Photo-sensitized oxidation in natural water via OH radicals

A method is presented for determining production and consumption rates of ^.OH radicals produced photochemically in natural surface waters. It is based on the determination of the kinetics by which the concentration of a specified trace compound decreases during irradiation. In samples from Lake Greifensee (Switzerland) low production rates for ^.OH limit its possible effects. In addition, fast consumptions by the natural dissolved organic solutes and by the bicarbonate protect organic micropollutants from oxidation by ^.OH. Neither direct nor indirect H₂O₂ photolysis was a significant source of ^.OH in the lakewater studied lacking iron, whereas nitrate photolysis could have been a source. Comparison with reaction kinetic formulations allows generalizations for other types of waters.     

The aqueous oxidation of SO2 by OH radicals

The reaction of aqueous hydroxyl radicals with bisulphite ion has been proposed as a significant and in some cases a primary pathway by which SO₂ is oxidized to sulphate in tropospheric cloud. It is shown that many uncertainties still remain with regard to the kinetics and mechanism of this oxidation process. Illustrative computations indicate that reaction schemes taking account of the aqueous oxidation of SO₂ by OH in existing models of cloud chemistry are inconsistent with the mechanisms on which they are based and yield a wide range of oxidation rates. Calculations are presented which suggest that the reaction of the SO₅⁻ radical with bisulphite ion is rate determining, although the rate constant remains uncertain. The products of this reaction must be identified before the mechanism of sulphate production can be elucidated and the relevance of this process to tropospheric SO₂ oxidation reliably assessed.     

Flue Gas NOX Reduction Using Ammonia Radical Injection

A novel technique has been developed for flue gas NO_X reduction through the injection of plasma-treated ammonia and its decomposition products. Numerical investigation of the chemical kinetics shows nearly complete NO removal when ammonia radicals are injected into the flue gas. The feasibility of this new technique was experimentally explored on a small-scale laboratory combustor at Carnegie Mellon University and concurrently on a larger-scale combustor at the DOE Pittsburgh Energy Technology Center. Preliminary experimental results have shown that ammonia plasma-injection is more effective than simple ammonia-injection at low flue gas temperatures. NO_X reduction of 85 to 90 percent was achieved at a low plasma power input. This technique is expected to provide additional opportunities for inexpensive and effective NO_X reduction in stationary sources.     

Kinetics of OH radical reactions with dibenzo-p-dioxin and selected chlorinated dibenzo-p-dioxins

The pulsed laser photolysis/pulsed laser-induced fluorescence (PLP/PLIF) technique has been applied to obtain rate coefficients for OH + dioxin (DD) (k1), OH + 2-chlorodibenzo-p-dioxin (2-CDD) (k2), OH + 2,3-dichlorodibenzo-p-dioxin (2,3-DCDD) (k3), OH + 2,7-dichlorodibenzo-p-dioxin (2,7-DCDD) (k4), OH + 2,8-dichlorodibenzo-p-dioxin (2,8-DCDD) (k5), OH + 1,2,3,4-tetrachlorodibenzo-p-dioxin (1,2,3,4-TCDD) (k6), and OH + octachlorodibenzo-p-dioxin (OCDD) (k7) over an extended range of temperature. The atmospheric pressure (740 +/- 10 Torr) rate measurements are characterized by the following Arrhenius parameters (in units of cm3 molecule(-1) s(-1), error limits are 1 omega): k1(326-907 K) = (1.70+/-0.22) x 10(-12)exp(979+/-55)/T, k2(346-905 K) = (2.79+/-0.27) x 10(-12)exp(784+/-54)/T, k3(400-927 K) = 10(-12)exp(742+/-67)/T, k4(390-769 K) = (1.10+/-0.10) x 10(-12)exp(569+/-53)/T, k5(379-931 K) = (1.02+/-0.10) x 10(-12)exp(580+/-68)/T, k6(409-936 K) = (1.66+/-0.38) x 10(-12)exp(713+/-114)/T, k7(514-928 K) = (3.18+/-0.54) x 10(-12)exp(-667+/-115)/T. The overall uncertainty in the measurements, taking into account systematic errors dominated by uncertainty in the substrate reactor concentration, range from a factor of 2 for DD, 2-CDD, 2,3-DCDD, 2,7-DCDD, and 2,8-DCDD to +/- a factor of 4 for 1,2,3,4-TCDD and OCDD. Negative activation energies characteristic of an OH addition mechanism were observed for k1-k6. k7 exhibited a positive activation energy. Cl substitution was found to reduce OH reactivity, as observed in prior studies at lower temperatures. At elevated temperatures (500 K < T < 500 K), there was no experimental evidence for a change in reaction mechanism from OH addition to H abstraction. Theoretical calculations suggest that H abstraction will dominate OH reactivity for most if not all dioxins (excluding OCDD) at combustion temperatures (>1000 K). For OCDD, the dominant reaction mechanism at all temperatures is OH addition followed by Cl elimination.     

