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.     

Environmental photochemical fate of pesticides ametryn and imidacloprid in surface water (Paranapanema River,São Paulo,Brazil)

In addition to direct photolysis studies, in this work the second-order reaction rate constants of pesticides imidacloprid (IMD) and ametryn (AMT) with hydroxyl radicals (HO^●), singlet oxygen (¹O₂), and triplet excited states of chromophoric dissolved organic matter (³CDOM^*) were determined by kinetic competition under sunlight. IMD and AMT exhibited low photolysis quantum yields: (1.23?±?0.07)?×?10^–2 and (7.99?±?1.61)?×?10^–3 mol Einstein^?1, respectively. In contrast, reactions with HO^● radicals and ³CDOM* dominate their degradation, with ¹O₂ exhibiting rates three to five orders of magnitude lower. The values of k_IMD,HO● and k_AMT,HO● were (3.51?±?0.06)?×?10⁹ and (4.97?±?0.37)?×?10⁹ L mol^?1 s^?1, respectively, while different rate constants were obtained using anthraquinone-2-sulfonate (AQ2S) or 4-carboxybenzophenone (CBBP) as CDOM proxies. For IMD this difference was significant, with k_IMD,3AQ2S*?=?(1.02?±?0.08)?×?10⁹ L mol^?1 s^?1 and k_IMD,3CBBP*?=?(3.17?±?0.14)?×?10⁸ L mol^?1 s^?1; on the contrary, the values found for AMT are close, k_AMT,3AQ2S*?=?(8.13?±?0.35)?×?10⁸ L mol^?1 s^?1 and k_AMT,3CBBP*?=?(7.75?±?0.80)?×?10⁸ L mol^?1 s^?1. Based on these results, mathematical simulations performed with the APEX model for typical levels of water constituents (NO₃^?, NO₂^?, CO₃^2?, TOC, pH) indicate that the half-lives of these pesticides should vary between 24.1 and 18.8 days in the waters of the Paranapanema River (São Paulo, Brazil), which can therefore be impacted by intensive agricultural activity in the region.

     

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.     

Rate Constants for the Reaction of OH with Halocarbons in the Presence of O2 + N2

Rates of CO₂ production in the reaction CO + OH and CO + OH + halocarbon have been used to determine rate constants for some OH + halocarbon reactions at 29.5°C relative to that of k(CO + OH) = 2.69 × 10^?13 cm³ molecule^?1 sec^?1. The following rate constants were obtained: k(OH + CH₃Cl) = 3.1 ± 0.8, k(OH + CH₂Cl₂) = 2.7 ± 1.0, k(OH + C₂H₅Cl) = 44.0 ± 25, k(OH + CICH₂CH₂CI) = 6.5, (<29) and k(OH + CH₃CCl₃) = 2.1 (<5.7) cm³ molecule^?1 sec^?1 × 10^?14. The k values, CH₂Cl₂ excepted, are in substantial agreement with determinations made in nonoxygen environments. The present results for CH₂Cl₂ are almost certainly in error due to difficulties with the competitive approach used.     

Benzene in Auto Exhaust

The gas phase thermal decomposition rates of the C₁ and C₂-substituted peroxyacyl nitrates (RC(O)OONO₂), PAN (R = CH₃), PPN (R = C₂H₅) and vinyl-PAN (R = CH₂ = CH-) have been measured at ambient temperature (288 - 299 K) and 1 atm. of air. Our results for PAN (k = A exp (-E_a/RT), log₁₀ (A, s^-1) = 16.2 ± 1.6, E_a = 26.9 ± 2.1 kcal / mol, k₂₉₈ = 3.0 × 10^?4S^?1) are consistent with literature data. Thermal decomposition rates for PPN and vinyl-PAN are similar to that for PAN, with k₂₉₈ = 3.0 × 10^?4S^?1 for PAN, 3.4 × 10^?4S^?1 for PPN and 3.0 × 10^?4S^?1 for vinyl-PAN. Implications for the atmospheric persistence of PPN and vinyl-PAN as compared to that of PAN are briefly discussed.     

