Gas-Phase Nitrous Acid (HONO) Is Controlled by Surface Interactions of Adsorbed Nitrite (NO2-) on Common Indoor Material Surfaces.

Nitrous acid (HONO) is a household pollutant exhibiting adverse health effects and a major source of indoor OH radicals under a variety of lighting conditions. The present study focuses on gas-phase HONO and condensed-phase nitrite and nitrate formation on indoor surface thin films following heterogeneous hydrolysis of NO2, in the presence and absence of light, and nitrate (NO3-) photochemistry. These thin films are composed of common building materials including zeolite, kaolinite, painted walls, and cement. Gas-phase HONO is measured using an incoherent broadband cavity-enhanced ultraviolet absorption spectrometer (IBBCEAS), whereby condensed-phase products, adsorbed nitrite and nitrate, are quantified using ion chromatography. All of the surface materials used in this study can store nitrogen oxides as nitrate, but only thin films of zeolite and cement can act as condensed-phase nitrite reservoirs. For both the photo-enhanced heterogeneous hydrolysis of NO2 and nitrate photochemistry, the amount of HONO produced depends on the material surface. For zeolite and cement, little HONO is produced, whereas HONO is the major product from kaolinite and painted wall surfaces. An important result of this study is that surface interactions of adsorbed nitrite are key to HONO formation, and the stronger the interaction of nitrite with the surface, the less gas-phase HONO produced.

HONO Production from Gypsum Surfaces Following Exposure to NO2 and HNO3: Roles of Relative Humidity and Light Source.

Nitrous acid (HONO) is a toxic household pollutant and a major source of indoor OH radicals. The high surface-to-volume ratio and diverse lighting conditions make the indoor photochemistry of HONO complex. This study demonstrates surface uptake of NO2 and gaseous HNO3 followed by gas-phase HONO generation on gypsum surfaces, model system for drywall, under reaction conditions appropriate for an indoor air environment. Tens of parts per billion of steady-state HONO are detected under these experimental conditions. Mechanistic insight into this heterogeneous photochemistry is obtained by exploring the roles of material compositions, relative humidities, and light sources. NO2 and HNO3 are adsorbed onto drywall surfaces, which can generate HONO under illumination and under dark conditions. Photoenhanced HONO generation is observed for illumination with a solar simulator as well as with the common indoor light sources such as compact fluorescence light and incandescent light bulbs. Incandescent light sources release more HONO and NO2 near the light source compared to the solar radiation. Overall, HONO production on the gypsum surface increases with the increase of RH up to 70% relative humidity; above that, the gaseous HONO level decreases due to surface loss. Heterogeneous hydrolysis of NO2 is predicted to be the dominant HONO generation channel, where NO2 is produced through the photolysis of surface-adsorbed nitrates. This hydrolysis reaction predominantly occurs in the first layer of surface-adsorbed water.

Formic acid catalyzed isomerization and adduct formation of an isoprene-derived Criegee intermediate: experiment and theory.

Isoprene is the most abundant non-methane hydrocarbon emitted into the Earth's atmosphere. Ozonolysis is an important atmospheric sink for isoprene, which generates reactive carbonyl oxide species (R1R2C[double bond, length as m-dash]O+O-) known as Criegee intermediates. This study focuses on characterizing the catalyzed isomerization and adduct formation pathways for the reaction between formic acid and methyl vinyl ketone oxide (MVK-oxide), a four-carbon unsaturated Criegee intermediate generated from isoprene ozonolysis. syn-MVK-oxide undergoes intramolecular 1,4 H-atom transfer to form a substituted vinyl hydroperoxide intermediate, 2-hydroperoxybuta-1,3-diene (HPBD), which subsequently decomposes to hydroxyl and vinoxylic radical products. Here, we report direct observation of HPBD generated by formic acid catalyzed isomerization of MVK-oxide under thermal conditions (298 K, 10 torr) using multiplexed photoionization mass spectrometry. The acid catalyzed isomerization of MVK-oxide proceeds by a double hydrogen-bonded interaction followed by a concerted H-atom transfer via submerged barriers to produce HPBD and regenerate formic acid. The analogous isomerization pathway catalyzed with deuterated formic acid (D2-formic acid) enables migration of a D atom to yield partially deuterated HPBD (DPBD), which is identified by its distinct mass (m/z 87) and photoionization threshold. In addition, bimolecular reaction of MVK-oxide with D2-formic acid forms a functionalized hydroperoxide adduct, which is the dominant product channel, and is compared to a previous bimolecular reaction study with normal formic acid. Complementary high-level theoretical calculations are performed to further investigate the reaction pathways and kinetics.

