Kinetics of heterogeneous ozone reactions

Earlier results on ozone destruction on solid surfaces gave apparent first order kinetics. Estimating the reaction kinetics from our data on ozone destruction on various powders (silica-gel, alumina, wood ash, coal ash, Saharan sand, calcite), we found that only calcite and wood ash exhibited such a behaviour. Removal of ozone by other powders used showed two straight lines in ln c-t plot with two different half-lives, t'(1/2) < t'(1/2). Comparing the kinetic constants for ozone removal on silica-gel and that of ozone reactions with polynuclear aromatic hydrocarbons (PAHs) adsorbed in submonolayer coverage on the same powder, the first reaction seems to be more likely in the case of pyrene and particularly fluoranthene. Enhanced ozone destruction on airborne aerosols could be an additional reason for fluoranthene stability in the real atmosphere.     

Studies on reactions of ozone with alkenes

In the last years, a continuous increase of the O3 concentration has been recorded in the lower atmospheric layers. Photochemical reactions with NO(x), CO and organic compounds are the main sources of O3 in the troposphere. In this work, an attempt was made to determine the impact of alkenes on the O3 concentration in the troposphere. A study on the gas-phase reactions of 03 with 1-hexene, 1-heptene and 1-nonene was made. The reactions were carried out at room temperature under atmospheric pressure. Ozone was formed by the ultraviolet radiation emitted by a mercury lamp, in order to simulate the atmospheric conditions. The changes with time in the concentration of O3, 1-alkenes and formed aldehydes were investigated. Qualitative and quantitative analyses were done by means of the gas chromatography and colorimetry. The following products were identified: pentanal from 1-hexene; hexanal from 1-heptene; oktanal from 1-nonene. For each of the reactions, HCHO was also determined as a product. The reaction rate constants were calculated and obtained in units of 10(-17) cm(-3) molecule(-1) s(-1): 1.94-0.99 for 1-hexene, 5.54-4.51 for 1-heptene and 1.54-0.76 for 1-nonene. Based on the results obtained, an explanation of O3 concentration variations in the planetary boundary layer can be given. Last year a considerable increase of O3 concentration on the roads of Western Europe was recorded. This increase could have resulted from the decrease of alkene concentration in the air due to common use of the catalytic converters in cars. The unsaturated hydrocarbons rapidly oxidize on the catalyst. In Eastern Europe, where the amount of cars equipped with catalytic converters is smaller than in Western Europe, the alkene content in the exhaust fumes results in a decrease of the O3 concentration in the troposphere.     

Impact of ozone mini-holes on the heterogeneous destruction of stratospheric ozone

A comprehensive study of ozone mini-holes over the mid-latitudes of both hemispheres is presented, based on model simulations with the coupled climate-chemistry model ECHAM4.L39(DLR)/CHEM representing atmospheric conditions in 1960, 1980, 1990 and 2015. Ozone mini-holes are synoptic-scale regions of strongly reduced total ozone, directly associated with tropospheric weather systems. Mini-holes are supposed to have chemical and dynamical impacts on ozone levels. Since ozone levels over northern mid-latitudes show a negative trend of approximately -4%/decade and since it exists a negative correlation between total column ozone and erythemally active solar UV-radiation reaching the surface it is important to understand and assess the processes leading to the observed ozone decline. The simulated mini-hole events are validated with a mini-hole climatology based on daily ozone measurements with the TOMS (total ozone mapping spectrometer) instrument on the satellite Nimbus-7 between 1979 and 1993. Furthermore, possible trends in the event frequency and intensity over the simulation period are assessed. In the northern hemisphere the number of mini-hole events in early winter decreases between 1960 and 1990 and increases towards 2015. In the southern hemisphere a positive trend in mini-hole event frequency is detected between 1960 and 2015 in spring associated with the increasing Antarctic Ozone Hole. Finally, the impact of mini-holes on the stratospheric heterogeneous ozone chemistry is investigated. For this purpose, a computer-based detection routine for mini-holes was developed for the use in ECHAM4.L39(DLR)/CHEM. This method prevents polar stratospheric cloud formation and therefore heterogeneous ozone depletion inside mini-holes. Heterogeneous processes inside mini-holes amount to one third of heterogeneous ozone destruction in general over northern mid- and high-latitudes during winter (January-April) in the simulation.     

