has been derived to high precision and is found to be unusually long: 1.84 Å, the longest S-N bond reported to date for an R-SNO compound. The present structural and, particularly, reactivity investigations of this elusive molecule provide a firm foundation to better understand its potential physiological chemistry and propensity to undergo S-N bond cleavage in vivo.

A Molecular Precursor to Phosphaethyne and Its Application in Synthesis of the Aromatic 1,2,3,4-Phosphatriazolate Anion.

Dibenzo-7-phosphanorbornadiene Ph3PC(H)PA (1, A = C14H10, anthracene) is reported here as a molecular precursor to phosphaethyne (HC≡P), produced together with anthracene and triphenylphosphine. HCP generated by thermolysis of 1 has been observed by molecular beam mass spectrometry, laser-induced fluorescence, microwave spectroscopy, and nuclear magnetic resonance (NMR) spectroscopy. In toluene, fragmentation of 1 has been found to proceed with activation parameters of ΔH(⧧) = 25.5 kcal/mol and ΔS(⧧) = -2.43 eu and is accompanied by formation of an orange insoluble precipitate. Results from computational studies of the mechanism of HCP generation are in good agreement with experimental data. This high-temperature method of HCP generation has pointed to new reaction chemistry with azide anion to produce the 1,2,3,4-phosphatriazolate anion, HCPN3(-), for which structural data have been obtained in a single-crystal X-ray diffraction study. Negative-ion photoelectron spectroscopy has shown the adiabatic detachment energy for this anion to be 3.555(10) eV. The aromaticity of HCPN3(-) has been assessed using nucleus-independent chemical shift, quantum theory of atoms in molecules, and natural bond orbital methods.

Observation of the simplest Criegee intermediate CH2OO in the gas-phase ozonolysis of ethylene.

Ozonolysis is one of the dominant oxidation pathways for tropospheric alkenes. Although numerous studies have confirmed a 1,3-cycloaddition mechanism that generates a Criegee intermediate (CI) with form R1R2COO, no small CIs have ever been directly observed in the ozonolysis of alkenes because of their high reactivity. We present the first experimental detection of CH2OO in the gas-phase ozonolysis of ethylene, using Fourier transform microwave spectroscopy and a modified pulsed nozzle, which combines high reactant concentrations with rapid sampling and sensitive detection. Nine other product species of the O3 + C2H4 reaction were also detected, including formaldehyde, formic acid, dioxirane, and ethylene ozonide. The presence of all these species can be attributed to the unimolecular and bimolecular reactions of CH2OO, and their abundances are in qualitative agreement with published mechanisms and rate constants.

Millimeter-wave optical double resonance schemes for rapid assignment of perturbed spectra, with applications to the C̃ (1)B(2) state of SO2.

Millimeter-wave detected, millimeter-wave optical double resonance (mmODR) spectroscopy is a powerful tool for the analysis of dense, complicated regions in the optical spectra of small molecules. The availability of cavity-free microwave and millimeter wave spectrometers with frequency-agile generation and detection of radiation (required for chirped-pulse Fourier-transform spectroscopy) opens up new schemes for double resonance experiments. We demonstrate a multiplexed population labeling scheme for rapid acquisition of double resonance spectra, probing multiple rotational transitions simultaneously. We also demonstrate a millimeter-wave implementation of the coherence-converted population transfer scheme for background-free mmODR, which provides a ∼10-fold sensitivity improvement over the population labeling scheme. We analyze perturbations in the C̃ state of SO2, and we rotationally assign a b2 vibrational level at 45,328 cm(-1) that borrows intensity via a c-axis Coriolis interaction. We also demonstrate the effectiveness of our multiplexed mmODR scheme for rapid acquisition and assignment of three predissociated vibrational levels of the C̃ state of SO2 between 46,800 and 47,650 cm(-1).

Gas-phase structure determination of dihydroxycarbene, one of the smallest stable singlet carbenes.

Carbenes are reactive molecules of the form R(1)-:C-R(2) that play a role in topics ranging from organic synthesis to gas-phase oxidation chemistry. We report the first experimental structure determination of dihydroxycarbene (HO-:C-OH), one of the smallest stable singlet carbenes, using a combination of microwave rotational spectroscopy and high-level coupled-cluster calculations. The semi-experimental equilibrium structure derived from five isotopic variants of HO-:C-OH contains two very short CO single bonds (ca. 1.32 Å). Detection of HO-:C-OH in the gas phase firmly establishes that it is stable to isomerization, yet it has been underrepresented in discussions of the CH2O2 chemical system and its atmospherically relevant isomers: formic acid and the Criegee intermediate CH2OO.