Remediation of acid mine drainage (AMD)-contaminated soil by Phragmites australis and rhizosphere bacteria

Experiments were conducted to assess the impact of citric acid (CA) and rhizosphere bacteria on metal uptake in Phragmites australis cultured in a spiked acid mine drainage (AMD) soil. Rhizosphere iron-oxidizing bacteria (Fe(II)OB) enhanced the formation of Fe plaque on roots, which decreased the uptake of Fe and Mn. CA inhibited the growth of Fe(II)OB, decreased the formation of metal plaque, raised the metal mobility in soil, and increased the accumulation of metals in all tissues of the reeds. The higher the CA dosage, the more metals accumulated into reeds. The total amount of metals in reeds increased from 7.8?±?0.5?×?10^?6 mol plant^?1 (Mn), 1.4?±?0.1?×?10^?3 mol plant^?1 (Fe), and 1.0?±?0.1?×?10^?4 mol plant^?1 (Al) in spiked soil without CA to 22.2?±?0.5?×?10^?6 mol plant^?1 (Mn), 3.5?±?0.06?×?10^?3 mol plant^?1 (Fe), and 5.0?±?0.2?×?10^?4 mol plant^?1 (Al) in soil added with 33.616 g C₆H₈O₇·H₂O for per kilogram soil. CA could be effective at enhancing the phytoremediation of metals from AMD-contaminated soil.     

Role of some organic inhibitors on the oxidation of dissolved sulfur dioxide by oxygen in rainwater medium

In August 2012, eight rainwater samples were collected and analyzed for pH and metal ions, viz., iron, copper, and manganese. The pH was within the range 6.84–7.65. The rate of oxidation of dissolved sulfur dioxide was determined using these rainwater samples as reaction medium. Kinetics was defined by the rate law: ?d[S(IV)]/dt = R _o = k _o[S(IV)]], where k _o is the first-order rate constant and R _o is the rate of the reaction. The effect of two volatile organic compounds—ethanol and 2-butanol—was examined and found to inhibit the oxidation as defined by the rate law: k _obs = k _o/(1 + B [Inh]), where k _obs is the first-order rate constant in the presence of the inhibitor, [Inh] is the concentration of the inhibitor, and B is the inhibitor parameter—an empirical constant. In the pH range of collected rainwater samples, the values of first-order rate constants ranged from 3.1?×?10^?5 to 1.5?×?10^?4 s^?1 at 25 °C. The values of inhibition parameter were found to be (5.99?±?3.91?×?10⁴) (ethanol) and (3.95?±?2.36)?×?10⁴ (2-butanol) at 25 °C.     

Identification and quantification of methyl halide sources in a lowland tropical rainforest

In conjunction with the OP3 campaign in Danum Valley, Malaysian Borneo, flux measurements of methyl chloride (CH₃Cl) and methyl bromide (CH₃Br) were performed from both tropical plant branches and leaf litter in June and July 2008. Live plants were mainly from the Dipterocarpaceae family whilst leaf litter samples were representative mixtures of different plant species. Environmental parameters, including photosynthetically-active radiation, total solar radiation and air temperature, were also recorded. The dominant factor determining magnitude of methyl halide fluxes from living plants was plant species, with specimens of the genus Shorea showing persistent high emissions of both gases, e.g. Shorea pilosa: 65 ± 17 ng CH₃Cl h^?1 g^?1 (dry weight foliage) and 2.7 ± 0.6 ng CH₃Br h^?1 g^?1 (dry weight foliage). Mean CH₃Cl and CH₃Br emissions across 18 species of plant were 19 (range, <LOD ?76) and 0.4 (<LOD ?2.9) ng h^?1 g^?1 respectively; fluxes from leaf litter were 1–2 orders of magnitude smaller per dry mass. CH₃Cl and CH₃Br fluxes were weakly correlated. Overall, the findings suggest that tropical rainforests make an important contribution to global terrestrial emissions of CH₃Cl, but less so for CH₃Br.     

