John J. Orlando

The reactions of ozone with alkenes: An important source of HOx in the boundary layer

The reactions of ozone with alkenes have been shown recently to lead to the direct production of OH radi- cals. Organic peroxy radicals (RO2) probably accompany the production of OH. In this paper, we draw attention to the potential importance of these reactions in the primary production of HOx (HOx = OH, HO2 and RO2) radicals in various regions of the boundary layer. The reactions of ozone with anthropogenic alkenes are shown to be the most important source of HOx in many urban settings during the day and evening, and a significant source at night. The majority of HO x comes from trace quantities of alkenes with internal double bonds. Reaction of 03 with isoprene and terpenes can be an important source of HOx in forested regions; we show that these reactions are the dominant radical source in the late afternoon and into the night. This additional HOx source is expected to increase predicted OH concentrations compared to those calculated by models that do not include the O3-alkene source.

Contribution of isoprene to chemical budgets: A model tracer study with the NCAR CTM MOZART‐4

[1] We present a study of the sensitivity of isoprene emission calculations in a global chemistry transport model (CTM) to input land cover characteristics and analyze the impacts of changes in isoprene on the tropospheric budgets of atmospheric key species. The CTM Model for Ozone and Related Chemical Species, version 4 (MOZART-4) includes the online calculation of isoprene emissions based on the Model of Emissions of Gases and Aerosols from Nature (MEGAN), which is driven by three different land parameter inputs. We also included a tagging scheme in the CTM, which keeps track of the production of carbon containing species from isoprene oxidation. It is found that the amount of tropospheric carbon monoxide (CO), formaldehyde (HCHO) and peroxyacetylnitrate (PAN) explained by isoprene oxidation ranges from 9–16%, 15–27%, and 22–32%, depending on the isoprene emissions scenario. Changes in the global tropospheric burden with different land cover inputs can reach up to 10% for CO, 15% for HCHO, and 20% for PAN. Changes for ozone are small on a global scale, but regionally differences are as large as 3DU in the tropospheric column and as large as 5 ppbv in the surface concentrations. Our results demonstrate that a careful integration of isoprene emissions and chemistry in CTMs is very important for simulating the budgets of a number of atmospheric trace gases. We further demonstrate that the model tagging scheme has the capability of improving conventional methods of constraining isoprene emissions from space-borne HCHO column observations, especially in regions where a considerable part of the variability in the HCHO column is not related to isoprene.

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.

Direct evidence for chlorine-enhanced urban ozone formation in Houston, Texas

Urban air pollution is characterized by high ozone levels, formed when volatile organic compounds (VOCs) are oxidized in the presence of nitrogen oxides (NOx). VOC and NOx emissions controls have traditionally been implemented to reduce urban ozone formation, however, a separate chemical species implicated in ozone formation in Houston, TX and possibly other urban areas is the chlorine radical (Cl ). Cl enhances tropospheric VOC oxidation, but is not included in models used to develop air quality attainment plans. We present results of a three-fold approach to elucidate the importance of Cl in urban ozone formation: (1) the first direct evidence of chlorine chemistry in the urban troposphere, (2) enhanced ozone formation (>75 parts per 10 9 (ppb/h) observed when small amounts of chlorine (Cl2) are injected into captive ambient air, and (3) enhanced ozone formation (B16 ppb) predicted by regional photochemical models employing Cl chemistry. These results suggest that reducing chlorine emissions should be considered in urban ozone management strategies. r 2003 Elsevier Science Ltd. All rights reserved.

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.

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)%.

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.

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.

The atmospheric chemistry of glycolaldehyde

The chemistry of glycolaldehyde (hydroxyacetaldehyde) relevant to the troposphere has been investigated using UV absorption spectrometry and FTIR absorption spectrometry in an environmental chamber. Quantitative UV absorption spectra have been obtained for the first time. The UV spectrum peaks at 277 nm with a maximum cross section of (5.5± 0.7)×10−20 cm2 molecule−1. Studies of the ultraviolet photolysis of glycolaldehyde (λ = 285 ± 25 nm) indicated that the overall quantum yield is > 0.5 in one bar of air, with the major products being CH2OH and HCO radicals. Rate coefficients for the reactions of Cl atoms and OH radicals with glycolaldehyde have been determined to be (7.6± 1.5)×10−11 and (1.1± 0.3)×10−11 cm3 molecule−1 s−1, respectively, in good agreement with the only previous study. The lifetime of glycolaldehyde in the atmosphere is about 1.0 day for reaction with OH, and > 2.5 days for photolysis, although both wet and dry deposition should also be considered in future modeling studies.

Measurements of the Henry's law coefficients of 2-methyl-3-buten-2-ol, methacrolein, and methylvinyl ketone

Using an equilibrium headspace technique, Henrys law coefficients were measured for methacrolein (H = 6.5 ± 0.7 M atm-1) and methylvinyl ketone (41 ± 7.0 M atm-1) in water at 25 °C. In addition, 2-methyl-3-buten-2-ol was studied at 30 °C in water and in an aqueous ionic solution representative of plant tissue. Similar values were found in deionized water (65 ± 3.5 M atm-1) and in a 0.05 mol kg-1 Ca2+, K+, NO3-, SO42- solution (62 ± 0.8 M atm-1). These Henrys Law coefficients are too small to allow for significant partitioning of methacrolein, methylvinyl ketone or methylbutenol into cloud water under equilibrium conditions.