Sonolytic reactions of phenanthrene in organic extraction solutions

Ultrasonic extraction is a common method used to extract semi-volatile and nonvolatile organic compounds such as polycyclic aromatic hydrocarbons (PAHs) from solid matrices. However, ultrasonic energy has been suspected to lead to undesired reactions of the solute and thus affect qualitative and quantitative results. In this paper, sonolytic reactions of phenanthrene in common organic extraction solutions were examined using a 20 kHz ultrasonic probe under conditions commonly used for ultrasonic extraction. Extraction parameters including phenanthrene concentration, solvent type, pulse length, and sonication time were investigated. Hexane:acetone (1:1 V/V) resulted in less phenanthrene degradation than dichloromethane (DCM):acetone (1:1 V/V). Initial solute concentration, length of sonication time, and solvent type affected the degradation of phenanthrene. Reaction byproducts including methylphenanthrene and methylnaphthalene detected after sonication indicate that phenanthrene reacts by both direct pyrolysis and reaction with methyl or ethyl radicals formed from solvent pyrolysis.     

Formation of 9,10-phenanthrenequinone by atmospheric gas-phase reactions of phenanthrene

Phenanthrene is a 3-ring polycyclic aromatic hydrocarbon which exists mainly in the gas-phase in the atmosphere. Recent concern over the presence of 9,10-phenanthrenequinone in ambient particles led us to study the products of the gas-phase reactions of phenanthrene with hydroxyl radicals, nitrate radicals and ozone. The formation yields of 9,10-phenanthrenequinone were measured to be ∼3%, 33±9%, and ∼2% from the OH radical, NO₃ radical and O₃ reactions, respectively. Calculations suggest that daytime OH radical-initiated and nighttime NO₃ radical-initiated reactions of gas-phase phenanthrene may be significant sources of 9,10-phenanthrenequinone in ambient atmospheres. In contrast, the ozone reaction with phenanthrene is unlikely to contribute significantly to ambient 9,10-phenanthrenequinone.     

Photocatalytic reactions of phenanthrene at TiO2/water interfaces

The photocatalytic oxidation of phenanthrene was investigated in aqueous TiO2 suspensions under UV light irradiation. Chemical oxygen demand (COD) measurements, UV-Vis spectrophotometer, IR spectrometer and gas chromatography-mass spectrometry (GC-MS) analytical techniques were used to monitor the reaction process. Some factors affecting the photodegradation rate were studied and some aromatic intermediates were detected during the reaction process. Fast and complete mineralization of phenanthrene was achieved in this reaction system.     

Hydroxyl radical and ozone initiated photochemical reactions of 1,3-butadiene

1,3-Butadiene, classified as hazardous in the 1990 Clean Air Act Amendments, is an important ambient air pollutant. Understanding its atmospheric transformation is useful for its own sake, and is also helpful for eliciting isoprene's fate in the atmosphere (isoprene dominates the biogenic emissions in US). In this paper, samples from both hydroxyl- and ozone-initiated photooxidation of 1,3-butadiene were analyzed by derivatization with O- (2,3,4,5,6-pentafluorobenzyl)-hydroxylamine followed by separation and detection by gas chromatography/ion trap mass spectrometry to detect and identify carbonyl compounds. The following carbonyls were observed: formaldehyde, acrolein, glycolaldehyde, glycidaldehyde, 3-hydroxy-propanaldehyde, hydroxy acetone, and malonaldehyde, which can be classified into three categories: epoxy carbonyls, hydroxyl carbonyls, and di-carbonyls. Three non-carbonyls, furan, 1,3-buatdiene monoxide, and 1,3-butadiene diepoxide, were also found. To confirm their identities, both commercially available and synthesized standards were used. To investigate the mechanism of 1,3-butadiene, separate batch reactor experiments for acrolein and 1,3-butadiene monoxide were carried out. Time series samples for several products were also taken. When necessary, computational chemistry methods were also employed. Based on these results, various schemes for the reaction mechanism are proposed.     

