Estimated photochemical ozone creation potentials (POCPs) of CF3CFCH2 (HFO-1234yf) and related hydrofluoroolefins (HFOs)

The photochemical ozone creation potentials (POCPs) for CF₃CFCH₂ and other commercially significant hydrofluoroolefins have been estimated for the first time. CF₃CFCH₂ (HFO-1234yf) has a POCP of 7.0 which is less than that for ethane (12.3) and greater than for methane (0.6). Methane and ethane have sufficiently low POCPs that they are usually considered unreactive with respect to ozone formation in urban areas and accordingly are exempt from volatile organic compound (VOC) emission regulations. Estimated POCPs for other hydrofluoroolefins are: CH₂CF₂, 18.0; CF₂CF₂, 12.5; CH₂CHCF₃, 10.7; CF₂CFCF₃, 5.4; Z-CHFCFCF₃, 5.6; E-CHFCFCF₃, 7.3; CH₂CHCF₂CF₃, 6.6; and t-CHFCHCF₃, 6.4.     

Evidence of the water-cage effect on the photolysis of NO3− and FeOH2+. Implications of this effect and of H2O2 surface accumulation on photochemistry at the air–water interface of atmospheric droplets

Experiments are conducted to determine the effect of a cage of water molecules on the photolysis quantum yields of nitrate, FeOH²⁺, and H₂O₂. Results suggest that the quantum yields of nitrate and FeOH²⁺ are decreased by the recombination of photo-fragments ( OH +  NO₂ and Fe²⁺ +  OH, respectively) before they leave the surrounding cage of water molecules. However, no evidence is found for an enhanced quantum yield for H₂O₂. Therefore, the photolysis of nitrate and FeOH²⁺ could be enhanced if the cage of the solvent molecules is incomplete, as is the case at the air–water interface of atmospheric droplets. The photolysis rate constant distribution within nitrate, FeOH²⁺, and H₂O₂ aerosols is calculated by combining the expected quantum yield data in the bulk and at the interface with Mie theory calculations of light intensity. The photolysis rate constant of nitrate and FeOH²⁺ would be significantly higher at the surface than in the bulk if quantum yields are enhanced at the surface. In the case of H₂O₂, the photolysis rate constant would be enhanced by surface accumulation. The results concerning the expected rates of photolysis of these photoactive species are applied to the assessment of the reaction between benzene and OH in the presence of OH scavengers in an atmospherically relevant scenario. For a droplet of 1 μm radius, a large fraction of the total OH-benzene reaction (15% for H₂O₂, 20% for nitrate, and 35% for FeOH²⁺) would occur in the surface layer, which accounts for just 0.15% of the droplet volume.     

Atmospheric fate of acrylic acid and acrylonitrile: Rate constants with Cl atoms and OH radicals in the gas phase

Rate coefficients for the reactions of hydroxyl radicals and chlorine atoms with acrylic acid and acrylonitrile have been determined at 298 K and atmospheric pressure. The decay of the organics was followed using a gas chromatograph with a flame ionization detector (GC-FID) and the rate constants were determined using a relative rate method with different reference compounds. Room temperature rate constants are found to be (in cm³ molecule⁻¹ s⁻¹): k₁(OH+CH₂CHC(O)OH)=(1.75±0.47)×10⁻¹¹, k₂(Cl+CH₂CHC(O)OH)=(3.99±0.84)×10⁻¹⁰, k₃(OH+CH₂CHCN)=(1.11±0.33)×10⁻¹¹ and k₄(Cl+CH₂CHCN)=(1.11±0.23)×10⁻¹⁰ with uncertainties representing ±2σ. This is the first kinetic study for these reactions under atmospheric pressure. The rate coefficients are compared with previous determinations taking into account the effect of pressure on the rate constants. The effect of substituent atoms or groups on the overall rate constants is analyzed in comparison with other unsaturated compounds in the literature. In addition, atmospheric lifetimes based on the homogeneous sinks of acrylic acid and acrylonitrile are estimated and compared with other tropospheric sinks for these compounds.     