A photochemical reactor for studies of atmospheric chemistry

A photochemical reactor for studies of atmospheric kinetics and spectroscopy has been built at the Copenhagen Center for Atmospheric Research. The reactor consists of a vacuum FTIR spectrometer coupled to a 100 L quartz cylinder by multipass optics mounted on electropolished stainless steel end flanges, surrounded by UV-A, UV-C and broadband sun lamps in a temperature-controlled housing. The combination of a quartz vessel and UV-C lamps allows higher concentrations of O(¹D) and OH than can be generated by similar chambers. The reactor is able to produce radical concentrations of ca. 8 × 10¹¹ cm^?3 for OH, 3 × 10⁶ cm^?3 for O(¹D), 3.3 × 10¹⁰ cm^?3 for O(³P) and 1.6 × 10¹² cm^?3 for Cl. The reactor can be operated at pressures from 10^?3 to 10³ mbar and temperatures from 240 to 330 K. As a test of the system we have studied the reaction CHCl₃ + Cl using the relative rate technique and find k_CHCl3+Cl/k_CH4+Cl = 1.03 ± 0.11, in good agreement with the accepted value.     

Photocatalytic degradation of contaminants of concern with composite NF-TiO2 films under visible and solar light

This study reports the synthesis and characterization of composite nitrogen and fluorine co-doped titanium dioxide (NF-TiO₂) for the removal of contaminants of concern in wastewater under visible and solar light. Monodisperse anatase TiO₂ nanoparticles of different sizes and Evonik P25 were assembled to immobilized NF-TiO₂ by direct incorporation into the sol–gel or by the layer-by-layer technique. The composite films were characterized with X-ray diffraction, high-resolution transmission electron microscopy, environmental scanning electron microscopy, and porosimetry analysis. The photocatalytic degradation of atrazine, carbamazepine, and caffeine was evaluated in a synthetic water solution and in an effluent from a hybrid biological concentrator reactor (BCR). Minor aggregation and improved distribution of monodisperse titania particles was obtained with NF-TiO₂-monodisperse (10 and 50 nm) from the layer-by-layer technique than with NF-TiO₂?+?monodisperse TiO₂ (300 nm) directly incorporated into the sol. The photocatalysts synthesized with the layer-by-layer method achieved significantly higher degradation rates in contrast with NF-TiO₂-monodisperse titania (300 nm) and slightly faster values when compared with NF-TiO₂-P25. Using NF-TiO₂ layer-by-layer with monodisperse TiO₂ (50 nm) under solar light irradiation, the respective degradation rates in synthetic water and BCR effluent were 14.6 and 9.5?×?10^?3?min^?1 for caffeine, 12.5 and 9.0?×?10^?3?min^?1 for carbamazepine, and 10.9 and 5.8?×?10^?3?min^?1 for atrazine. These results suggest that the layer-by-layer technique is a promising method for the synthesis of composite TiO₂-based films compared to the direct addition of nanoparticles into the sol.     

Relative kinetic measurements of rate coefficients for the gas-phase reactions of Cl atoms and OH radicals with a series of methyl alkyl esters

Relative kinetic studies have been performed on the reactions of Cl atoms with a series of methyl alkyl esters in a 405-liter borosilicate glass chamber at (298 ± 3) K and one atmosphere of synthetic air using in situ FTIR spectroscopy to monitor the reactants. Rate coefficients (in units of cm³ molecule^?1 s^?1) were determined for the following compounds: methyl acetate (2.48 ± 0.58) × 10^?12; methyl propanoate (1.68 ± 0.36) × 10^?11; methyl butanoate (4.77 ± 0.87) × 10^?11; methyl pentanoate (7.84 ± 1.15) × 10^?11; methyl hexanoate (1.09 ± 0.31) × 10^?10; methyl heptanoate (1.56 ± 0.37) × 10^?10; methyl cyclohexane carboxylate (3.32 ± 0.76) × 10^?10; methyl-2-methyl butanoate (9.41 ± 1.39) × 10^?11.In addition rate coefficients (in units of 10^?11 cm³ molecule^?1 s^?1) have been obtained for the reactions of OH radicals with the following compounds: methyl butanoate (3.55 ± 0.71), methyl pentanoate (5.41 ± 1.08), and methyl-2-methyl butanoate (4.08 ± 0.82).Using the kinetic rate data tropospheric lifetimes for the methyl alkyl esters with respect to their reactions with OH, and Cl have been estimated for typical ambient air concentrations of these oxidants.     