Laboratory studies of heterogeneous reactions of SO2

The rates of removal of gaseous SO₂ over solids commonly found in urban aerosols were measured in the laboratory. A tubular flow reactor, in which the walls of an inner, concentric cylinder were coated with the solid of interest, was used in these studies. Analysis of the data, using models that specifically accounted for mass transport in the laboratory system, yielded collision efficiencies or the fraction of gas-solid collisions that are effective in removing SO₂. Experimentally measured collision efficiencies for fresh solid coatings range from < 10⁻⁶ to 10⁻³. Wet chemical and X-ray photoelectron spectroscopic results indicate that, to within an experimental error of a factor of 2, gaseous SO₂ is converted to adsorbed sulfate on most of the solids examined.As the time of SO₂ exposure increased we found that the rates of SO₂ removal gradually diminished until, with prolonged exposure, the solids completely lost their ability to remove this species from the gas phase. The relative humidity of the reaction mixture was found to be important in determining the total amount, but not the initial rate, of SO₂ uptake, with SO₂ uptake increasing at higher humidities. Overall, selected solids removed up to several tenths of a gram of SO₂ per gram solid from humidified reaction mixtures. Further reaction could be induced by exposure to small amounts of ammonia.The saturation type of behavior observed on prolonged exposure to SO₂ led to the suspicion that fly ash materials examined in this study, as received, may already have undergone substantial reaction with SO₂, before or during collection. Further experiments on these materials, involving washing those as received materials with distilled water to remove soluble sulfates, supported this contention. Of the six fly ash materials examined, initial collision efficiencies for four of these materials were increased by factors ranging from 2 to > 300 by the water pretreatment. The other two materials exhibited high initial collision efficiencies (~10⁻⁴) that were unaffected, to within experimental error, by the water washing.Atmospheric projection of results from this study suggests that freshly emitted aerosols can be quite effective in converting gaseous SO₂ to particulate sulfate. The capacity limited nature of the reactions suggests that these processes will be most important at or near emission sources, although further, non-source interactions can be induced by atmospheric ammonia.     

Heterogeneous reactions of ozone with pyrene, 1-hydroxypyrene and 1-nitropyrene adsorbed on particles

This work deals with the kinetic study of the reactions of ozone with pyrene, 1-hydroxypyrene and 1-nitropyrene, adsorbed on model particles. Experiments were performed at room temperature and atmospheric pressure, using a quasi-static flow reactor in the absence of light. Compounds were extracted from particles using pressurized fluid extraction (PFE) and concentration measurements were performed using gas chromatography/mass spectrometry (GC/MS). The pseudo-first order rate constants were obtained from the fit of the experimental decay of particulate polycyclic compound concentrations versus reaction time. Experiments were performed at three different O₃ concentrations from which second order rate constants were calculated. The following rate constant values were obtained at 293 K: k(O₃ + Pyrene) = (3.2 ± 0.7) × 10^?16 cm³ molecule^?1 s^?1; k(O₃ + 1OHP) = (7.7 ± 1.4) ×10 ^?16 cm³ molecule^?1 s^?1; and k(O₃ + 1NP) = (2.2 ± 0.5) × 10^?17 cm³ molecule^?1 s^?1, for pyrene, 1-hydroxypyrene and 1-nitropyrene adsorbed on silica particles. The variation in the rate constants demonstrates the strong influence of the substituent (OH or NO₂) on the heterogeneous reactivity of pyrene. The pyrene particulate concentration was also varied in order to check how this parameter may influence the experiments. Finally, oxidation products were investigated for all reactions and some were detected and identified for the first time for ozone heterogeneous reaction with pyrene adsorbed on particles.     

Light-induced heterogeneous ozone processing on organic coated particles: Kinetics and condensed-phase products

For the first time we investigated the effect of solar irradiation upon the heterogeneous ozonation of adsorbed 3,4,5-trimethoxybenzaldehyde on solid surface. Light-induced heterogeneous reactions between gas-phase ozone and 3,4,5-trimethoxybenzaldehyde adsorbed on silica particles were performed and the consecutive reaction products were identified. At an ozone mixing ratio of 250 ppb, the loss of 3,4,5-trimethoxybenzaldehyde ranged from 1.0 · 10^?6 s^?1 in the dark to 2.9 · 10^?5 s^?1 under light irradiation. Such large enhancement of 29 times clearly shows the importance of light (λ > 300 nm) during the heterogeneous ozonolysis on organic coated particles.The reaction products identified in this study (3,4,5-trimethoxybenzoic acid, syringic acid, methyl 3,4,5-trimethoxybenzoate) absorb light in the spectral window (λ > 300 nm) which implies that light-induced heterogeneous ozone processing can have an influence on the aerosol surfaces by changing their physico-chemical properties.The main identified product of the heterogeneous reactions between gas-phase ozone and 3,4,5-trimethoxybenzaldehyde under dark conditions and in presence of light was 3,4,5-trimethoxybenzoic acid. For this reason we estimated the carbon yield of 3,4,5-trimethoxybenzoic acid. Carbon yields of 3,4,5-trimethoxybenzoic acid decreased with increasing ozone mixing ratio; from 40% at 250 ppb to 15% at ≥2.5 ppm under dark conditions. At ozone mixing ratio (250 ppb–1 ppm), carbon yields of 3,4,5-trimethoxybenzaldehyde are relatively higher in the experiment under dark condition than under simulated solar light.     