Uptake of acetone,acetaldehyde and ethanol in cold sulfuric acid solutions containing organic material: Carbon accretion mechanisms

The solubilities of acetone, ethanol and acetaldehyde in cold ternary solutions composed of 38.4–75.0 wt% sulfuric acid in water with additional dissolved organic material have been measured over the temperature range 214.4–238.5 K using a Knudsen cell reactor. The solubility of acetaldehyde in H₂SO₄/H₂O is enhanced by an order of magnitude by the presence of ethanol or acetone. The reactive uptake of acetaldehyde is enhanced by the presence of formaldehyde in acid solution. No significant formation of acetals from ethanol with carbonyl partners was observed. The solubility of acetone is unaffected by the presence of ethanol in solution and vice versa. Only polymerization of small aldehydes offers a potentially significant route to the accretion of organic material into acidic particles in the upper troposphere. The acid-catalyzed polymerization of aldehydes, RC(H)O + R′C(H)O, proceeds through the hydrated forms of the aldehydes, is optimized at acidities around 40 wt% H₂SO₄, and can potentially accumulate significant amounts (>20%) of organic material by mass in upper tropospheric particles.     

Release of gaseous bromine from the photolysis of nitrate and hydrogen peroxide in simulated sea-salt solutions

We have carried out a series of laboratory experiments to investigate the oxidation of bromide (Br⁻) by hydroxyl radical (OH) in solutions used to mimic sea-salt particles. Aqueous halide solutions with nitrate or hydrogen peroxide (HOOH) as a photochemical source of OH were illuminated with 313 nm light and the resulting gaseous bromine (Br*(g)) was collected. While illumination of these solutions nearly always formed gaseous bromine (predominantly Br₂ based on modeling results), there was no evidence for the release of gaseous chlorine. The rate of Br*(g) release increased (up to a plateau value) with increasing concentrations of bromide and was enhanced at lower pH values for both nitrate and HOOH solutions. Increased ionic strength in nitrate solutions inhibited Br*(g) release and the extent of inhibition was dependent upon the salt used. In HOOH solutions, however, no ionic strength effects were observed and the presence of Cl⁻ strongly enhanced Br*(g) release.Overall, for conditions typical of aged, deliquesced, sea-salt particles, the efficiencies of gaseous bromine release, expressed as mole of Br*(g) released per mole of OH photochemically formed, were typically 20–30%. Using these reaction efficiencies, we calculated the Br₂(g) release rate from aged, ambient sea-salt particles due to OH oxidation to be approximately 0.07 pptv h⁻¹ with the main contributions from nitrate photolysis and partitioning of gas-phase OH into the particle. While our solution conditions are simplified compared to ambient particles, this estimated rate of Br₂ release is high enough to suggest that OH-mediated reactions in sea-salt particles could be a significant source of reactive bromine to the marine boundary layer.     

High-resolution mass spectrometry analysis of secondary organic aerosol generated by ozonolysis of isoprene

The chemical composition of secondary organic aerosol (SOA) generated from the ozonolysis of isoprene (C₅H₈) in the presence of an OH scavenger was examined using high-resolution electrospray ionization mass spectrometry (ESI-MS) in the mass range m/z = 50–1000. The chemical composition of SOA is complex, with more than 1000 assigned peaks observed in the positive and negative ion mode spectra. Only a small fraction of peaks correspond to known products of isoprene oxidation, such as pyruvic acid, glycolic acid, methylglyoxal, etc. The absolute majority of the detected peaks correspond to highly oxidized oligomeric constituents of SOA, with an average O:C molar ratio of 0.6. The corresponding organic mass (OM) to organic oxygen (OO) ratio is 2.4. Approximately 8% of oxygen atoms in SOA are in the form of peroxides, as quantified with an iodide test. Double bond equivalency (DBE) factors, representing the sum of all double bonds and rings, increase by 1 for every 1–2 additional carbon atoms in the molecule. The number of unoxidized CC double bonds is estimated to be less than 10%; the remaining DBE is due to CO carbonyl groups. Kendrick analysis suggests that the prevalent oligomer building blocks are small carbonyls with a C₁–C₂ skeleton. Formaldehyde (CH₂O) is identified as the most common repetitive building block in the observed oligomeric compounds.     