Effect of land-use change on CH4 and N2O emissions from freshwater marsh in Northeast China

The wetlands play an important role in global carbon and nitrogen storage, and they are also natural sources of greenhouse gases such as methane (CH₄) and nitrous oxide (N₂O). Land-use change is an important factor affecting the exchange of greenhouse gases between wetlands and the atmosphere. However, few studies have investigated the effect of land-use change on CH₄ and N₂O emissions from freshwater marsh in China. Therefore, a field study was carried out over a year to investigate the seasonal changes of the emissions of CH₄ and N₂O at three sites (Deyeuxia angustifolia marsh, dryland and rice field) in the Sanjiang Plain of Northeast China. Marsh was the source of CH₄ showing a distinct temporal variation. Maximum fluxes occurred in June and the highest value was 20.69 ± 2.57 mg CH₄ m^?2 h^?1. The seasonal change of N₂O fluxes from marsh was not obvious, consisted of a series of emission pulses. The marsh acted as a N₂O sink during winter, while became a N₂O source in the growing season. The results showed that gas exchange between soil/snow and the atmosphere in the winter season contributed greatly to the annual budgets. The winter season CH₄ flux was about 3.24% of the annual flux and the winter uptake of N₂O accounted for 13.70% of the growing-season emission. Conversion marsh to dryland resulted in a shift from a strong CH₄ source to a weak sink (from 199.12 ± 39.04 to ?1.37 ± 0.68 kg CH₄ ha^?1 yr^?1), while increased N₂O emissions somewhat (from 4.07 ± 1.72 to 4.90 ± 1.52 kg N₂O ha^?1 yr^?1). Conversion marsh to rice field significantly decreased CH₄ emission from 199.12 ± 39.04 to 94.82 ± 9.86 kg CH₄ ha^?1 yr^?1 and N₂O emission from 4.07 ± 1.72 to 2.09 ± 0.79 kg N₂O ha^?1 yr^?1.     

Atmospheric chemistry of HFE-7000 (CF(3)CF (2)CF (2)OCH (3)) and 2,2,3,3,4,4,4-heptafluoro-1-butanol (CF (3)CF (2)CF (2)CH (2)OH): kinetic rate coefficients and temperature dependence of reactions with chlorine atoms