CH Stretch Activation of CH3CHOO: Deep Tunneling to Hydroxyl Radical Products.

Alkene ozonolysis, an important source of hydroxyl (OH) radicals in the Earth's troposphere, proceeds through unimolecular decay of Criegee intermediates. In this work, infrared activation of the methyl-substituted Criegee intermediate, syn-CH3CHOO, in the CH stretch fundamental region (2850-3150 cm-1) is shown to result in unimolecular decay to OH radical products. These excitation energies correspond to only half of the transition state barrier height, and thus the resultant 1,4 H atom transfer that leads to OH products occurs exclusively by quantum mechanical tunneling. Infrared action spectra recorded with UV laser-induced fluorescence detection of the OH products reveal the four CH stretch fundamentals and CO stretch overtone predicted to have strong transition strength. The vibrational band origins, relative intensities, and transition types derived from rotational band contour analyses are in good accord with theory. Distinctly different Lorentzian line broadening of the observed features is attributed to mode-specific anharmonic couplings predicted theoretically between spectroscopically bright and nearby dark states. The measured OH product state distribution shows a strong λ-doublet preference arising from pπ orbital alignment, which is indicative of the vinyl hydroperoxide intermediate along the reaction pathway. The unimolecular decay of syn-CH3CHOO at ca. 3000 cm-1 is predicted to be quite slow (ca. 105 s-1) using statistical Rice-Ramsperger-Kassel-Marcus theory with tunneling and much slower than observed at higher energies.

Four-Carbon Criegee Intermediate from Isoprene Ozonolysis: Methyl Vinyl Ketone Oxide Synthesis, Infrared Spectrum, and OH Production.

The reaction of ozone with isoprene, one of the most abundant volatile organic compounds in the atmosphere, produces three distinct carbonyl oxide species (RR'COO) known as Criegee intermediates: formaldehyde oxide (CH2OO), methyl vinyl ketone oxide (MVK-OO), and methacrolein oxide (MACR-OO). The nature of the substituents (R,R' = H, CH3, CH═CH2) and conformations of the Criegee intermediates control their subsequent chemistry in the atmosphere. In particular, unimolecular decay of MVK-OO is predicted to be the major source of hydroxyl radicals (OH) in isoprene ozonolysis. This study reports the initial laboratory synthesis and direct detection of MVK-OO through reaction of a photolytically generated, resonance-stabilized monoiodoalkene radical with O2. MVK-OO is characterized utilizing infrared (IR) action spectroscopy, in which IR activation of MVK-OO with two quanta of CH stretch at ca. 6000 cm-1 is coupled with ultraviolet detection of the resultant OH products. MVK-OO is identified by comparison of the experimentally observed IR spectral features with theoretically predicted IR absorption spectra. For syn-MVK-OO, the rate of appearance of OH products agrees with the unimolecular decay rate predicted using statistical theory with tunneling. This validates the hydrogen atom transfer mechanism and computed transition-state barrier (18.0 kcal mol-1) leading to OH products. Theoretical calculations reveal an additional roaming pathway between the separating radical fragments, which results in other products. Master equation modeling yields a thermal unimolecular decay rate for syn-MVK-OO of 33 s-1 (298 K, 1 atm). For anti-MVK-OO, theoretical exploration of several unimolecular decay pathways predicts that isomerization to dioxole is the most likely initial step to products.

Primary vs. secondary H-atom abstraction in the Cl-atom reaction with n-pentane.

