Aerosol formation from the gas-phase reaction of hydroxyl radical with the natural hydrocarbon bornyl acetate

The gas-phase reaction of bornyl acetate (1,7,7-trimethyl-bicyclo[2,2,1]-heptan-2-ol-acetate) with hydroxyl radical has been studied in a smog chamber. It was found that the reaction of bornyl acetate with OH radicals leads to organic aerosols. The chemical composition of the aerosol was studied. On the basis of mass spectral data 1,7,7-trimethyl-6-acetyloxy-bicyclo[2.2.1]-heptan-2,3-dione has been tentatively identified in irradiated CH₃ONO–NO–air–bornyl acetate mixtures. The aerosol carbon yield, the fraction of the carbon initially present that is converted to aerosol, has been estimated to be ≅5%.     

A study of gas-phase mercury speciation using detailed chemical kinetics.

Mercury speciation in combustion-generated flue gas was modeled using a detailed chemical mechanism consisting of 60 reactions and 21 species. This speciation model accounts for the chlorination and oxidation of key flue-gas components, including elemental mercury (Hg0). Results indicated that the performance of the model is very sensitive to temperature. Starting with pure HCl, for lower reactor temperatures (less than approximately 630 degrees C), the model produced only trace amounts of atomic and molecular chlorine (Cl and Cl2), leading to a drastic underprediction of Hg chlorination compared with experimental data. For higher reactor temperatures, model predictions were in good accord with experimental data. For conditions that produce an excess of Cl and Cl2 relative to Hg, chlorination of Hg is determined by the competing influences of the initiation step, Hg + Cl = HgCl, and the Cl recombination reaction, 2Cl = Cl2. If the Cl recombination reaction is faster, Hg chlorination will eventually be dictated by the slower pathway Hg + Cl2 = HgCl2.     

Unique products from the reaction of naphthalene with the hydroxyl radical

Many of the products of the reaction of naphthalene (Naph) with the OH radical in a reaction chamber were identified. Previously unidentified products included 1,2-naphthoquinone (NQ), oxygenated indenes and benzopyrones. Possible pathways for the formation of 1,2-NQ and 1,4-NQ are proposed. In the chamber reactions, more 1,2-NQ than 1,4-NQ partitioned to the particle phase. From this result we infer that, in the atmosphere, the percentage of 1,2-NQ in the particle phase should be greater than that for the 1,4-NQ. Because both of these compounds are considered to be toxic, and since they appear in both the gas and particle phases in the reaction chamber, and by implication in the atmosphere, it is considered important that both the gas and particle phases of these two compounds should be measured to assess their impact on human health.     

Computer estimation of the Atmospheric gas-phase reaction rate of organic compounds with hydroxyl radicals and ozone

The Atmospheric Oxidation Program (AOP) is a computer program that estimates the rate constant for the atmospheric, gas-phase reaction between photochemically produced hydroxyl radicals (OH) and organic chemicals. It also estimates the rate constant for the gas-phase reaction between ozone and olefinic/acetylenic compounds. AOP, which uses estimation methods developed by Atkinson and co-workers, estimates more accurate rate constants than the PCFAP (Fate of Atmospheric Pollutants) program that was part of the U.S. EPA's Graphical Exposure Modeling System (GEMS). Due to its superior predictive ability, the EPA is currently using AOP to evaluate the atmospheric fate of compounds defined under Sections 4, 5 and 6 of the Toxic Substances Control Act (TSCA).     

Prediction of hydroxyl radical reaction rates with organic compounds in the gas phase

Room temperature rate constants for the gas phase reaction of OH radicals with organic substrates can be estimated by means of a statistically significant correlation with the corresponding rate constants in liquid water.     