Indoor ozone/terpene reactions as a source of indoor particles

This paper reports effects of reactions between ozone and selected terpenes on the concentrations and size distributions of airborne particles in a typical indoor setting. The studies were conducted in adjacent, identical offices. In the first set of experiments, known concentrations of ozone and a selected terpene (either d-limonene, α-terpinene, or a terpene-based cleaner whose major constituent is α-pinene) were deliberately introduced into one of the offices while the other office served as a control. Subsequent particle formation and redistribution were monitored with an eight-channel optical particle counter. Particle formation was observed in each terpene system, but was greatest in the case of d-limonene. The number of particles in the 0.1–0.2 μm diameter size range was as much as 20 times larger in the office with deliberately supplemented ozone and d-limonene than in the office serving as the control. The concentration differences in the larger size ranges developed with time, indicating the importance of coagulation and condensation processes in this indoor environment. In the second set of experiments, d-limonene was deliberately introduced into one of the offices, but ozone was not supplemented in either office; instead, the indoor ozone concentrations were those that happened to be present (primarily as a consequence of outdoor-to-indoor transport). In the office that contained supplemental d-limonene, the concentrations of the 0.1–0.2 μm particles tracked those of indoor ozone (the limiting reagent) and were as much as 10 times greater than levels measured in the comparable office that did not contain supplemental d-limonene. The results demonstrate that ozone/terpene reactions can be a significant source of sub-micron particles in indoor settings, and further illustrate the potential for reactions among commonly occurring indoor pollutants to markedly influence indoor environments.     

The effects of ozone/limonene reactions on indoor secondary organic aerosols

An indoor air quality model was used to predict dynamic particle mass concentrations based on homogeneous chemical mechanisms and partitioning of semi-volatile products to particles. The ozone–limonene reaction mechanism was combined with gas-phase chemistry of common atmospheric organic and inorganic compounds and incorporated into the indoor air quality model. Experiments were conducted in an environmental chamber to investigate secondary particle formation resulting from ozone/limonene reactions. Experimental results indicate that significant fine particle growth occurs due to the interaction of ozone and limonene and subsequent intermediate by-products. Secondary particle mass concentrations were estimated from the measured particle size distribution. Predicted particle mass concentrations were in good agreement with experimental results—generally within ∼25% at steady-state conditions. Both experimental and predicted results suggest that air exchange rate plays a significant role in determining secondary fine particle levels in indoor environments. Secondary particle mass concentrations are predicted to increase substantially with lower air exchange rates, an interesting result given a continuing trend toward more energy efficient buildings. Lower air exchange rates also shifted the particle size distribution toward larger particle diameters. Secondary particle mass concentrations are also predicted to increase with higher outdoor ozone concentrations, higher outdoor particle concentrations, higher indoor limonene emission rates, and lower indoor temperatures.     

Modeling of ozone reactions on aircraft-related soot in the upper troposphere and lower stratosphere

Several studies in modeling atmospheric processes have suggested that heterogeneous chemistry on soot emitted from high altitude aircraft could affect stratospheric ozone depletion. However, these modeling studies were limited because they did not adequately consider the decrease in reaction probability with time as the surface of the soot becomes “poisoned” by its interactions with various gases. Here we extend UIUC's two-dimensional chemical-transport model to investigate possible effects of heterogeneous reactions of ozone on aircraft-generated carbon particles, including a treatment of soot poisoning in the model. We generally follow literature recommendations for ozone uptake probabilities and determine the available active sites on soot given partial pressures of the reactants, temperature, and time since soot emission in order to investigate ozone decrease. The regeneration of soot active sites is also taken into account in this study. We find that, even if active sites on soot surfaces are regenerated, upper troposphere and lower stratosphere ozone losses on aircraft emitted soot occurring through heterogeneous reactions are insignificant once poisoning effects are considered.     