磷酸铵镁Mg(OH)_2碱促热解动力学

考察了Mg(OH)_2辅助碱性介质对磷酸铵镁降解的媒介作用,解析了磷酸铵镁热解动力学过程。Mg(OH)_2碱促热解为3步降解过程,5 K/min时,其最大吸热峰为390 K;Mg(OH)_2碱促热解动力学降解模型符合自催化降速模型,存在2种以上的降解途径;氨基剥离所需活化能在34.4~112.9 k J/mol之间,Mg(OH)_2碱促热解动力学方程为:f(α)=k_0(1-α)~n,线性相关系数为0.994,反应级数为6.12。     

Ambient Air Hydroxyl Radical Concentrations: Measurements and Model Predictions

This review critically examines the limited number of recent near surface measurements of hydroxyl radicals in urban and rural areas. Predictions of average tropospheric, hemispheric and/or zonal concentrations of hydroxyl radicals derived from methylchloroform and carbon monoxide concentrations and models are discussed. The limitations of one- and two-dimensional model predictions of zonal and seasonal hydroxyl concentrations receive detailed consideration. Of especial concern on a regional scale are cloud cover effects on the penetration of ultraviolet radiation below 320 μm which is critical to the subsequent reactions forming hydroxyl radicals.     

Photo-Induced OH reactions of naphthalene and its oxidation products on SiO2

The photo-induced degradation of naphthalene, 1,4-naphthoquinone, 1-naphthol and 1-NO₂ naphthalene, adsorbed on silica gel, and with the addition of nitrogenous air pollutants e.g. NO₂ (as KNO₂) was investigated. Results indicate that compounds adsorbed onto a solid carrier are degraded when irradiated with UV light (λ > 290 nm) in the presence of nitrites. The key species initiating the naphthalene degradation is the OH-radical which is generated through the photolysis of NO₂. Reaction products identified were 2-formyl-cinnamaldehyde, 1,4-naphthoquinone, nitronaphthol, o-phthaldialdehyde, phthalide and nitronaphthalene. A mass balance between 40–50% was achieved. Under the same irradiation conditions, 1-NO₂ naphthalene is mainly degraded by direct photolysis while degradation of 1-naphthol and 1,4-naphthoquinone proceeds via the reaction with OH-radicals. Identified products were hydroxy-nitro-nitroso- and quinones compounds.     

磷酸铵镁碱促(Mg(OH)_2)热解产物氨氮去除性能

磷酸铵镁热解循环技术可以有效降低磷酸铵镁结晶技术的药剂费用。同时,在磷酸铵镁热解过程中添加Mg(OH)2碱促媒介,能阻止磷酸氢镁向焦磷酸镁的转化,并降低热解产物氨氮去除过程中上清液磷酸盐的残留。在Mg(OH)2∶NH+4摩尔比为1∶1,热解温度为110℃条件下,热解产物沉氨效率接近84%,上清液磷酸盐残留量为0.02 mg/L。     

Evidence for destruction of PCBs by the OH radical in urban atmospheres

Evidence for reaction of polychlorinated biphenyl (PCB) congeners with the hydroxyl (OH) radical in the troposphere was observed in diurnal variations in ambient gas-phase PCB concentrations at three urban sampling sites located in the Chicago, IL; Baltimore, MD; and Jersey City, NJ urban/industrial areas. The magnitude of the depletion of individual PCB congeners decreased by about 10-20% for each additional chlorine substituent, reflecting slower reaction rates for higher MW congeners with the OH radical. Octa- and nonachlorobiphenyls, which are largely unreactive with the OH radical, were used as tracers to investigate the effects of dilution on diurnal variation. The environmental rate constants for disappearance of the PCBs range from about 1.0 day(-1) for trichlorobiphenyls to about 0.3 day(-1) for hexachlorobiphenyls. Assuming a OH radical concentration of 3 x 10(6) molecules cm (-3), the second-order rate constants for reaction of specific congeners with the OH radical are consistent with laboratory measurements. More importantly, the relative reactivity of PCB homologues agrees well with the relationship predicted by other researchers from laboratory measurements, suggesting that losses of PCBs during daytime tropospheric transport are due at least in part to reactions with the OH radical.     

