Geoffrey S. Tyndall

Atmospheric chemistry of small organic peroxy radicals

Global atmospheric models play a key role in international assessments of the human impact on global climate and air pollution. To increase the accuracy and facilitate comparison of results from such models, it is essential they contain up-to-date chemical mechanisms. To this end, we present an evaluation of the atmospheric chemistry of the four most abundant organic peroxy radicals: CH3O2, C2H5O2, CH3C(O)O2, and CH3C(O)CH2O2. The literature data for the atmospheric reactions of these radicals are evaluated. In addition, the ultraviolet absorption cross sections for the above radicals and for HO2 have been evaluated. The absorption spectra were fitted to an analytical formula, which enabled published spectra to be screened objectively. Published kinetic and product data were reinterpreted, or in some case reanalyzed, using the new cross sections, leading to a self-consistent set of kinetic, mechanistic, and spectroscopic data. Product studies were also evaluated. A set of peroxy radical reaction rate coefficients and products are recommended for use in atmospheric modeling. A three-dimensional global chemical transport model (the Intermediate Model for the Global Evolution of Species, IMAGES) was run using both previously recommended rate coefficients and the current set to highlight the sensitivity of key atmospheric trace species to the peroxy radical chemistry used in the model.

Product studies of the OH‐ and ozone‐initiated oxidation of some monoterpenes

The OH- and O3-initiated oxidation of five monoterpenes (myrcene, terpinolene, Δ3-carene, α-pinene, and β-pinene) has been studied in environmental chambers equipped with either a Fourier transform infrared spectrometer or a gas chromatography/flame ionization detector system. The OH-oxidation of myrcene and terpinolene is shown to lead to substantial yields of acetone (36 and 39%, respectively), while the acetone yield from the pinene compounds is quite small (4% and ∼2%, for α- and β-pinene, respectively). Formaldehyde has been identified as a major product (yields of 20–40%) in the OH-initiated oxidation of all five species. Formic acid was also observed in the OH-initiated oxidation of all five monoterpenes, with yields of 2% from β-pinene and 5–9% from the other species studied. The production of acetone from the reaction of monoterpenes with ozone in the presence of an OH scavenger was measured. The yields of acetone for the O3 reactions were α-pinene, 0.03±0.01; β-pinene, 0.009±0.009; Δ3-carene, 0.10±0.015; myrcene, 0.25±0.06; and terpinolene, 0.50±0.06. The mechanism leading to the production of these compounds is discussed, as is the atmospheric relevance of the results. In particular, an estimate of the contribution of monoterpene oxidation to observed atmospheric levels of acetone and formic acid is made.

Patterns in volatile organic compound emissions along a savanna-rainforest gradient in central Africa

In temperate regions the chemistry of the lower troposphere is known to be significantly affected by biogenic volatile organic compounds (VOCs) emitted by plants. The chemistry of the lower troposphere over the tropics, however, is poorly understood, in part because of the considerable uncertainties in VOC emissions from tropical ecosystems. Present global VOC models predict that base emissions of isoprene from tropical rainforests are considerably higher than from savannas. These global models of VOC emissions which rely mainly on species inventories are useful, but significant improvement might be made with more ecologically based models of VOC emissions by plants. Ecosystems along a successional transect from woodland savanna to primary rainforest in central Africa were characterized for species composition and vegetation abundance using ground surveys and remotely sensed data. A total of 336 species (mostly trees) at 13 sites were recorded, and 208 of these were measured for VOC emissions at near-optimal light and temperature conditions using a leaf cuvette and hand-held photoionization detector (PID). A subset of 59 species was also sampled using conventional VOC emission techniques in order to validate the PID technique. Results of ecological and VOC emission surveys indicate both phylogenetic and successional patterns along the savanna-rainforest transect. Genera and families of trees which tend to emit isoprene include Lophira, Irvingia, Albizia, Artocarpus, Ficus, Pterocarpus, Caesalpiniaceae, Arecaceae, and Moraceae. Other taxa tend to contain stored VOCs (Annonaceae and Asteraceae). Successional patterns suggest that isoprene emissions are highest in the relatively early successional Isoberlinia forest communities and progressively decrease in the later successional secondary and primary rainforest communities. Stored VOCs appear to increase along the savanna-rainforest succession, but these data are more tentative. These findings are consistent with successional patterns of isoprene and terpene fluxes in North American forests and highlight the feasibility of constructing better predictive models of VOC emissions.