Velocity map imaging (VMI) measurements and quasi-classical trajectory (QCT) calculations on a newly developed, global potential energy surface (PES) combine to reveal the detailed mechanisms of reaction of Cl atoms with n-pentane. Images of the HCl (v = 0, J = 1, 2 and 3) products of reaction at a mean collision energy of 33.5 kJ mol-1 determine the centre-of-mass frame angular scattering and kinetic energy release distributions. The HCl products form with relative populations of J = 0-5 levels that fit to a rotational temperature of 138 ± 13 K. Product kinetic energy release distributions agree well with those derived from a previous VMI study of the pentyl radical co-product [Estillore et al., J. Chem. Phys. 2010, 132, 164313], but the angular distributions show more pronounced forward scattering. The QCT calculations reproduce many of the experimental observations, and allow comparison of the site-specific dynamics of abstraction of primary and secondary H-atoms. They also quantify the relative reactivity towards Cl atoms of the three different H-atom environments in n-pentane.

Direct comparison of 3-centre and 4-centre HBr elimination pathways in methyl-substituted vinyl bromides.

Elimination of HBr from UV-photoexcited vinyl bromides can occur through both 3-centre and 4-centre transition states (TSs). The competition between these pathways is examined using velocity map imaging of HBr (v = 0-2, J) photofragments. The three vinyl bromides chosen for study have methyl substituents that block either the 3-centre or the 4-centre TS, or leave both pathways open. The kinetic energy distributions extracted from velocity map images of HBr from 193 nm photolysis of the three vinyl bromide compounds are approximately described by a statistical model of energy disposal among the degrees of freedom of the photoproducts, and are attributed to dissociation on the lowest electronic state of the molecule after internal conversion. Dissociation via the 4-centre TS gives greater average kinetic energy release than for the 3-centre TS pathway. The resonance enhanced multi-photon ionization (REMPI) schemes used to detect HBr restrict measurements to J ≤ 7 for v = 2 and J ≤ 15 for v = 0. Within this spectroscopic range, the HBr rotational temperature is colder for the 4-centre than for the 3-centre elimination pathway. Calculations of the intrinsic reaction coordinates and RRKM calculations of HBr elimination rate coefficients provide mechanistic insights into the competition between the pathways.

Dynamical Effects and Product Distributions in Simulated CN + Methane Reactions.

Dynamics of collisions between structured molecular species quickly become complex as molecules become large. Reactions of methane with halogen and oxygen atoms serve as model systems for polyatomic molecule chemical dynamics, and replacing the atomic reagent with a diatomic radical affords further insights. A new, full-dimensional potential energy surface for collisions between CN + CH4 to form HCN + CH3 is developed and then used to perform quasi-classical simulations of the reaction. Coupled-cluster energies serve as input to an empirical valence bonding (EVB) model, which provides an analytical function for the surface. Efficient sampling permits simulation of velocity-map ion images and exploration of dynamics over a range of collision energies. Reaction populates HCN vibration, and energy partitioning changes with collision energy. The reaction cross-section depends on the orientation of the diatomic CN radical. A two-dimensional extension of the cone of acceptance for an atom in the line-of-centers model appropriately describes its reactivity. The simulation results foster future experiments and diatomic extensions to existing atomic models of chemical collisions and reaction dynamics.

Evidence for concerted ring opening and C-Br bond breaking in UV-excited bromocyclopropane.

Photodissociation of gaseous bromocyclopropane via its A-band continuum has been studied at excitation wavelengths ranging from 230 nm to 267 nm. Velocity-map images of ground-state bromine atoms (Br), spin-orbit excited bromine atoms (Br(∗)), and C3H5 hydrocarbon radicals reveal the kinetic energies of these various photofragments. Both Br and Br(∗) atoms are predominantly generated via repulsive excited electronic states in a prompt photodissociation process in which the hydrocarbon co-fragment is a cyclopropyl radical. However, the images obtained at the mass of the hydrocarbon radical fragment identify a channel with total kinetic energy greater than that deduced from the Br and Br(∗) images, and with a kinetic energy distribution that exceeds the energetic limit for Br + cyclopropyl radical products. The velocity-map images of these C3H5 fragments have lower angular anisotropies than measured for Br and Br(∗), indicating molecular restructuring during dissociation. The high kinetic energy C3H5 signals are assigned to allyl radicals generated by a minor photochemical pathway which involves concerted C-Br bond dissociation and cyclopropyl ring-opening following single ultraviolet (UV)-photon absorption. Slow photofragments also contribute to the velocity map images obtained at the C3H5 radical mass, but the corresponding slow Br atoms are not observed. These features in the images are attributed to C3H5 (+) from the photodissociation of the C3H5Br(+) molecular cation following two-photon ionization of the parent compound. This assignment is confirmed by 118-nm vacuum ultraviolet ionization studies that prepare the molecular cation in its ground electronic state prior to UV photodissociation.

