A large-scale experiment on mass transfer of trichloroethylene from the unsaturated zone of a sandy aquifer to its interfaces

A large-scale experiment was conducted to investigate the transport of trichloroethylene (TCE) vapors in the unsaturated zone and to determine the mass transfer to the groundwater and the atmosphere. The experiment involved injection of 5 1 of TCE in the unsaturated zone under controlled conditions, with multidepth sampling of gas and water through the unsaturated zone and across the capillary zone into underlying groundwater. The mass transfer of TCE vapors from the vadose zone to the atmosphere was quantified using a vertical flux chamber. A special soil water sampler was used to monitor transport across the capillary fringe. Experimental data indicated that TCE in the unsaturated zone was mainly transported to the atmosphere and this exchange reduced significantly the potential for groundwater pollution. The maximum measured TCE flux to the atmosphere was about 3 g/m(2)/day. Observed and calculated fluxes based on vertical TCE vapor concentration gradients and Fick's law were in good agreement. This confirms that TCE vapor transport under the experimental conditions was governed essentially by molecular diffusion. TCE vapors also caused a lower, but significant contamination of the underlying groundwater by dispersion across the capillary fringe with a corresponding maximum flux of about 0.1 g/m(2)/day. This mass transfer to groundwater is partly uncertain due to an inadvertent entry of some nonaqueous phase liquid (NAPL) from the source area into the saturated zone. Application of an analytical solution to estimate the TCE flux from the unsaturated zone to the groundwater indicated that this phenomenon is not only influenced by molecular diffusion but also by vertical dispersion. The mass balance indicates that, under the given experimental conditions (e.g. proximity of the source emplacement relative to the soil surface, relatively high permeable porous medium), nearly 95% of the initial TCE mass was transferred to the atmosphere.     

The variation of street air levels of PAH and other mutagenic PAC in relation to regulations of traffic emissions and the impact of atmospheric processes

The occurrence of particle associated PAH and other mutagenic PAC was determined in 1996 in the street air of Copenhagen. In addition, particle extracts were tested for mutagenicity. The measurements were compared with previous measurements in 1992/1993. The levels had decreased in this period. The decrease was caused by an implementation of light diesel fuels for buses and the exchange of older petrol-driven passenger cars with catalystequipped new ones. About 65% of the reduction was caused by the application of the light diesel fuels. Under special conditions, chemical processes in the atmosphere produced many more mutagens than the direct emissions. The concentrations of S-PAC and N-PAC were 10 times lower than those of PAH, while the levels of oxy-PAH were in the same order of magnitude as those of PAH. Benzanthrone, an oxy-PAH, is proposed to be formed in the atmosphere in addition to direct emissions. Benzo(a)pyrene, often applied as an air quality criteria indicator, was photochemically degraded in the atmosphere. A strong increase in the mutagenic activities was observed to coincide with a depletion of benzo(a)pyrene.     

A tool for estimating the contribution of water-soluble organic compounds to the particle mass and condensational growth in the atmosphere

In this theoretical investigation, we elucidated several factors governing the ability of organic, water-soluble vapours to participate into either the secondary organic aerosol formation or particle condensational growth in the atmosphere. The corresponding requirements for physico-chemical properties of the vapour were estimated. These estimates were also compared with the properties of several difunctional carboxylic acids identified in the atmosphere. Our analysis indicates that while many of the considered compounds are expected to contribute to the total particle mass in the atmosphere, their role in particle condensational growth process remains uncertain. This uncertainty emerges from the fact that the saturation ratio of an organic vapour does not alone determine its ability to act as a “nonvolatile” compound. Instead, the hygroscopic and chemical properties of the vapour, as well as the particle pH and size, have also to be taken into account.     