A new model of tropospheric hydroxyl radical concentrations

A chemistry model of the global troposphere is presented which focuses on the hydroxyl radical, OH. Global distributions of OH are calculated based on known chemical reaction pathways, experimentally measured values of precursor species O3, H2O, NOx (defined as NO+NO2), CO, CH4, and actinic flux (which includes the effects of cloud cover and O3 column absorption). Model grid resolution is 1 km in altitude by 10 degrees latitude, and zonally divided into land or ocean. Species are calculated as seasonal averages. Global annual mean OH in the troposphere (up to 14 km altitude) is calculated to be 9.2 x 10(5) molcm(-3) with averages of 9.8 x 10(5) in the northern hemisphere, and 8.5 x 10(5) in the southern hemisphere. Global CO and CH(4) oxidation rates by OH are calculated to be 1840 Tgyear(-1) and 580 Tgyear(-1), respectively. OH is found to be most sensitive to O3 and H2O concentrations, as well as to the photolysis rate of O3 to O1D. Sensitivity of CO and CH4 oxidation rates to cloud presence shows an inverse relationship to cloud amount and optical depth. Model results are shown to be consistent with results from two other published models.     

Stable carbon kinetic isotope effects for the production of methacrolein and methyl vinyl ketone from the gas-phase reactions of isoprene with ozone and hydroxyl radicals

The stable-carbon kinetic isotope effects (KIEs) associated with the production of methacrolein (MACR) and methyl vinyl ketone (MVK) from the reactions of isoprene with ozone and OH radicals were studied in a 25 L reaction chamber at (298±2) K and ambient pressure. The time dependence of both the stable-carbon isotope ratios and the concentrations was determined using a gas chromatography combustion isotope ratio mass spectrometry (GCC-IRMS) system. The average yields of ¹³C-containing MACR and MVK generated from the ozone reaction of ¹³C-containing isoprene differed by ?3.6‰ and ?4.5‰, respectively, from the yields for MACR and MVK containing only ¹²C. For MACR and MVK generated from the OH-radical oxidation of isoprene the corresponding values were ?3.8‰ and ?2.2‰, respectively. These values indicate a significant depletion in the ¹³C abundance of MACR and MVK upon their formation relative to isoprene’s pre-reaction ¹³C/¹²C ratio, which is supported by theoretical interpretations of the oxidation mechanism of isoprene and its ¹³C-substituted isotopomers. Numerical model calculations of the isoprene + O₃ reaction predicted a similar depletion in ¹³C for both reaction products upon production. Furthermore, the model predicts mixing ratios and stable carbon delta values for isoprene, MACR, and MVK that were in agreement with the experimental results. The combined knowledge of isotope enrichment values with KIEs will reduce uncertainties in determinations of the photochemical histories of isoprene, MACR, and MVK in the troposphere. The studies presented here were conducted with using isoprene without any artificial isotope enrichment or depletion and it is therefore very likely that these results are directly applicable to the interpretation of studies of isoprene oxidation using stable carbon isotope ratio measurements.     

Degradation of gas-phase propylene glycol monomethyl ether acetate by ultraviolet/ozone process: A kinetic study

A pilot-scale plug-flow reactor was built to investigate its performance in treating airborne propylene glycol monomethyl ether acetate (PGMEA) via ozonation, ultraviolet (UV) photolysis and UV/O3 technologies. Governing factors, such as the initial molar ratio of ozone (O3) to PG-MEA, UV volumetric electric power input, and moisture content in the influent airstream, were investigated. A 1-L batch reactor was used to investigate some photodegradation characteristics of PGMEA in advance. Experiments were conducted at a fixed influent PGMEA concentration of approximately 50 ppm and an ambient temperature of 26 degrees C. A gas space time of 85 sec in the plug-flow reactor was kept for either ozonation or photolysis reaction, whereas a gas space time of 170 sec was used for the UV/O3 degradation. Results show that an initial molar ratio of O3 to PGMEA of >2.91 and an UV volumetric electric input power of 0.294 W/L(-1) sufficed to obtain PGMEA decompositions of >90% by UV/O3. Kinetic analyses indicate that all types of PGMEA decomposition are pseudo-first order with respect to its concentration. Moisture content (relative humidity = 15-99%) and UV volumetric electric input power (0.147 and 0.294 W/L(-1)) were major factors that strongly affect the PGMEA degradation rate.     