OH and HO2 measurements in a forested region of north-western Greece

Boundary layer concentrations of hydroxyl (OH) and hydroperoxyl (HO₂) radicals were measured at 1180 m elevation in a mountainous, forested region of north-western Greece during the AEROsols formation from BIogenic organic Carbon (AEROBIC) field campaign held in July–August 1997. In situ measurements of OH radicals were made by laser-induced fluorescence (LIF) at low pressure, exciting in the (0, 0) band of the A–X system at 308 nm. HO₂ radicals were monitored by chemical titration to OH upon the addition of NO, with subsequent detection by LIF. The instrument was calibrated regularly during the field campaign, and demonstrated a sensitivity towards OH and HO₂ of 5.2×10⁵ and 2.4×10⁶ molecule cm⁻³, respectively, for a signal integration period of 2.5 min and a signal-to-noise ratio of 1. Diurnal cycles of OH and HO₂ were measured on 10 days within a small clearing of a forest of Greek Fir (Abies Borisi-Regis). In total 4165 OH data points and 1501 HO₂ data points were collected at 30 s intervals. Noon-time OH and HO₂ concentrations were between 4–12×10⁶ and 0.4–9×10⁸ molecule cm⁻³, respectively. The performance of the instrument is evaluated, and the data are interpreted in terms of correlations with controlling variables. A significant correlation (r²=0.66) is observed between the OH concentration and the rate of photolysis of ozone, J(O¹D). However, OH persisted into the early evening when J(O¹D) had fallen to very low values, consistent with the modelling study presented in the following paper (Carslaw et al., 2001, OH and HO₂ radical chemistry in a forest region of north-western Greece. Atmospheric Environment 35, 4725–4737) that predicts a significant radical source from the ozonolysis of biogenic alkenes. Normalisation of the OH concentrations for variations in J(O¹D) revealed a bell-shaped dependence of OH upon NO_x (NO+NO₂), which peaked at [NO_x] ∼1.75 ppbv. The diurnal variation of HO₂ was found to be less correlated with J(O¹D) compared to OH.     

Rapidly eliminating pathogenic microorganisms in large air space using spraying *OH radicals

A new method for rapidly eliminating pathogenic microorganisms in large air space using spraying *OH radicals is presented in this paper With a physical method of strong electric-field discharge, large numbers of *OH radicals were produced by the oxygen activated particles of O2+, O(1D), O(3P), etc., and the introducing reagent HO2-. The gram-positive bacteria Bacillus subtilis, the gram-negative bacteria Serratia marcescens, and Bacillus spores were used for the eliminating experiments. Results show that the different microorganisms were rapidly killed by *OH radicals with a concentration of 0.8 mg/L and spraying density of 21 microL/m2 within 4 sec. Cell morphological changes were also observed under microscope. The cells of B. subtilis and Bacillus spores in their cellular wall, cellular membrane, or cell protoplasm were greatly destroyed when being exposed to a killing dosage of *OH radicals.     

Kinetic study of the OH reaction with some hydrochloroethers under simulated atmospheric conditions

Using the relative rate technique, rate constants for the gas-phase reactions of hydroxyl radicals with 2-chloroethyl methyl ether (k₁), 2-chloroethyl ethyl ether (k₂) and bis(2-chloroethyl) ether (k₃) have been measured. Experiments were carried out at (298 ± 2) K and atmospheric pressure using synthetic air as bath gas. Using n-pentane and n-heptane as reference compounds, the following rate constants were derived: k₁ = (5.2 ± 1.2) × 10^?12, k₂ = (8.3 ± 1.9) × 10^?12 and k₃ = (7.6 ± 1.9) × 10^?12, in units of cm³ molecule^?1 s^?1. This is the first experimental determination of k₂ and k₃ under atmospheric pressure. The rate constants obtained are compared with previous literature data and the observed trends in the relative rates of reaction of hydroxyl radicals with the ethers studied are discussed. The atmospheric implications of the results are considered in terms of lifetimes and fates of the hydrochloroethers studied.     