Effects of sintering atmosphere on cement clinkers produced from chromium-bearing sludge

The purpose of this study was to investigate the effects of sintering atmosphere (oxidizing and reducing) on the polymorphs of dicalcium silicates (Ca2SiO4, C2S) and on the chromium leaching of the belite-rich clinkers made from a chromium-bearing sludge. This sludge was generated in an electroplating factory, and in addition to chromium, it contained nickel, copper and zinc. In the clinker production, air was used as the oxidizing atmosphere, and carbon monoxide, which was produced by burning graphite with an insufficient amount of oxygen, was employed as the reducing atmosphere. Dicalcium silicates were substantially formed under both kinds of sintering atmosphere, but there was some nonhydraulic gamma-C2S in the clinkers produced under the oxidizing atmosphere. In addition, the amount of gamma-C2S decreased with the chromium-bearing sludge addition, whereas that of beta-C2S increased. The clinkers produced under the reducing atmosphere had less residual chromium, a finding that shows that more chromium was evaporated. However, the reducing atmosphere can decrease the proportion of hexavalent chromium (Cr(VI)) in the resulting clinkers. For other heavy metals, the residual amounts of nickel and copper generally increased with the sludge addition, but zinc was absent in most of the clinkers produced under the reducing atmosphere. This implies that the evaporation of zinc is much more significant than that of the other heavy metals under a reducing atmosphere. In the leaching tests, the concentrations of nickel, copper, and zinc were below the detection limits in all the leachates. In terms of chromium, the total leaching concentration was highly related to Cr(VI). The clinkers produced under the oxidizing atmosphere had high leaching concentrations of chromium, and thus failed to meet the regulatory limit. In contrast, the reducing atmosphere was effective in decreasing the chromium leaching, and it therefore makes the resulting cement clinkers more environmentally sound.     

Sulfur processing in the marine atmospheric boundary layer: A review and critical assessment of modeling uncertainties

Sulfur is an extremely motile and vital element in the Earth's biogeochemical environment, one whose active redox chemistry maintains small reservoirs in the atmosphere and biosphere yet large fluxes through both. Essential for life, intimately linked to the climate state, and an important component of air quality, sulfur and its transport and processing in the atmosphere have been the subject of active research for several decades. This review article describes the current state of our understanding of the atmospheric sulfur cycle, focusing on the marine atmospheric boundary layer, with the aim of identifying the largest roots of uncertainty that most inhibit accurate simulation of sulfur cycling in the atmosphere. An overview of the emissions by phytoplankton and shipping, dispersion and entrainment in the marine boundary layer, and chemical processing by aerosols, clouds, and dry deposition is presented. Analysis of 20 contemporary modeling studies suggests that the greatest ambiguity in global sulfur cycling derives from (in descending order) wet deposition of aerosol sulfate, dry deposition of sulfur dioxide to the Earth's surface, and the heterogeneous oxidation of SO₂ in aerosols and clouds.     

The relationship of the monoterpene composition in the atmosphere,the foliar emission gas and the leaf oil of Pinus densiflora

Monoterpene compositions in the atmosphre in a pine forest, the foliar emission gas and the leaf oil of pine needles were compared. The emission rate of each monoterpene from pine needles was proportional to the product of the amount in the leaf oil and the approximate vapor pressure. The composition of monoterpenes in the atmosphere is rather well explained by considering foliar emission rates with the supposition that the terpenes emitted into the atmosphere react with ozone and photochemically with nitrogen oxides.     

Carbon Monoxide in the Atmosphere

A carbon monoxide analyzer has been developed which is capable of continuous measurement of the carbon monoxide concentration in the atmosphere. The operating principle of the instrument is the reaction of carbon monoxide with hot mercuric oxide followed by the photometric determination of the mercury vapor produced. Oxygenated hydrocarbons and olefins are quantitatively detected. Those normally present are in the ambient atmosphere in low concentrations relative to CO. Hydrogen and methane in the atmosphere do not interfere with the CO analysis. Measurements of atmospheric CO concentrations in California, Greenland, and Oregon seem to indicate that CO content is an air mass characteristic. North Pacific marine air mass concentrations may be as low as about 0.040 parts per million (ppm) CO, while the air mass over continental California seems to be characterized by CO levels of 0.5-1.0 ppm or greater.     