Kinetics and mechanisms of the reactions of chlorine atoms with ethane, propane, and n-butane

Absolute (flash photolysis) and relative (FTIR-smog chamber and GC) rate techniques were used to study the gas-phase reactions of Cl atoms with C2H6 (k1), C3H8 (k3), and n-C4H10 (k2). At 297 ± 1 K the results from the two relative rate techniques can be combined to give k2/k1 = (3.76 ± 0.20) and k3/k1 = (2.42 ± 0.10). Experiments performed at 298–540 K give k2/k1 = (2.0 ± 0.1)exp((183 ± 20)/T). At 296 K the reaction of Cl atoms with C3H8 produces yields of 43 ± 3% 1-propyl and 57 ± 3% 2-propyl radicals, while the reaction of Cl atoms with n-C4H10 produces 29 ± 2% 1-butyl and 71 ± 2% 2-butyl radicals. At 298 K and 10–700 torr of N2 diluent, 1- and 2-butyl radicals were found to react with Cl2 with rate coefficients which are 3.1 ± 0.2 and 2.8 ± 0.1 times greater than the corresponding reactions with O2. A flash-photolysis technique was used to measure k1 = (5.75 ± 0.45) × 10−11 and k2 = (2.15 ± 0.15) × 10−10 cm3 molecule−1 s−1 at 298 K, giving a rate coefficient ratio k2/k1 = 3.74 ± 0.40, in excellent agreement with the relative rate studies. The present results are used to put other, relative rate measurements of the reactions of chlorine atoms with alkanes on an absolute basis. It is found that the rate of hydrogen abstraction from a methyl group is not influenced by neighboring groups. The results are used to refine empirical approaches to predicting the reactivity of Cl atoms towards hydrocarbons. Finally, relative rate methods were used to measure rate coefficients at 298 K for the reaction of Cl atoms with 1- and 2-chloropropane and 1- and 2-chlorobutane of (4.8 ± 0.3) × 10−11, (2.0 ± 0.1) × 10−10, (1.1 ± 0.2) × 10−10, and (7.0 ± 0.8) × 10−11 cm3 molecule−1 s−1, respectively.

Absorption cross sections for water vapor from 183 to 193 nm

Absorption cross sections for water vapor at 184.9 nm were measured using a standard low-pressure mercury lamp light source, optically filtered to isolate the spectral region near the emission line. The light from the source was detected using a solar-blind phototube. Experiments were performed over a wide range of water column amounts, using neat water vapor and water vapor/nitrogen mixtures, with four methods to determine the water vapor concentration in the cell. Absorption cross sections were measured between 0 and 80°C. The results from these experiments yield a cross section of 7.14 (± 0.2) × 10−20 cm² molecule−1 at 25°C with a small positive temperature dependence (about 4% between 0 and 80°C). This result is about 30% greater than current recommendations. The absorption spectrum was also measured at discrete wavelengths between 183 and 193 nm using a scanning double monochromator system. The cross section of deuterated water vapor was determined at 184.9 nm. The implications of the changes in the H2O cross section for the calibration of atmospheric OH and HO2 measurements using the 184.9 nm photolysis of water vapor are discussed.

Rate and mechanism of the reactions of OH and Cl with 2-methyl-3-buten-2-ol

An environmental chamber/Fourier transform infrared system was used to determine the rate coefficient k1 for the gas-phase reaction of OH with 2-methyl-3-buten-2-ol (MBO, (CH3)2C(OH)CH=CH2), relative to the rate of its reaction with ethylene (k2) and propylene (k3). Experiments performed at 295±1 K, in 700 torr total pressure of air, gave k1 = (6.9±1.0) × 10−11 cm3 molecule−1 s−1. At 295±1 K, the reaction of OH with MBO yielded, on a per mole basis, (52±5)% acetone, (50±5)% glycolaldehyde, and (35±4)% formaldehyde. The production of acetone from the oxidation of MBO may be of significance globally. The kinetics and mechanism of the reaction of chlorine atoms with MBO (k15) have also been studied at 700 torr total pressure of air and 295±1 K. The rate coefficient was determined using a relative rate technique, with ethane (k16), ethylene (k17), and cyclohexane (k18) as reference compounds. The value of k15 was found to be (3.3±0.4) × 10−10 cm3 molecule−1 s−1 at 295 K. The major carbon-containing products obtained in the Cl-atom oxidation of MBO were acetone (47±5)%, chloroacetaldehyde (53±5)%, HCOCl (<11%), and formaldehyde (6 ± 2)%.

Formation of Low Volatility Organic Compounds and Secondary Organic Aerosol from Isoprene Hydroxyhydroperoxide Low-NO Oxidation.