Computational Study of Competition between Direct Abstraction and Addition-Elimination in the Reaction of Cl Atoms with Propene.

Quasi-classical trajectory calculations on a newly constructed and full-dimensionality potential energy surface (PES) examine the dynamics of the reaction of Cl atoms with propene. The PES is an empirical valence bond (EVB) fit to high-level ab initio energies and incorporates deep potential energy wells for the 1-chloropropyl and 2-chloropropyl radicals, a direct H atom abstraction route to HCl + allyl radical (CH2CHCH2(•)) products (Δ(r)H(298K)(⊖) = −63.1 kJ mol(-1)), and a pathway connecting these regions. In total, 94 000 successful reactive trajectories were used to compute distributions of angular scattering and HCl vibrational and rotational level populations. These measures of the reaction dynamics agree satisfactorily with available experimental data. The dominant reaction pathway is direct abstraction of a hydrogen atom from the methyl group of propene occurring in under 500 fs. Less than 10% of trajectories follow an addition­elimination route via the two isomeric chloropropyl radicals. Large amplitude motions of the Cl about the propene molecular framework couple the addition intermediates to the direct abstraction pathway. The EVB method provides a good description of the complicated PES for the Cl + propene reaction despite fitting to a limited number of ab initio points, with the further advantage that dynamics specific to certain mechanisms can be studied in isolation by switching off coupling terms in the EVB matrix connecting different regions of the PES.

The chemical assessment of surfaces and air (CASA) study: using chemical and physical perturbations in a test house to investigate indoor processes.

The Chemical Assessment of Surfaces and Air (CASA) study aimed to understand how chemicals transform in the indoor environment using perturbations (e.g., cooking, cleaning) or additions of indoor and outdoor pollutants in a well-controlled test house. Chemical additions ranged from individual compounds (e.g., gaseous ammonia or ozone) to more complex mixtures (e.g., a wildfire smoke proxy and a commercial pesticide). Physical perturbations included varying temperature, ventilation rates, and relative humidity. The objectives for CASA included understanding (i) how outdoor air pollution impacts indoor air chemistry, (ii) how wildfire smoke transports and transforms indoors, (iii) how gases and particles interact with building surfaces, and (iv) how indoor environmental conditions impact indoor chemistry. Further, the combined measurements under unperturbed and experimental conditions enable investigation of mitigation strategies following outdoor and indoor air pollution events. A comprehensive suite of instruments measured different chemical components in the gas, particle, and surface phases throughout the study. We provide an overview of the test house, instrumentation, experimental design, and initial observations - including the role of humidity in controlling the air concentrations of many semi-volatile organic compounds, the potential for ozone to generate indoor nitrogen pentoxide (N2O5), the differences in microbial composition between the test house and other occupied buildings, and the complexity of deposited particles and gases on different indoor surfaces.

The persistence of smoke VOCs indoors: Partitioning, surface cleaning, and air cleaning in a smoke-contaminated house.

Wildfires are increasing in frequency, raising concerns that smoke can permeate indoor environments and expose people to chemical air contaminants. To study smoke transformations in indoor environments and evaluate mitigation strategies, we added smoke to a test house. Many volatile organic compounds (VOCs) persisted days following the smoke injection, providing a longer-term exposure pathway for humans. Two time scales control smoke VOC partitioning: a faster one (1.0 to 5.2 hours) that describes the time to reach equilibrium between adsorption and desorption processes and a slower one (4.8 to 21.2 hours) that describes the time for indoor ventilation to overtake adsorption-desorption equilibria in controlling the air concentration. These rates imply that vapor pressure controls partitioning behavior and that house ventilation plays a minor role in removing smoke VOCs. However, surface cleaning activities (vacuuming, mopping, and dusting) physically removed surface reservoirs and thus reduced indoor smoke VOC concentrations more effectively than portable air cleaners and more persistently than window opening.