Background, aim, and scope  The adverse environmental impacts of chlorinated hydrocarbons on the Earth’s ozone layer have focused attention on the effort to replace these compounds by nonchlorinated substitutes with environmental acceptability. Hydrofluoroethers (HFEs) and fluorinated alcohols are currently being introduced in many applications for this purpose. Nevertheless, the presence of a great number of C–F bonds drives to atmospheric long-lived compounds with infrared absorption features. Thus, it is necessary to improve our knowledge about lifetimes and global warming potentials (GWP) for these compounds in order to get a complete evaluation of their environmental impact. Tropospheric degradation is expected to be initiated mainly by OH reactions in the gas phase. Nevertheless, Cl atoms reaction may also be important since rate constants are generally larger than those of OH. In the present work, we report the results obtained in the study of the reactions of Cl radicals with HFE-7000 (CF₃CF₂CF₂OCH₃) (1) and its isomer CF₃CF₂CF₂CH₂OH (2). Materials and methods  Kinetic rate coefficients with Cl atoms have been measured using the discharge flow tube–mass spectrometric technique at 1 Torr of total pressure. The reactions of these chlorofluorocarbons (CFCs) substitutes have been studied under pseudo-first-order kinetic conditions in excess of the fluorinated compounds over Cl atoms. The temperature ranges were 266–333 and 298–353 K for reactions of HFE-7000 and CF₃CF₂CF₂CH₂OH, respectively. Results  The measured room temperature rate constants were k(Cl+CF₃CF₂CF₂OCH₃) = (1.24 ± 0.28) × 10⁻¹³ cm³ molecule⁻¹ s⁻¹and k(Cl+CF₃CF₂CF₂CH₂OH) = (8.35 ± 1.63) × 10⁻¹³ cm³ molecule⁻¹ s⁻¹ (errors are 2σ + 10% to cover systematic errors). The Arrhenius expression for reaction 1 was k ₁(266–333 K) = (6.1 ± 3.8) × 10⁻¹³exp[−(445 ± 186)/T] cm³ molecule⁻¹ s⁻¹ and k ₂(298–353 K) = (1.9 ± 0.7) × 10⁻¹²exp[−(244 ± 125)/T] cm³ molecule⁻¹ s⁻¹ (errors are 2σ). The reactions are reported to proceed through the abstraction of an H atom to form HCl and the corresponding halo-alkyl radical. At 298 K and 1 Torr, yields on HCl of 0.95 ± 0.38 and 0.97 ± 0.16 (errors are 2σ) were obtained for CF₃CF₂CF₂OCH₃ and CF₃CF₂CF₂CH₂OH, respectively. Discussion  The obtained kinetic rate constants are related to the previous data in the literature, showing a good agreement taking into account the error limits. Comparing the obtained results at room temperature, k ₁ and k ₂, HFE-7000 is significantly less reactive than its isomer C₃F₇CH₂OH. A similar behavior has been reported for the reactions of other fluorinated alcohols and their isomeric fluorinated ethers with Cl atoms. Literature data, together with the results reported in this work, show that, for both fluorinated ethers and alcohols, the kinetic rate constant may be considered as not dependent on the number of –CF₂– in the perfluorinated chain. This result may be useful since it is possible to obtain the required physicochemical properties for a given application by changing the number of –CF₂– without changes in the atmospheric reactivity. Furthermore, lifetimes estimations for these CFCs substitutes are calculated and discussed. The average estimated Cl lifetimes are 256 and 38 years for HFE-7000 and C₃H₇CH₂OH, respectively. Conclusions  The studied CFCs’ substitutes are relatively short-lived and OH reaction constitutes their main reactive sink. The average contribution of Cl reactions to global lifetime is about 2% in both cases. Nevertheless, under local conditions as in the marine boundary layer, τ _Cl values as low as 2.5 and 0.4 years for HFE-7000 and C₃H₇CH₂OH, respectively, are expected, showing that the contribution of Cl to the atmospheric degradation of these CFCs substitutes under such conditions may constitute a relevant sink. In the case of CF₃CF₂CF₂OCH₃, significant activation energy has been measured, thus the use of kinetic rate coefficient only at room temperature would result in underestimations of lifetimes and GWPs. Recommendations and perspectives  The results obtained in this work may be helpful within the database used in the modeling studies of coastal areas. The knowledge of the atmospheric behavior and the structure–reactivity relationship discussed in this work may also contribute to the development of new environmentally acceptable chemicals. New volatile materials susceptible of emission to the troposphere should be subject to the study of their reactions with OH and Cl in the range of temperature of the troposphere. The knowledge of the temperature dependence of the kinetic rate constants, as it is now reported for the case of reactions 1 and 2, will allow more accurate lifetimes and related magnitudes like GWPs. Nevertheless, a better knowledge of the vertical Cl tropospheric distribution is still required.     