Effects of uncertainty in the reaction of the hydroxyl radical with nitrogen dioxide on model-simulated ozone control strategies

We evaluated the effect of a 20% reduction in the rate constant of the reaction of the hydroxyl radical with nitrogen dioxide to produce nitric acid (OH+NO₂→HNO₃) on model predictions of ozone mixing ratios ([O₃]) and the effectiveness of reductions in emissions of volatile organic compounds (VOC) and nitrogen oxides (NO_x) for reducing [O₃]. By comparing a model simulation with the new rate constant to a base case scenario, we found that the [O₃] increase was between 2 and 6% for typical rural conditions and between 6 and 16% for typical urban conditions. The increases in [O₃] were less than proportional to the reduction in the OH+NO₂ rate constant because of negative feedbacks in the photochemical mechanism. Next, we used two different approaches to evaluate how the new OH+NO₂ rate constant changed the effectiveness of reductions in emissions of VOC and NO_x: first, we evaluated the effect on [O₃] sensitivity to small changes in emissions of VOC (d[O₃]/dE_VOC) and NO_x (d[O₃]/dE_NOx); and secondly, we used the empirical kinetic modeling approach to evaluate the effect on the level of emissions reduction necessary to reduce [O₃] to a specified level. Both methods showed that reducing the OH+NO₂ rate constant caused control strategies for VOC to become less effective relative to NO_x control strategies. We found, however, that d[O₃]/dE_VOC and d[O₃]/dE_NOx did not quantitatively predict the magnitude of the change in the control strategy because the [O₃] response was nonlinear with respect to the size of the emissions reduction. We conclude that model sensitivity analyses calculated using small emissions changes do not accurately characterize the effect of uncertainty in model inputs (in this case, the OH+NO₂ rate constant) on O₃ attainment strategies. Instead, the effects of changes in model inputs should be studied using large changes in precursor emissions to approximate realistic attainment scenarios.     

FTIR gas-phase kinetic study on the reactions of some acrylate esters with OH radicals and Cl atoms

Acrylate esters are α,β-unsaturated esters that contain vinyl groups directly attached to the carbonyl carbon. These compounds are widely used in the production of plastics and resins. Atmospheric degradation processes of these compounds are currently not well understood. The kinetics of the gas phase reactions of OH radicals with methyl 3-methylacrylate and methyl 3,3-dimethylacrylate were determined using the relative rate technique in a 50 L Pyrex photoreactor using in situ FTIR spectroscopy at room temperature (298?±?2 K) and atmospheric pressure (708?±?8 Torr) with air as the bath gas. Rate coefficients obtained were (in units cm³ molecule^?1 s^?1): (3.27?±?0.33)?×?10^?11 and (4.43?±?0.42)?×?10^?11, for CH₃CH═CHC(O)OCH₃ and (CH₃)₂CH═CHC(O)OCH_3, respectively. The same technique was used to study the gas phase reactions of hexyl acrylate and ethyl hexyl acrylate with OH radicals and Cl atoms. In the experiments with Cl, N₂ and air were used as the bath gases. The following rate coefficients were obtained (in cm³ molecule^?1 s^?1): k₃ (CH₂═CHC(O)O(CH₂)₅CH₃?+?Cl)?=?(3.31?±?0.31)?×?10^?10, k₄(CH₂═CHC(O)OCH₂CH(CH₂CH₃)(CH₂)₃CH₃?+?Cl)?=?(3.46?±?0.31)?×?10^?10, k₅(CH₂═CHC(O)O(CH₂)₅CH₃?+?OH)?=?(2.28?±?0.23)?×?10^?11, and k₆(CH₂═CHC(O)OCH₂CH(CH₂CH₃)(CH₂)₃CH₃?+?OH)?=?(2.74?±?0.26)?×?10^?11. The reactivity increased with the number of methyl substituents on the double bond and with the chain length of the alkyl group in –C(O)OR. Estimations of the atmospheric lifetimes clearly indicate that the dominant atmospheric loss process for these compounds is their daytime reaction with the hydroxyl radical. In coastal areas and in some polluted environments, Cl atom-initiated degradation of these compounds can be significant, if not dominant. Maximum Incremental Reactivity (MIR) index and global warming potential (GWP) were also calculated, and it was concluded that these compounds have significant MIR values, but they do not influence global warming.     