Modelling the fate of persistent organic pollutants in Europe: parameterisation of a gridded distribution model

A regionally segmented multimedia fate model for the European continent is described together with an illustrative steady-state case study examining the fate of gamma-HCH (lindane) based on 1998 emission data. The study builds on the regionally segmented BETR North America model structure and describes the regional segmentation and parameterisation for Europe. The European continent is described by a 5 degrees x5 degrees grid, leading to 50 regions together with four perimetric boxes representing regions buffering the European environment. Each zone comprises seven compartments including; upper and lower atmosphere, soil, vegetation, fresh water and sediment and coastal water. Inter-regions flows of air and water are described, exploiting information originating from GIS databases and other georeferenced data. The model is primarily designed to describe the fate of Persistent Organic Pollutants (POPs) within the European environment by examining chemical partitioning and degradation in each region, and inter-region transport either under steady-state conditions or fully dynamically. A test case scenario is presented which examines the fate of estimated spatially resolved atmospheric emissions of lindane throughout Europe within the lower atmosphere and surface soil compartments. In accordance with the predominant wind direction in Europe, the model predicts high concentrations close to the major sources as well as towards Central and Northeast regions. Elevated soil concentrations in Scandinavian soils provide further evidence of the potential of increased scavenging by forests and subsequent accumulation by organic-rich terrestrial surfaces. Initial model predictions have revealed a factor of 5-10 underestimation of lindane concentrations in the atmosphere. This is explained by an underestimation of source strength and/or an underestimation of European background levels. The model presented can further be used to predict deposition fluxes and chemical inventories, and it can also be adapted to provide characteristic travel distances and overall environmental persistence, which can be compared with other long-range transport prediction methods.     

A review of the characteristics of nanoparticles in the urban atmosphere and the prospects for developing regulatory controls

The likely health and environmental implications associated with atmospheric nanoparticles have prompted considerable recent research activity. Knowledge of the characteristics of these particles has improved considerably due to an ever growing interest in the scientific community, though not yet sufficient to enable regulatory decision making on a particle number basis. This review synthesizes the existing knowledge of nanoparticles in the urban atmosphere, highlights recent advances in our understanding and discusses research priorities and emerging aspects of the subject. The article begins by describing the characteristics of the particles and in doing so treats their formation, chemical composition and number concentrations, as well as the role of removal mechanisms of various kinds. This is followed by an overview of emerging classes of nanoparticles (i.e. manufactured and bio-fuel derived), together with a brief discussion of other sources. The subsequent section provides a comprehensive review of the working principles, capabilities and limitations of the main classes of advanced instrumentation that are currently deployed to measure number and size distributions of nanoparticles in the atmosphere. A further section focuses on the dispersion modelling of nanoparticles and associated challenges. Recent toxicological and epidemiological studies are reviewed so as to highlight both current trends and the research needs relating to exposure to particles and the associated health implications. The review then addresses regulatory concerns by providing an historical perspective of recent developments together with the associated challenges involved in the control of airborne nanoparticle concentrations. The article concludes with a critical discussion of the topic areas covered.     

Development of a regional hydrologic soil model and application to the Beerze--Reusel drainage basin

The soil compartment is an important interface between the atmosphere and the subsurface hydrosphere. In this paper a conceptual approach for regional hydrologic soil modelling (RHSM) is presented, which provides two important qualities for modelling. First, the soil compartment is directly coupled to the atmosphere via the land surface and to the aquifers. Second, extremely fine (5cm vertical) resolutions of the soil system can be realized at regional scales (several hundreds of km(2)). This high-resolution modelling could be achieved by parallel computation techniques. The RHSM approach is applied to the Beerze-Reusel drainage basin, which belongs to the Meuse River basin. Moisture transport in the soil system was calculated with extremely high vertical resolution at a regional scale based on rainfall-evaporation data for the year 2000. As a result, highly resolved regional groundwater recharge pattern addressing the heterogeneity of soil systems could be determined.     