助溶剂甲醇对化学物质紫外光解的影响

在研究微量有机化学污染物的光化学行为时,为了增加难溶于水的药品溶解度,往往采用甲醇作助溶剂。但对反应体系中助溶剂甲醇对光化学反应的影响缺乏系统研究。以高压汞灯为光源,以萘普生(NP)及其光解产物NP_1和NP_2为研究对象,通过对比纯水、1%甲醇和1%异丙醇反应体系中的实验结果发现,少量助溶剂甲醇在紫外光照下对羟基自由基的淬灭效果接近于异丙醇。结合检测到的中间产物,推导了NP的光解路径主要为光致脱羧、羟基自由基夺氢、自由基耦合等反应过程。在助溶剂甲醇存在的体系中,甲醇能与萘普生发生酯化反应,并通过淬灭羟基自由基,影响萘普生的光解反应,从而导致实验结果误读。因此,在光化学实验研究中,应该排除助溶剂甲醇对实验结果的影响。     

利用激光诱导荧光技术测量大气氢氧自由基的研究进展

激光诱导荧光技术是目前测量大气ＯＨ 自由基中使用最多的方法。本文总结了２０ 多年的研究成果,论述了激光诱导荧光技术用于测定大气ＯＨ 自由基的研究进展,讨论了测量中的不确定因素及其对策,并着重介绍了目前最常用的基于气体扩张的低压荧光（ＦＡＧＥ） 技术。     

OH and HO2 radical chemistry in a forested region of north-western Greece

This paper explores several aspects of the chemistry of a forested region in north-western Greece, from data collected during the AEROBIC97 campaign. An observationally constrained box model has been constructed to enable comparisons between modelled concentrations of OH and HO₂ and those determined by the fluorescence assay by gas expansion (FAGE) technique. These results represent the first comparison of measured and modelled OH concentrations in such an environment. The modelled OH concentrations are, on average,∼50% of those measured (range of 16–61%) over 4 days of model and measurement comparison. Possible reasons for the model-measurement discrepancy are discussed. A rate of production analysis illustrates the dominance of isoprene and the monoterpenes on OH loss, as well as the significance of the ozonolysis of biogenic species as an OH source. The measured and modelled [HO₂]/[OH] ratio averaged between 11:00 and 15:00 h is much higher than has been found previously for similar NO_x concentrations,∼75 and 340, respectively, cf. 10–20. The high ratio reflects the rapid recycling through the OH–HO₂ oxidation chain, involving biogenic species. The high biogenic concentrations result in a midday OH lifetime of∼0.15 s. Finally, for the conditions encountered during the campaign, there is high net photochemical ozone production, peaking at∼20 ppbv h⁻¹ around 09:00 h.     

综合利用镁合金废料制氢及氢氧化镁

AZ31B镁合金废片-盐水体系能同时产生氢气和Mg(OH)2,研究了催化剂,Cl-浓度和反应温度对该体系产氢速率的影响,并分析了产物Mg(OH)2的化学成分和形态结构。实验结果表明,与Cu Cl2相比,铜线作为催化剂不仅产氢的速率高,且不会污染形成的Mg(OH)2。Cl-浓度和反应温度影响对体系产氢速率有较大影响,当温度为60℃,盐水浓度为4 mol/L时产氢速率达到最大值为62 m L/(min·g)。采用扫描电镜、X射线能谱仪表征分析中间产物,探讨Cl-点蚀反应的机理,对产物氢氧化镁粉末进行表征和物相分析的结果表明,产物为高纯度Mg(OH)2纳米材料,其晶体结构和分散性随温度的增加变好,80℃下获得规整的六方晶体。     

Total OH reactivity and VOC analyses for gasoline vehicular exhaust with a chassis dynamometer