Photocatalytic oxidation of gas-phase elemental mercury by nanotitanosilicate fibers

Photocatalytic fibers were generated from the continuous evaporation of titanium tetraisopropoxide with tetraethyl orthosilicate through a flame burner. The morphology, the crystal form, and the components of the nanotitanosilicate fibers were analyzed by Raman spectroscopy, Field emission-scanning electron microscope, X-ray diffraction, and Brunauer-Emmett-Teller surface area analysis. The nanotitanosilicates prepared by three different carrier gases (air, N(2), and Ar) were tested for their photocatalytic ability to remove/oxidize gas-phase elemental mercury. Under UV black light, the Hg(0) capture efficiencies were 78%, 86%, and 85% for air, N(2), and Ar, respectively. For air, the value was close to 90%, even under household fluorescent light. The Hg(0) capture efficiency by nanotitanosilcate was measured under fluorescent light, UV black light, and sunlight.     

A study of atmospheric mercury dispersion

Mercury deposition — dry and wet — and re-emission from the ground has been studied around two chlor-alkali plants in Sweden. The experimental data are:

• 1.(1) monthly precipitation samples up to ca. 35km from the sources;
• 2.(2) air samples from 14 tests of 45 min duration each taken at 15–20 sites up to 5 km distance from the source;
• 3.(3) emission data from the tests mentioned above under (2).

Re-emission and dry deposition has been determined approximately for the test area in each of the test cases by a trial and error procedure implying numerical solution of the atmospheric diffusion equation, varying the boundary conditions until optimum agreement is obtained between measured and calculated air concentration.The result of a budget calculation for the 5 × 5 km² square area surrounding the source is that the average rate of re-emission is likely to be of about the same magnitude as the sum of wet and dry deposition over the study area. Considering the likely uncertainty in the calculations this result implies that less than ca. 5% of the Hg-emission from the source is net deposited with ca. 3 km distance from the source, although the sum of wet and dry deposition over the area is roughly 25% of the emission. Dry deposition over this area close to the source is found to be of order 20 times larger than the corresponding wet deposition.     

Carbonyl products of the gas-phase reaction of ozone with 1-alkenes

Carbonyl products have been identified and their formation yields measured in experiments involving the gas-phase reaction of ozone with the 1-alkenes (RCH = CH 2) 3-methyl-l-butene (R = i-propyl), 4-methyl-l-pentene (R = i-butyl), 3-methyl-l-pentene (R= s-butyl), 3,3-dimethyl-l-butene (R = t-butyl) and styrene (R = C₆H₅) at ambient T and p = 1 atm of air. Sufficient cyclohexane was added to scavenge OH in order to minimize reactions of OH with the alkenes and with their carbonyl products. Formation yields (carbonyl formed/ozone reacted) of primary carbonyls were close to the value of 1.0 that is consistent with the mechanism: O₃ + RCH = CH₂ → α(HCHO + RCHOO) + (1 - α) (H₂COO + RCHO), where formaldehyde and RCHO are the primary carbonyls and H₂COO and RCHOO are the biradicals. Measured sums of the primary carbonyl formation yields were 1.006 ± 0.053 (1 S.D.) for formaldehyde + methylpropanal from3-methyl-l-butene(α = 0.494 ± 0.049), 1.025 ± 0.017 for formaldehyde + 2-methylbutanal from 3-methyl-l-pentene (α = 0.384 ± 0.013),1.147 ± 0.050 for formaldehyde + 3-methylbutanal from 4-methyl-l-pentene (α = 0.384 ± 0.020), 0.986 ± 0.014 for formaldehyde + 2,2-dimethylpropanal from 3,3-dimethyl-l-butene (α = 0.320 ± 0.012) and 0.980 ± 0.086 for formaldehyde + benzaldehyde from styrene (α = 0.347 ± 0.059). Carbonyls other than the primary carbonyls were identified; formation pathways are proposed that involve subsequent reactions of the monosubstituted biradicals RCHOO. Similarities and differences between branched-chain 1-alkenes and n-alkyl-substituted 1-alkenes are discussed.     