Effects of high angular momentum on the unimolecular dissociation of CD2CD2OH: theory and comparisons with experiment.

This paper explores the dynamics of a highly rotationally and vibrationally excited radical, CD2CD2OH. The radical is produced from the 193 nm photodissociation of 2-bromoethanol-d4, so it is imparted with high angular momentum and high vibrational energy and subsequently dissociates to several product channels. This paper focuses on characterizing its angular momentum and modeling its effect on the product channels, including the HOD + vinyl-d3 product channel resulting from a frustrated dissociation of the radical originally en route to OH + ethene-d4 that instead results in D atom abstraction. Our impulsive model of the initial photodissociation shows that, for some cases, upward of 200 au of angular momentum is imparted, which greatly affects the dynamics of the competing product channels. Using a permutationally invariant potential energy surface and quasiclassical trajectories, we simulated the dissociation dynamics of CD2CD2OH and compared these results to those of Kamarchik et al. (J. Phys. Chem. Lett. 2010, 1, 3058-3065), who studied the dynamics of CH2CH2OH with zero angular momentum. We found that the recoil translational energy distribution for radicals that dissociated to OH + C2D4 matched experiment closely only when high angular momentum of the initial radical was explicitly included in the trajectory calculations. Similarly, the rate constant for dissociation changes when rotational energy was added to the vibrational energy in the initial conditions. Lastly, we applied the sketch-map dimensionality reduction technique to analyze mechanistic information leading to the vinyl + water product channel. Projecting the ab initio intrinsic reaction coordinates onto the lower dimensional space identified with sketch map offers new insight into the dynamics when one looks at the simulated trajectories in the lower dimensional space. Further analysis shows that the transition path resembles a frustrated dissociation of the OH + ethene radical adduct, followed instead by branching to vinyl + water when the leaving OH group encounters a nearby D atom on the ethene moiety. This characterization is in accord with the one made previously. We show that the transition path bifurcation between the two similar channels occurs at carbon-oxygen distances and oxygen-abstracted deuterium distances of 2-2.5 Å controlled by the C-O-D bond angle with large angles preferentially branching to the water plus vinyl product state. The experimental branching ratios were not reproduced by theory, however, due partly to the insufficient quality of the fitted potential surface. We also have evidence of a minor product channel, HD + vinoxy-d3, from our molecular dynamics simulations that allows us to assign the HD signal in prior experimental work.

Photoproduct channels from BrCD2CD2OH at 193 nm and the HDO + vinyl products from the CD2CD2OH radical intermediate.

We present the results of our product branching studies of the OH + C(2)D(4) reaction, beginning at the CD(2)CD(2)OH radical intermediate of the reaction, which is generated by the photodissociation of the precursor molecule BrCD(2)CD(2)OH at 193 nm. Using a crossed laser-molecular beam scattering apparatus with tunable photoionization detection, and a velocity map imaging apparatus with VUV photoionization, we detect the products of the major primary photodissociation channel (Br and CD(2)CD(2)OH), and of the secondary dissociation of vibrationally excited CD(2)CD(2)OH radicals (OH, C(2)D(4)/CD(2)O, C(2)D(3), CD(2)H, and CD(2)CDOH). We also characterize two additional photodissociation channels, which generate HBr + CD(2)CD(2)O and DBr + CD(2)CDOH, and measure the branching ratio between the C-Br bond fission, HBr elimination, and DBr elimination primary photodissociation channels as 0.99:0.0064:0.0046. The velocity distribution of the signal at m/e = 30 upon 10.5 eV photoionization allows us to identify the signal from the vinyl (C(2)D(3)) product, assigned to a frustrated dissociation toward OH + ethene followed by D-atom abstraction. The relative amount of vinyl and Br atom signal shows the quantum yield of this HDO + C(2)D(3) product channel is reduced by a factor of 0.77 ± 0.33 from that measured for the undeuterated system. However, because the vibrational energy distribution of the deuterated radicals is lower than that of the undeuterated radicals, the observed reduction in the water + vinyl product quantum yield likely reflects the smaller fraction of radicals that dissociate in the deuterated system, not the effect of quantum tunneling. We compare these results to predictions from statistical transition state theory and prior classical trajectory calculations on the OH + ethene potential energy surface that evidenced a roaming channel to produce water + vinyl products and consider how the branching to the water + vinyl channel might be sensitive to the angular momentum of the ß-hydroxyethyl radicals.