Temperature-dependent rate coefficients for the reactions of Cl atoms with methyl methacrylate,methyl acrylate and butyl methacrylate at atmospheric pressure

Rate coefficients for the gas-phase reactions of Cl atoms with a series of unsaturated esters CH₂C(CH₃)C(O)OCH₃ (MMA), CH₂CHC(O)OCH₃ (MAC) and CH₂C(CH₃)C(O)O(CH₂)₃CH₃ (BMA) have been measured as a function of temperature by the relative technique in an environmental chamber with in situ FTIR detection of reactants. The rate coefficients obtained at 298 K in one atmosphere of nitrogen or synthetic air using propene, isobutene and 1,3-butadiene as reference hydrocarbons were (in units of 10^?10 cm³ molecule^?1 s^?1) as follows: k_(Cl+MMA) = 2.82 ± 0.93, k_(Cl+MAC) = 2.04 ± 0.54 and k_(Cl+BMA) = 3.60 ± 0.87. The kinetic data obtained over the temperature range 287–313 K were used to derive the following Arrhenius expressions (in units of cm³ molecule^?1 s^?1): k_(Cl+MMA) = (13.9 ± 7.8) × 10^?15 exp[(2904 ± 420)/T], k_(Cl+MAC) = (0.4 ± 0.2) × 10^?15 exp[(3884 ± 879)/T], k_(Cl+BMA) = (0.98 ± 0.42) × 10^?15 exp[(3779 ± 850)/T]. All the rate coefficients display a slight negative temperature dependence which points to the importance of the reversibility of the addition mechanism for these reactions. This work constitutes the first kinetic and temperature dependence study of the reactions cited above.An analysis of the available rates of addition of Cl atoms and OH radicals to the double bond of alkenes and unsaturated and oxygenated volatile organic compounds (VOCs) at 298 K has shown that they can be related by the expression: log k_OH = 1.09 log k_Cl ? 0.10. In addition, a correlation between the reactivity of unsaturated VOCs toward OH radicals and Cl atoms and the HOMO of the unsaturated VOC is presented. Tropospheric implications of the results are also discussed.     

Effects of manganese-toxicity on immune-related organs of cocks

Manganese (Mn) is a trace element known to be essential for maintaining the proper function and regulation of many biochemical and cellular reactions. However, little is known about the effect of excessive amounts of Mn in immune damage of birds. The aim of this study was to investigate the effect of dietary Mn on immune damage in birds. 50-day-old male Hy-line cocks were fed either a commercial diet or a Mn-supplemented diet containing 600, 900, and 1800 mg kg^?1 MnCl₂. After being treated with Mn for 30, 60 and 90 d, the serum and immune organs (spleen, thymus, and bursa of Fabricius (BF)) were collected respectively and examined for Mn contents, activities of superoxide dismutase (SOD) and glutathione peroxidase (GSH-Px), malondialdehyde (MDA) contents, ability to resist OH. In addition, DNA damage and apoptosis were observed in cock immune system treated with Mn. The results showed that the contents of Mn and MDA in immune organs and serum were increased, while the activities of SOD, GSH-Px, and ability to resist OH were decreased in the Mn treatment groups. The extensive damage was observed in the immune organs. DNA single strand break and DNA–protein crosslink showed time and dosage effect in lymphocytes of immune organs. It indicated that Mn exposure resulted in oxidative damage of birds immune system by altering antioxidant defense enzyme systems, lipid peroxidation, and apoptosis.     

Observation of an unusual electronically distorted semiquinone radical of PCB metabolites in the active site of prostaglandin H synthase-2

The activation of the metabolites of airborne polychlorinated biphenyls (PCBs) into highly reactive radicals is of fundamental importance. We found that human recombinant prostaglandin H synthase-2 (hPGHS-2) biotransforms dihydroxy-PCBs, such as 4-chlorobiphenyl-2′,5′-hydroquinone (4-CB-2′,5′-H₂Q), into semiquinone radicals via one-electron oxidation. Using electron paramagnetic resonance (EPR) spectroscopy, we observed the formation of the symmetric quartet spectrum (1:3:3:1 by area) of 4-chlorobiphenyl-2′,5′-semiquinone radical (4-CB-2′,5′-SQ⁻) from 4-CB-2′,5′-H₂Q. This spectrum changed to an asymmetric spectrum with time: the change can be explained as the overlap of two different semiquinone radical species. Hindered rotation of the 4-CB-2′,5′-SQ⁻ appears not to be a major factor for the change in lineshape because increasing the viscosity of the medium with glycerol produced no significant change in lineshape. Introduction of a fluorine, which increases the steric hindrance for rotation of the dihydroxy-PCB studied, also produced no significant changes. An in silico molecular docking model of 4-CB-2′,5′-H₂Q in the peroxidase site of hPGHS-2 together with ab initio quantum mechanical studies indicate that the close proximity of a negatively charged carboxylic acid in the peroxidase active site may be responsible for the observed perturbation in the spectrum. This study provides new insights into the formation of semiquinones from PCB metabolites and underscores the potential role of PGHS-2 in the metabolic activation of PCBs.     