Kinetic studies of the reaction of hydroxyl radicals with trichloroethylene and tetrachloroethylene

Rate coefficients are reported for the gas-phase reaction of the hydroxyl radical (OH) with C2HCl3 (k(1)) and C2Cl4 (k2) over an extended temperature range at 740+/-10 Torr in a He bath gas. These absolute rate measurements were accomplished using a laser photolysis/laser-induced fluorescence (LP/LIF) technique under slow flow conditions. The simple Arrhenius equation adequately describes the low temperature data for k1 (<650 K) and the entire data set for k2 and is given by (in units of cm3 molecule(-1) s(-1)): k1(291 - 650 K) = (9.73+/-1.15) x 10(-13) exp (158.7+/-44.0)/T, k2(293 - 720 K ) = (1.53+/-0.14) x 10(-12) exp (-688.2+/-67.5)/T. Error limits are 2sigma values. The room temperature values for k1 and k2 are within +/-2sigma of previous data using different techniques. The Arrhenius activation energies for k1 and k2 are a factor of 2-3 lower than previously reported values. The experimental measurements for both k1 and k2 in conjunction with transition state and variation transition state theory calculations infer an OH addition mechanism. The lack of a measurable kinetic isotope effect for k1 is consistent with this mechanism. Insight into the subsequent reactions of the chemically activated intermediate are presented in the form of potential energy diagrams derived from ab initio calculations.     

Modeling of the long-term tropospheric trends of hydroxyl radical for the Northern Hemisphere

The MGO 2D (altitude–longitude) channel photochemical transport model has been applied to elucidate the spatial and temporal behavior of the hydroxyl radical in the troposphere of the northern temperate belt for the pre-industrial (1850) period and the last few decades (1960 and 1995). The relation between the tropospheric OH content and the carbon monoxide, methane, nitrogen oxides emissions during 1850–1995 is studied. The distribution of the carbon monoxide concentration is calculated and validated using the observational data collected in the different locations because of the geographical non-homogeneity of its emissions. The response of the hydroxyl radical concentrations to the non-homogeneity of the CO and other atmospheric species distribution is estimated. The carbon monoxide and methane contributions to the hydroxyl photochemical sink are also evaluated. Because the changes of OH in the troposphere alternate the intensity of methane and carbon monoxide oxidation, the CO, CH₄ and OH lifetime evolution due to the increase of anthropogenic pollution intensity is analyzed and discussed.     

Optimizing electron spin resonance detection of hydroxyl radical in water

The parameters affecting the electron spin resonance (ESR) detection of hydroxyl free radical in water are studied and optimized. The hydroxyl radical is generated by the Fenton reaction with iron (II) ammonium sulfate and hydrogen peroxide reacting in a phosphate buffer using N-tert-butyl-alpha-phenylnitron as the spin trap. The concentrations of Fe²⁺, H₂O₂, and phosphate buffer are the parameters studied. The Taguchi method and the orthogonal experiment design were used to evaluate the effects of these parameters on the ESR signal intensity. By the analysis of the signal-to-noise ratio and the analysis of variance, the order of importance of the various parameters on the hydroxyl radical formation is determined for optimal ESR detection of hydroxyl radical. The results will help the development of water purification technologies using hydroxyl free radical as a green oxidant.