Total OH reactivity for the exhaust gas of gasoline vehicles was measured for the first time under nine different driving conditions with a chassis dynamometer at the National Institute for Environmental Studies (NIES). Along with the total OH reactivity measurements, analysis of trace species such as CH₄, CO, NO, NO₂, and 56 kinds of volatile organic compounds (VOCs), including two aldehydes, was carried out. The ratio of alkanes to alkenes in the exhaust gas turned out to depend on the condition of the driving cycles. There were a considerable number of unidentified peaks obtained during GC analysis. About 15–30% of the total carbon was unidentified species. The chemical compositions of vehicular exhaust were found to depend on the temperature of the engine or catalysts. The contribution of OH reactivity to the species obtained depended on the temperature condition for the engine. The calculated total OH reactivity for VOCs was compared with ozone formation potential (OFP) and it turned out that there are in good correlation, while the correlation for “Cold” start deviates from that for “Hot” start. The measured and calculated OH reactivities were compared with each other. For all driving cycles, the calculated OH reactivity was confirmed to be an underestimation, implying the existence of unknown species in the exhaust gas. The percentage contribution of OH reactivity to the unknown species during “Cold” start was about 17.5%, which was almost the same as that for “Hot” start at 17.0%. However, the absolute value of OH reactivity for “Cold” start was about ten times higher than that for “Hot” start.     

Kinetics of the reactions of soot surface-bound polycyclic aromatic hydrocarbons with the OH radicals

The kinetics of the heterogeneous reaction of OH radicals with 15 polycyclic aromatic hydrocarbons (PAHs) present in laboratory generated simulated kerosene combustion soot was studied at T = 290 K in a low pressure discharge-flow reactor combined with an electron-impact mass spectrometer. The kinetics of soot-bound PAH consumption in reaction with OH were monitored using off-line HPLC measurements of their concentrations in soot samples as a function of time of exposure to OH. Concentration of OH radicals in the gas phase was measured by mass spectrometry. The first-order rate constants measured for the individual PAH at T = 290 K ranged from 0.02 to 0.04 s^?1 and were found to be independent of the OH concentration ([OH] = (0.34–2.5) × 10¹² molecule cm^?3) and of the molecular structure of the PAH. In addition, the uptake coefficient of OH on soot surface and the diffusion coefficient of OH in He were measured to be 0.19 ± 0.03 (calculated with geometric surface area) and (615 ± 80) Torr cm² s^?1, respectively. Comparison of the results on the PAH + OH reaction to those from previous studies carried out on different carbonaceous substrates, indicates probable dependence of the heterogeneous reactivity of PAH toward OH on the substrate nature. Rapid reaction with OH can be an important potential pathway of the atmospheric degradation of non-volatile PAH present mainly in the particulate phase in the atmosphere.     

Plasma versus Thermal Effects in Flue Gas NOx Reduction Using Ammonia Radical Injection

Abstract

A plasma-assisted ammonia injection technique was previously demonstrated as having the potential to remove NO_x from combustion flue gases at SCR-comparable levels without the use of catalysts. However, these demonstrations did not prove the advantage of plasma assistance because they did not explicitly account for enhanced radical production by bulk thermal heating. An experiment using hot ammonia injection was performed to separate this thermal effect from the effect of radical production via interaction with a plasma. Under excess air conditions, results show that a thermal effect does provide improved NO_x reduction, but not to the level achieved with the use of a plasma source. However, heating the injection gases provides only a minor improvement in NO_x reduction at NH₃/NO_x ratios and temperatures typical of commercial cold SNCR applications. The plasma effect in ammonia radical injection was also found to be significant, accounting for an additional 15% to 35% of absolute NO_x reduction beyond any thermal benefit at typical excess air conditions. The ammonia radical injection technique continues to show promise as an effective NO_x reduction alternative.     

Weekday/Weekend Differences in OH Reactivity with VOCs and CO in Baltimore,Maryland

ABSTRACT

This study compared the first-order frequencies for OH associated with volatile organic compounds (VOCs) and CO (hereafter called OH reactivity with VOCs or CO), the product of the VOC or CO concentration, and their respective k_OH value, on an average weekday with that on an average weekend day at a core urban site in Baltimore, MD. The average daytime concentrations were calculated for each of the 55 available Photochemical Assessment Monitoring Station (PAMS) VOCs using data from the Baltimore site. The data were sorted in descending order to highlight the important species based on concentration. The OH reactivity with VOCs was sorted in descending order to identify the important species based on the magnitude of the OH reactivity. A similar process was followed for the OH reactivity with CO. The contribution of the significant species to the weekday/weekend difference in OH reactivity was examined.

The OH reactivity with C₅H₈ was the largest among the OH reactivity with the PAMS' VOCs and was the same on the weekday and weekend. The weekday/weekend difference in OH reactivity with VOCs was entirely due to differences in concentrations of the anthropogenic VOCs. The OH reactivity with VOCs was 11% larger on the weekday. When OH reactivity with CO was included, the OH reactivity was 13% larger on the weekday.