The vertical distribution of NO3 in the atmospheric boundary layer

Recent urban measurements suggest that NO₃ concentrations vary significantly with altitude in the lowest few hundred metres of the atmosphere. Calculations using a one-dimensional boundary layer model show that NO₃ concentrations are small near the ground and increase with altitude to a maximum near the top of the nocturnal boundary layer (NBL). These results show that the NBL is not well mixed, and that where there are surface sources and sinks two-box models of the NBL are inadequate, and surface measurements are not representative and may lead to an underestimate of the oxidising capacity of the atmosphere.     

Analysis of experiments on ion-induced nucleation and aerosol formation in the presence of UV light and ionizing radiation

Under typical atmospheric conditions, sulfuric acid and water vapors are likely most important species in the nucleation of new aerosol particles. The main source of H₂SO₄ in the atmosphere is oxidation of SO₂. Hence, an understanding of the subsequent chemical reactions followed by aerosol-particles formation is of fundamental importance. Here we analyze the results of laboratory experiments in Svensmark et al. (2007) in which (i) the formation of neutral aerosol particles was observed at reported sulfuric acid concentrations well below the range where the binary homogeneous nucleation in a mixture of H₂SO₄–H₂O vapors could be important and (ii) an electron catalytic effect on particle nucleation was suggested as an explanation of the experimental results and as a potential source of aerosol-particles formation in the Earth's atmosphere. In the article we give an interpretation of these experimental data based on a known mechanism of the neutral particles formation via ion-induced nucleation followed by recombination of charged clusters. The main results of our investigation are the following: (i) the observed neutral particles were likely formed via the recombination of ion clusters; (ii) the phenomena of electron photodetachment from ion clusters under UV radiation was improbable in conditions of this experiment and likely unrealized for typical negative ion clusters found in the Earth's atmosphere. In total, these experiments and model investigations show that far more and specially directed laboratory experiments are needed to clarify the ways by which cosmic rays and solar radiation may link to the Earth's climate.     

Processes driving the short-term variability of polycyclic aromatic hydrocarbons in the Baltimore and northern Chesapeake Bay atmosphere,USA

Polycyclic aromatic hydrocarbons (PAHs) were measured in the Baltimore and adjacent Chesapeake Bay in July 1997. Time series of 4- and 12-h samples were taken at two sites 15 km apart in order to evaluate the influence of a number of processes on the short-term variability of PAH in the Baltimore and northern Chesapeake Bay atmospheres. PAH concentrations were 2–3-fold higher in the Baltimore atmosphere than in the adjacent Chesapeake Bay atmosphere. For example, gas-phase phenanthrene and pyrene concentrations were 12.5 and 2.14 ng m⁻³ in the Baltimore site and 5.57 and 0.548 ng m⁻³ in the Chesapeake Bay, respectively. The influence of wind direction, wind speed and temperature was evaluated by multiple linear regressions which indicated that atmospheric gas-phase PAH concentrations over the Chesapeake Bay were significantly higher when the air mass was from the urban/industrial Baltimore area. Furthermore, the increase of gas-phase low-MW PAH concentrations with temperature and wind speed suggests that volatilization from the bay is an important source of pollutants to the atmosphere, at least when air masses are not influenced by the Baltimore urban and industrial area. Indeed, while on the long-term, the Chesapeake Bay is a receptor of atmospherically deposited PAHs, on the short-term and during appropriate meteorological conditions, the bay acts as a source of pollutants to the atmosphere. Aerosol-phase PAH concentrations and temporal trends showed a strong dependence on aerosol soot content due to the high affinity of PAHs to the graphitic structure of soot. These results confirm the important influence of urban areas as a source of pollution to adjacent aquatic environments and as a driving factor of the short-term variability, either directly by transport of urban-generated pollutants or by volatilization of previously deposited pollutants. Conversely, the complex diurnal trends of gas-phase PAHs at the Baltimore site suggests that degradation processes dominate the diurnal trends of PAHs in urban atmospheres. This conclusion is supported by estimated rate constants for PAH reaction with OH radicals which show good agreement with reported values within a factor of two.     