Influence of iron and copper oxides on polychlorinated diphenyl ether formation in heterogeneous reactions

Polychlorinated diphenyl ether (PCDE) has attracted great attention recently as an important type of environmental pollutant. The influence of iron and copper oxides on formation of PCDEs was investigated using laboratory-scale flow reactors under air and under nitrogen at 350 °C, a temperature corresponding to the post-combustion zone of a municipal solid waste incinerator. The results show that the 2,2′,3,4,4′,5,5′,6-otachlorodiphenyl ether (OCDE) formed from the condensation of pentachlorophenol (PCP) and 1,2,4,5-tetrachlorobenzene (Cl₄Bz) is the predominant congener formed on the SiO₂/Fe₂O₃ surface with and without oxygen. This indicated that HCl elimination between PCP and 1,2,4,5-Cl₄Bz molecules formed 2,2′,3,4,4′,5,5′,6-OCDE in the presence of Fe₂O_3. On the other hand, decachlorodiphenyl ether, nonachlorodiphenyl ether, and OCDE were the dominant products on the SiO₂/CuO surface without oxygen, although the 2,2′,3,4,4′,5,5′,6-OCDE was the dominant product on the SiO₂/CuO surface with oxygen. Therefore, the presence of Fe₂O₃ and CuO influences the formation and homologue distribution of PCDEs, which shifted towards the lower chlorinated species. Fe₂O₃ can promote both the condensation and dechlorination reaction without oxygen_. On the contrary, with oxygen, Fe₂O₃ suppresses the condensation of chlorobenzene and chlorophenol to form PCDEs and polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/Fs). CuO can increase the formation of lower chlorinated PCDEs and PCDDs without oxygen. In conclusion, the different fly ash components have a major influence on PCDE emissions.     

Sorption of phenanthrene by soils contaminated with heavy metals

The fate of polycyclic aromatic hydrocarbons (PAHs) in soils with co-contaminants of heavy metals has yet to be elucidated. This study examined sorption of phenanthrene as a representative of PAHs by three soils contaminated with Pb, Zn or Cu. Phenanthrene sorption was clearly higher after the addition of heavy metals. The distribution coefficient (K(d)) and the organic carbon-normalized distribution coefficient (K(oc)) for phenanthrene sorption by soils spiked with Pb, Zn or Cu (0-1000 mg kg(-1)) were approximately 24% larger than those by unspiked ones, and the higher contents of heavy metals added into soils resulted in the larger K(d) and K(oc) values. The enhanced sorption of phenanthrene in the case of heavy metal-contaminated soils could be ascribed to the decreased dissolved organic matter (DOM) in solution and increased soil organic matter (SOM) as a consequence of DOM sorption onto soil solids. Concentrations of DOM in equilibrium solution for phenanthrene sorption were lower in the case of the heavy metal-spiked than unspiked soils. However, the decreased DOM in solution contributed little to the enhanced sorption of phenanthrene in the presence of metals. On the other hand, the sorbed DOM on soil solids after the addition of heavy metals in soils was found to be much more reactive and have far stronger capacity of phenanthrene uptake than the inherent SOM. The distribution coefficients of phenanthrene between water and the sorbed DOM on soil solids (K(ph/soc)) were about 2-3 magnitude larger than K(d) between water and inherent SOM, which may be the dominant mechanism of the enhanced sorption of phenanthrene by soils with the addition of heavy metals.     

Estimation of apparent rate coefficients for phenanthrene and pentachlorophenol interacting with sediments

To gain information on organic pollutants in water-sediment systems, a compartment model was applied to study the sorption course of phenanthrene and pentachlorophenol (PCP) in sediments. The model described the time-dependent interaction of phenanthrene and PCP with operationally defined reversible and irreversible (or slowly reversible) sediment fractions. The interactions between these fractions were described using first order differential equations. By fitting the models to the experimental data, apparent rate constants were obtained using numerical optimization software. The model optimizations showed that the amount of reversible phase increased rapidly in the first 10 d with the sorption time, then decreased after 10 d, while the amount of irreversible phase increased in the total sorption course. That suggested the mass transport between reversible phase and irreversible phase. The extraction efficiency with hot methanol ranged from 36% to 103% for phenanthrene and from 65% to 101% for PCP, with the trend of decreasing with sorption time.