Photochemistry of phenanthrene,pyrene, and fluoranthene in ice and snow

Although polycyclic aromatic hydrocarbons (PAHs) are common pollutants in snow, there is little quantitative data about their rates of photodegradation in this environment. To begin to address this gap, we have measured the degradation kinetics of phenanthrene, pyrene, and fluoranthene on ice, as these are the most abundant PAHs in arctic snow. Frozen aqueous solutions of individual PAHs, with and without added hydrogen peroxide (HOOH) as a source of hydroxyl radical (OH), were illuminated with simulated sunlight. For all three PAHs, direct photodecay is the main mechanism of degradation, while OH-initiated indirect photodegradation is a minor sink. Rate constants (±1 SE) for direct photodegradation extrapolated to midday, surface snow conditions at Summit, Greenland on the summer solstice are 3.8 (±0.8) × 10^?5, 28 (±3) × 10^?5, and 1.4 (±0.7) × 10^?5 s^?1 for phenanthrene, pyrene, and fluoranthene, respectively. Apparent quantum efficiencies for photodegradation with simulated sunlight were 3.8 (±0.8) × 10^?3, 4.3 (±0.5) × 10^?4, and 2 (±1) × 10^?5, respectively. Calculated PAH lifetimes in surface snow under Summit conditions are 1–19 h during mid-summer, but increase to >100 days in the dark winter. While the short photodegradation lifetimes in the summer suggest that there should be no appreciable PAH levels in this season, past measurements at Summit sometimes show significant levels of these PAHs in summer surface snow. This discrepancy is likely due to differences in PAH location between lab samples (where the PAHs are probably in quasi-liquid layers) and real snow (where PAHs are likely primarily associated with particulate matter).     

Composition and yields of secondary organic aerosol formed from OH radical-initiated reactions of linear alkenes in the presence of NOx: Modeling and measurements

The products and mechanism of secondary organic aerosol (SOA) formation from the OH radical-initiated reactions of linear alkenes in the presence of NO_x were investigated in an environmental chamber. The SOA consisted primarily of products formed through reactions initiated by OH radical addition to the CC double bond, including β-hydroxynitrates and dihydroxynitrates, as well as cyclic hemiacetals, dihydrofurans, and dimers formed from particle-phase reactions of dihydroxycarbonyls. 1,4-Hydroxynitrates formed through reactions initiated by H-atom abstraction also appeared to contribute. Product yields and OH radical and alkoxy radical rate constants taken from the literature or calculated using structure–reactivity methods were used to develop a quantitative chemical mechanism for these reactions. SOA yields were then calculated using this mechanism with gas-particle partitioning theory and estimated product vapor pressures for comparison with measured values. Calculated and measured SOA yields agreed very well at high carbon numbers when semi-volatile products were primarily in the particle phase, but diverged with decreasing carbon number to a degree that depended on the model treatment of dihydroxycarbonyls, which appeared to undergo reversible reactions in the particle phase. The results indicate that the chemical mechanism developed here provides an accurate representation of the gas-phase chemistry, but the utility of the SOA model depends on the partitioning regime. The results also demonstrate some of the advantages of studying simple aerosol-forming reactions in which the majority of products can be identified and quantified, in this case leading to insights into both gas- and particle-phase chemistry.     