Degradation kinetics of anthracene by ozone on mineral oxides

To further understand the role of substrates on the heterogeneous reactions of polycyclic aromatic hydrocarbons, the reactions of ozone with anthracene adsorbed on different mineral oxides (SiO₂, α-Al₂O₃ and α-Fe₂O₃) and on Teflon disc were investigated in dark at 20 °C. No reaction between ozone and anthracene on Teflon disc was observed when the ozone concentration was ～1.18 × 10¹⁴ molecules cm^?3. The reactions on mineral oxides exhibited pseudo-first-order kinetics for anthracene loss, and the pseudo-first-order rate constant (k_1,obs) displayed a Langmuir–Hinshelwood dependence on the gas-phase ozone concentration. The adsorption equilibrium constants for ozone (K_O3) on SiO₂-1, SiO₂-2, α-Al₂O₃ and α-Fe₂O₃ were (0.81 ± 0.26) × 10^?15 cm³, (2.83 ± 1.17) × 10^?15 cm³, (2.48 ± 0.77) × 10^?15 cm³ and (1.66 ± 0.45) × 10^?15 cm³, respectively; and the maximum pseudo-first-order rate constant (k_1,max) on these oxides were (0.385 ± 0.058) s^?1, (0.101 ± 0.0138) s^?1, (0.0676 ± 0.0086) s^?1 and (0.0457 ± 0.004) s^?1, respectively. Anthraquinone was identified as the main surface product of anthracene reacted with ozone. Comparison with previous research and the results obtained in this study suggest that the reactivity of anthracene with ozone and the lifetimes of anthracene adsorbed on mineral dust in the atmosphere are determined by the nature of the substrate.     

Principal component analysis to assess the efficiency and mechanism for enhanced coagulation of natural algae-laden water using a novel dual coagulant system

A novel dual coagulant system of polyaluminum chloride sulfate (PACS) and polydiallyldimethylammonium chloride (PDADMAC) was used to treat natural algae-laden water from Meiliang Gulf, Lake Taihu. PACS (Al_n(OH)_mCl_3n-m-2k(SO₄)_k) has a mass ratio of 10 %, a SO₄ ^2?/Al₃ ⁺ mole ratio of 0.0664, and an OH/Al mole ratio of 2. The PDADMAC ([C₈H₁₆NCl]_m) has a MW which ranges from 5?×?10⁵ to 20?×?10⁵ Da. The variations of contaminants in water samples during treatments were estimated in the form of principal component analysis (PCA) factor scores and conventional variables (turbidity, DOC, etc.). Parallel factor analysis determined four chromophoric dissolved organic matters (CDOM) components, and PCA identified four integrated principle factors. PCA factor 1 had significant correlations with chlorophyll-a (r?=?0.718), protein-like CDOM C1 (0.689), and C2 (0.756). Factor 2 correlated with UV₂₅₄ (0.672), humic-like CDOM component C3 (0.716), and C4 (0.758). Factors 3 and 4 had correlations with NH₃-N (0.748) and T-P (0.769), respectively. The variations of PCA factors scores revealed that PACS contributed less aluminum dissolution than PAC to obtain equivalent removal efficiency of contaminants. This might be due to the high cationic charge and pre-hydrolyzation of PACS. Compared with PACS coagulation (20 mg L^?1), the removal of PCA factors 1, 2, and 4 increased 45, 33, and 12 %, respectively, in combined PACS–PDADMAC treatment (0.8 mg L^?1?+?20 mg L^?1). Since PAC contained more Al (0.053 g/1 g) than PACS (0.028 g/1 g), the results indicated that PACS contributed less Al dissolution into the water to obtain equivalent removal efficiency.     

Spatial and temporal variations of nitrous oxide flux between coastal marsh and the atmosphere in the Yellow River estuary of China