Characterizing the rovibrational distribution of CD2CD2OH radicals produced via the photodissociation of 2-bromoethanol-d4.

This work characterizes the internal energy distribution of the CD(2)CD(2)OH radical formed via photodissociation of 2-bromoethanol-d(4). The CD(2)CD(2)OH radical is the first radical adduct in the addition of the hydroxyl radical to C(2)D(4) and the product branching of the OH + C(2)D(4) reaction is dependent on the total internal energy of this adduct and how that energy is partitioned between rotation and vibration. Using a combination of a velocity map imaging apparatus and a crossed laser-molecular beam scattering apparatus, we photodissociate the BrCD(2)CD(2)OH precursor at 193 nm and measure the velocity distributions of the Br atoms, resolving the Br((2)P(1/2)) and Br((2)P(3/2)) states with [2 + 1] resonance enhanced multiphoton ionization (REMPI) on the imaging apparatus. We also detect the velocity distribution of the subset of the nascent momentum-matched CD(2)CD(2)OH cofragments that are formed stable to subsequent dissociation. Invoking conservation of momentum and conservation of energy and a recently developed impulsive model, we determine the vibrational energy distribution of the nascent CD(2)CD(2)OH radicals from the measured velocity distributions.

The dissociation of vibrationally excited CH3OSO radicals and their photolytic precursor, methoxysulfinyl chloride.

The dissociation dynamics of methoxysulfinyl radicals generated from the photodissociation of CH(3)OS(O)Cl at 248 nm is investigated using both a crossed laser-molecular beam scattering apparatus and a velocity map imaging apparatus. There is evidence of only a single photodissociation channel of the precursor: S-Cl fission to produce Cl atoms and CH(3)OSO radicals. Some of the vibrationally excited CH(3)OSO radicals undergo subsequent dissociation to CH(3) + SO(2). The velocities of the detected CH(3) and SO(2) products show that the dissociation occurs via a transition state having a substantial barrier beyond the endoergicity; appropriately, the distribution of velocities imparted to these momentum-matched products is fit by a broad recoil kinetic energy distribution extending out to 24 kcal/mol in translational energy. Using 200 eV electron bombardment detection, we also detect the CH(3)OSO radicals that have too little internal energy to dissociate. These radicals are observed both at the parent CH(3)OSO(+) ion as well as at the CH(3)(+) and SO(2)(+) daughter ions; they are distinguished by virtue of the velocity imparted in the original photolytic step. The detected velocities of the stable radicals are roughly consistent with the calculated barriers (both at the CCSD(T) and G3B3 levels of theory) for the dissociation of CH(3)OSO to CH(3) + SO(2) when we account for the partitioning of internal energy between rotation and vibration as the CH(3)OSOCl precursor dissociates.

Assessing an impulsive model for rotational energy partitioning to acetyl radicals from the photodissociation of acetyl chloride at 235 nm.

This work uses the photodissociation of acetyl chloride to assess the utility of a recently proposed impulsive model when the dissociation occurs on an excited electronic state that is not repulsive in the Franck-Condon region. The impulsive model explicitly includes an average over the vibrational quantum states of acetyl chloride when it calculates an impact parameter for fission of the C-Cl bond, as well as the distribution of thermal energy in the photolytic precursor. The experimentally determined stability of the resulting acetyl radical to subsequent dissociation is the key observable that allows us to test the model's ability to predict the partitioning of energy between rotation and vibration of the radical. We compare the model's predictions for three different assumed geometries at which the impulsive force might act, generating the relative kinetic energy and the concomitant rotational energy in the acetyl radical. Assuming that the impulsive force acts at the transition state for C-Cl fission on the S(1) excited state gives a poor prediction; the model predicts far more energy in rotation of the acetyl radical than is consistent with the measured velocity map imaging spectrum of the stable radicals. The best prediction results from using a geometry derived from the classical trajectory calculations on the excited state potential energy surface. We discuss the insight gained into the excited state dissociation dynamics of acetyl chloride and, more generally, the utility of using the impulsive model in conjunction with excited state trajectory calculations to predict the partitioning of internal energy between rotation and vibration for radicals produced from the photolysis of halogenated precursors.