Mechanism of OH-initiated atmospheric photooxidation of the organophosphorus insecticide (C2H5O)3PS

O,O,O-triethyl phosphorothioate ((C₂H₅O)₃PS, TEPT) is a widely used organophosphorus insecticide. TEPT may be released into the atmosphere where it can undergo transport and chemical transformations, which include reactions with OH radicals, NO₃ radicals and O₃. The mechanism of the atmospheric reactions of TEPT has not been fully understood due to the short-lifetime of its oxidized radical intermediates, and the extreme difficulty in detection of these species experimentally. In this work, we carried out molecular orbital theory calculations for the OH radical-initiated atmospheric photooxidation of TEPT. The profile of the potential energy surface was constructed, and the possible channels involved in the reaction are discussed. The theoretical study shows that OH addition to the PS bond and H abstractions from the CH₃CH₂O moiety are energetically favorable reaction pathways. The dominant products TEP and SO₂ arise from the secondary reactions, the reactions of OH-TEPT adducts with O₂. The experimentally uncertain dominant product with molecular weight 170 is mostly due to (C₂H₅O)₂P(S)OH and not (C₂H₅O)₂P(O)SH.     

Photocatalytic degradation kinetics and mechanism of phenobarbital in TiO2 aqueous solution

5-Ethyl-5-phenylpyrimidine-2,4,6(1H, 3H, 5H)-trione is an anti-convulsant used to treat disorders of movement, e.g. tremors. This work deals with the transformation of phenobarbital by UV/TiO₂ heterogeneous photocatalysis, to assess the decomposition of the pharmaceutical compound, to identify intermediates, as well as to elucidate some mechanistic details of the degradation. The photocatalytic removal efficiency of 100 μm phenobarbital is about 80% within 60 min, while the degradation efficiency of phenobarbital was better in alkaline solution. The study on contribution of reactive oxidative species (ROSs) has shown that OH is responsible for the major degradation of phenobarbital, while the photohole, photoelectrons and the other ROSs have the minor contribution to the degradation. Finally, based on the identification of degradation intermediates, two main photocatalytic degradation pathways have been tentatively proposed, including the hydroxylation and cleavage of pyrimidine ring in the phenobarbital molecule respectively. Certainly, the phenobarbital can be mineralized when the photocatalytic reaction time prolongs.     

Kinetics and mechanism for omethoate degradation by catalytic ozonation with Fe(III)-loaded activated carbon in water

The kinetics and mechanism for degradation of omethoate (OMT) by catalytic ozonation with Fe(III)-loaded activated carbon (Fe@AC) were investigated in this study with focus on identification of degradation byproducts. The rate constants of OMT reacting with ozone and hydroxyl radical (OH) were determined to be 0.04 and 5.3 × 10⁸ M^?1 s^?1 at pH 7.5 and 20 °C, respectively. OMT was predominantly degraded by OH in the catalytic ozonation with Fe@AC. The high-molecular-weight degradation byproducts identified were O,O,O-trimethyl phosphoric ester (TMP), pyrrolidin-2-one, N-methyl-2-sulfanylacetamide, 2-(methylthio)acetamide, O,O,S-trimethylthiophosphate (STMP), and N-methyl-2-(methylthio)acetamide. Besides, low-molecular-weight organic acids and inorganic anions were also detected and quantified, including formic, acetic and oxalic acids as well as nitrate, sulfate and phosphate ions. In the catalytic ozonation, TMP and phosphate were two major P-containing byproducts resulting from OMT degradation. The toxicity of OMT solution gradually decreased during the catalytic ozonation, indicating that Fe@AC is a safe catalyst for OMT removal by ozone in water.     

Laboratory studies of the impact of aerosol composition on the heterogeneous oxidation of 1-propanol

The effects of (NH₄)₂SO₄, NH₄NO₃, NaCl, NH₄Cl, and Na₂SO₄ aerosols on the kinetics of 1-propanol oxidation in the presence of the hydroxyl radical have been investigated using the relative rate technique. p-Xylene was used as a reference compound. Two different aerosol concentrations that are typical of polluted urban conditions were tested. The total surface areas of aerosols were 1400 (condition I) and 3400 μm² cm⁻³ (condition II). Results indicate that aerosols promote the oxidation of 1-propanol, and the extents of the promoting effects depend on the aerosol composition and concentration. Increases in the relative rates of the 1-propanol/OH reaction vs. the p-xylene/OH reaction were only observed for (NH₄)₂SO₄ aerosol conditions I and II, NH₄NO₃ aerosol condition II, and NH₄Cl aerosol condition II. These results indicate that NH₄⁺ is the species promoting the oxidation of 1-propanol, and suggests the possibility of a strong interaction between NH₄⁺ and 1-propanol that can change the activation energy of the initial OH attack. These results have profound implications on the use of air quality models for the assessment of air pollution control strategies.