To investigate the spatial and seasonal variations of nitrous oxide (N₂O) fluxes and understand the key controlling factors, we explored N₂O fluxes and environmental variables in high marsh (HM), middle marsh (MM), low marsh (LM), and mudflat (MF) in the Yellow River estuary throughout a year. Fluxes of N₂O differed significantly between sampling periods as well as between sampling positions. During all times of day and the seasons measured, N₂O fluxes ranged from ?0.0051 to 0.0805 mg N₂O m^?2 h^?1, and high N₂O emissions occurred during spring (0.0278 mg N₂O m^?2 h^?1) and winter (0.0139 mg N₂O m^?2 h^?1) while low fluxes were observed during summer (0.0065 mg N₂O m^?2 h^?1) and autumn (0.0060 mg N₂O m^?2 h^?1). The annual average N₂O flux from the intertidal zone was 0.0117 mg N₂O m^?2 h^?1, and the cumulative N₂O emission throughout a year was 113.03 mg N₂O m^?2, indicating that coastal marsh acted as N₂O source. Over all seasons, N₂O fluxes from the four marshes were significantly different (p?<?0.05), in the order of HM (0.0256?±?0.0040 mg N₂O m^?2 h^?1)?>?MF (0.0107?±?0.0027 mg N₂O m^?2 h^?1)?>?LM (0.0073?±?0.0020 mg N₂O m^?2 h^?1)?>?MM (0.0026?±?0.0011 mg N₂O m^?2 h^?1). Temporal variations of N₂O emissions were related to the vegetations (Suaeda salsa, Phragmites australis, and Tamarix chinensis) and the limited C and mineral N in soils during summer and autumn and the frequent freeze/thaw cycles in soils during spring and winter, while spatial variations were mainly affected by tidal fluctuation and plant composition at spatial scale. This study indicated the importance of seasonal N₂O contributions (particularly during non-growing season) to the estimation of local N₂O inventory, and highlighted both the large spatial variation of N₂O fluxes across the coastal marsh (CV?=?158.31 %) and the potential effect of exogenous nitrogen loading to the Yellow River estuary on N₂O emission should be considered before the annual or local N₂O inventory was evaluated accurately.     

Effects of nitrogen application rate and a nitrification inhibitor dicyandiamide on methanotroph abundance and methane uptake in a grazed pasture soil

Methane-oxidizing bacteria (methanotrophs) in the soil are a unique group of methylotrophic bacteria that utilize methane (CH₄) as their sole source of carbon and energy which limit the flux of methane to the atmosphere from soils and consume atmospheric methane. A field experiment was conducted to determine the effect of nitrogen application rates and the nitrification inhibitor dicyandiamide (DCD) on the abundance of methanotrophs and on methane flux in a grazed pasture soil. Nitrogen (N) was applied at four different rates, with urea applied at 50 and 100 kg N ha^?1 and animal urine at 300 and 600 kg N ha^?1. DCD was applied at 10 kg ha^?1. The results showed that both the DNA and selected mRNA copy numbers of the methanotroph pmoA gene were not affected by the application of urea, urine or DCD. The methanotroph DNA and mRNA pmoA gene copy numbers were low in this soil, below 7.13?×?10³ g^?1 soil and 3.75?×?10³ μg^?1 RNA, respectively. Daily CH₄ flux varied slightly among different treatments during the experimental period, ranging from ?12.89 g CH₄ ha^?1 day^?1 to ?0.83 g CH₄ ha^?1 day^?1, but no significant treatment effect was found. This study suggests that the application of urea fertilizer, animal urine returns and the use of the nitrification inhibitor DCD do not significantly affect soil methanotroph abundance or daily CH₄ fluxes in grazed grassland soils.     

Influence of environmental factors on net N2 and N2O production in sediment of freshwater rivers

Denitrification is an important N removal process in aquatic systems but is also implicated as a potential source of global N₂O emissions. However, the key factors controlling this process as well as N₂O emissions remain unclear. In this study, we identified the main factors that regulate the production of net N₂ and N₂O in sediments collected from rivers with a large amount of sewage input in the Taihu Lake region. Net N₂ and N₂O production were strongly associated with the addition of NO₃ ^?-N and NH₄ ⁺-N. Specifically, NO₃ ^?-N controlled net N₂ production following Michaelis–Menten kinetics. The maximum rate of net N₂ production (V _max) was 116.3 μmol N₂-N m^?2 h^?1, and the apparent half-saturation concentration (k _m) was 0.65 mg N L^?1. N₂O to N₂ ratios increased from 0.18?±?0.03 to 0.68?±?0.16 with the addition of NO₃ ^?-N, suggesting that increasing NO₃ ^?-N concentrations favored the production of N₂O more than N₂. The addition of acetate enhanced net N₂ production and N₂O to N₂ ratios, but the ratios decreased by about 59.5 % when acetate concentrations increased from 50 to 100 mg C L^?1, suggesting that the increase of N₂O to N₂ ratios had more to do with the net N₂ production rate rather than acetate addition in this experiment. The addition of Cl^? did not affect the net N₂ production rates, but significantly enhanced N₂O to N₂ ratios (the ratios increased from 0.02?±?0.00 to 0.10?±?0.00), demonstrating that the high salinity effect might have a significant regional effect on N₂O production. Our results suggest that the presence of N-enriching sewage discharges appear to stimulate N removal but also increase N₂O to N₂ ratios.     