Gas-phase low volatility organic compounds (LVOC), produced from oxidation of isoprene 4-hydroxy-3-hydroperoxide (4,3-ISOPOOH) under low-NO conditions, were observed during the FIXCIT chamber study. Decreases in LVOC directly correspond to appearance and growth in secondary organic aerosol (SOA) of consistent elemental composition, indicating that LVOC condense (at OA below 1 μg m(-3)). This represents the first simultaneous measurement of condensing low volatility species from isoprene oxidation in both the gas and particle phases. The SOA formation in this study is separate from previously described isoprene epoxydiol (IEPOX) uptake. Assigning all condensing LVOC signals to 4,3-ISOPOOH oxidation in the chamber study implies a wall-loss corrected non-IEPOX SOA mass yield of ∼4%. By contrast to monoterpene oxidation, in which extremely low volatility VOC (ELVOC) constitute the organic aerosol, in the isoprene system LVOC with saturation concentrations from 10(-2) to 10 μg m(-3) are the main constituents. These LVOC may be important for the growth of nanoparticles in environments with low OA concentrations. LVOC observed in the chamber were also observed in the atmosphere during SOAS-2013 in the Southeastern United States, with the expected diurnal cycle. This previously uncharacterized aerosol formation pathway could account for ∼5.0 Tg yr(-1) of SOA production, or 3.3% of global SOA.

Actinometric and radiometric measurement and modeling of the photolysis rate coefficient of ozone to O (^1D) during Mauna Loa Observatory Photochemistry Experiment 2

The in situ photolysis rate coefficient of O3 to O(1D) has been measured at Mauna Loa Observatory using a new actinometric instrument based on the reaction of O(1D) with N2O and with a hemispherical radiometer. One minute averaged photolysis rate coefficients were determined with an overall uncertainty of approximately ±11% at the 1 σ level for the actinometer and ±15% at the 1 σ level for the radiometer. Over 120 days of data were collected with varying cloud cover, aerosol loadings, and overhead ozone representing the first set of long term measurements. Clear sky solar noon values vary between approximately 3.0 × 10−5 and 4.5 × 10−5 sec−1. Modeling of the photolysis rate coefficients was done using a discrete ordinate radiative transfer scheme and results were compared with the actinometric measurements. The quantum yields for O(1D) production are reevaluated from existing data and reported here. The comparisons were done using the quantum yields for the photolysis of ozone recommended by DeMore et al. [1994], the newer evaluation of Michelsen et al. [1994], and also with reevaluated values in this paper. An analysis of the measured photolysis rate coefficient of O3 to O(1D) and model simulations of the photolysis rate coefficient data from clear days during the study provides added insight into the choice of quantum yield data for use in photochemical models of the troposphere.

Contribution of Secondary VOC to the Composition of Aqueous Atmospheric Particles: A Modeling Approach

Measurements show that 20–60% of the carbon mass present in fine atmospheric particulate matter consists of water soluble organic compounds (WSOC). However, only 5–20% of this WSOC has been identified, mainly as dicarboxylic acids. Because of their high solubility in water, multifunctional secondary compounds derived from the gas-phase oxidation of volatile organic compounds (VOC) are suspected to be key contributors to the WSOC. To test this assumption, an estimate of aqueous uptake of secondary VOC was included in a highly detailed gas-phase mechanism which treats explicitly the formation of the secondary VOC from a set of representative primary species. Simulations were conducted for 2 scenarios, representing typical rural and urban areas. It was observed that the uptake of secondary VOC can lead to WSOC mass concentrations in the range of a few μC m−3, in fairly good agreement with typical WSOC mass concentrations measured. Speciation of WSOC was found to be mainly as tri- or higher multifunctional hydroxy-carbonyl species and hydroxy-hydroperoxide-carbonyl species, in urban and rural environments, respectively. However, it was also found that taking into account only the absorption of secondary VOC does not bring the carboxylic acids mass concentration in agreement with measurements. An attempt was made to explain this discrepancy by introducing chemistry occurring within deliquescent aerosols.

Mechanism of the OH‐initiated oxidation of methacrolein

The OH-initiated oxidation of methacrolein, a major product of isoprene oxidation, has been studied in an environmental chamber using FT-IR spectroscopy. Products observed (which account for more than 90% of the reacted carbon) were CO, CO2, hydroxyacetone, formaldehyde, and methacryloylperoxynitrate (MPAN). It is determined that the attack of OH on methacrolein occurs 55% of the time via addition to the double bond, and 45% via abstraction of the aldehydic hydrogen atom, in agreement with a previous study. The end products of the abstraction channel are identified and quantified for the first time, and the mechanism of their production discussed.