Atmospheric degradation of fluoroesters (FESs): Gas-phase reactivity study towards OH radicals at 298 K

Using the relative technique, rate coefficients have been measured for the gas phase reactions of hydroxyl radicals with four fluoroacetates, methyl trifluoroacetate (CF₃COOCH₃), ethyl trifluoroacetate (CF₃COOCH₂CH₃), methyl difluoroacetate (CF₂HCOOCH₃) and 2,2,2-trifluoroethyl trifluoroacetate (CF₃COOCH₂CF₃). Experiments were carried out at 296±2 K and atmospheric pressure (∼750 Torr) using nitrogen or synthetic air as bath gases. The following rate coefficients were derived for the reaction of OH radicals (in units of cm³ mol⁻¹ s⁻¹) with CF₃COOCH₃, k=(4.97±1.04)×10⁻¹⁴, CF₃COOCH₂CH₃, k=(2.64±0.59)×10⁻¹³, CF₂HCOOCH₃, k=(1.48±0.34)×10⁻¹³ and CF₃COOCH₂CF₃, (1.05±0.23)×10⁻¹³. The rate constants obtained are compared with previous literature data of other volatile organic compounds to establish reactivity trends. Atmospheric implications are discussed in terms of lifetimes and fates of the fluoroacetates in the troposphere.     

Concentration and characterization of airborne particles in Tehran’s subway system

Particulate matter is an important air pollutant, especially in closed environments like underground subway stations. In this study, a total of 13 elements were determined from PM₁₀ and PM_2.5 samples collected at two subway stations (Imam Khomeini and Sadeghiye) in Tehran’s subway system. Sampling was conducted in April to August 2011 to measure PM concentrations in platform and adjacent outdoor air of the stations. In the Imam Khomeini station, the average concentrations of PM₁₀ and PM_2.5 were 94.4?±?26.3 and 52.3?±?16.5 μg m^?3 in the platform and 81.8?±?22.2 and 35?±?17.6 μg m^?3 in the outdoor air, respectively. In the Sadeghiye station, mean concentrations of PM₁₀ and PM_2.5 were 87.6?±?23 and 41.3?±?20.4 μg m^?3 in the platform and 73.9?±?17.3 and 30?±?15 μg m^?3, in the outdoor air, respectively. The relative contribution of elemental components in each particle fraction were accounted for 43 % (PM₁₀) and 47.7 % (PM_2.5) in platform of Imam Khomeini station and 15.9 % (PM₁₀) and 18.5 % (PM_2.5) in the outdoor air of this station. Also, at the Sadeghiye station, each fraction accounted for 31.6 % (PM₁₀) and 39.8 % (PM_2.5) in platform and was 11.7 % (PM₁₀) and 14.3 % (PM_2.5) in the outdoor. At the Imam Khomeini station, Fe was the predominant element to represent 32.4 and 36 % of the total mass of PM₁₀ and PM_2.5 in the platform and 11.5 and 13.3 % in the outdoor, respectively. At the Sadeghiye station, this element represented 22.7 and 29.8 % of total mass of PM₁₀ and PM_2.5 in the platform and 8.7 and 10.5 % in the outdoor air, respectively. Other major crustal elements were 5.8 % (PM₁₀) and 5.3 % (PM_2.5) in the Imam Khomeini station platform and 2.3 and 2.4 % in the outdoor air, respectively. The proportion of other minor elements was significantly lower, actually less than 7 % in total samples, and V was the minor concentration in total mass of PM₁₀ and PM_2.5 in both platform stations.