Characterization of emissions during the heating of tyre contaminated scrap

In order to characterize the compounds (type and quantities) emitted during melting of organic contaminated scrap and to investigate the mechanism of their formation, an experimental set-up has been designed and built to study precisely the influence of temperature and gas atmosphere in the conditions of an electric arc furnace. These experiments lead to the determination of mass balances (C, H, O, S) and to the quantification of unburnt compounds (tars, carbon monoxide, volatile organic compounds (VOCs), benzene, toluene, ethylbenzene and xylenes (BTEX), polyaromatic compounds (PAHs)). Degradation conditions (gas atmosphere and temperature) corresponding to different areas in the electric furnace have also been investigated. Such experiments lead to a better understanding of degradation mechanisms; this interpretation is not possible from investigations performed in an industrial furnace since there are many uncontrolled parameters (large dispersion of the results).     

Ambient Carbon Monoxide And Its Fate in the Atmosphere

Carbon monoxide, the most abundant air pollutant found in the atmosphere generally exceeds that of all other pollutants combined (excluding C0₂). An estimated tonnage of >87 X 10⁶ of CO was emitted in the United States from major technological sources alone during 1966. More than 90% of the total CO emitted from fossil fuels is derived from gasoline powered motor vehicles. Other sources of CO include emissions from coal and fuel oil burning, aircraft and open burning. Some CO is also formed by certain vegetation and marine invertebrates (siphonophores). Chemical reactions of CO in the upper and lower atmosphere are discussed. Chemical oxidation of CO in the lower atmosphere by molecular oxygen is very slow. The exact duration of CO in the lower atmosphere is not known with certainty; however, the mean residence time has been variously estimated to be between 0.3 and 5.0 years. In the absence of scavenging processes the estimated world-wide CO emission would be sufficient to raise the’atmospheric level by 0.03 ppm per year, yet the background levels of CO in clean air do not appear to be increasing. Several potential sinks are discussed. Knowledge of the mechanism of process of removal of CO from the lower atmosphere is unsatisfactory; the process, at the present time, cannot be identified with certainty.     

An observation-based model for analyzing ozone precursor relationships in the urban atmosphere

An Observation-Based Model (OBM) is described, which uses in-situ atmospheric observations to determine the sensitivity of ozone concentrations in an urban atmosphere to changes in the emissions of ozone precursors (i.e., volatile organic compounds and nitrogen oxides). The model is formulated following the concept of Relative Incremental Reactivity (RIR) developed by Carter and Atkinson. In the OBM, however, observed concentrations rather than emission inventories are used to drive the photochemical simulations and thereby ensure that the calculations are carried out for the proper mix of nitrogen oxides and volatile organic compounds. From these calculations, a series of sensitivity factors, or RIRs, are inferred that can be used to (1) determine whether reducing emissions of nitrogen oxide or emissions of hydrocarbons would be most effective in abating ozone in a given urban area, and (2) identify the most critical subset of hydrocarbons present in an urban atmosphere causing ozone exceedances. Because the OBM is relatively easy and inexpensive to operate and makes use of data that are increasingly available, it can be used to analyze a wide array of ozone episodes and, thus, could prove to be a relatively cost-effective tool for the analysis of ozone precursor relationships in an urban atmosphere. On the other hand, because the OBM is diagnostic rather than prognostic, it cannot be used in a predictive mode to estimate exactly how much emission reduction is needed to reduce ozone concentrations. For this reason, the OBM should be viewed as a complement to, rather than a substitute for, more sophisticated gridded, emission-based models. To illustrate the characteristics of the OBM and to demonstrate its applicability, we first compare the results of the OBM to those obtained from a series of simulations of the Atlanta metropolitan area using the Urban Airshed Model (UAM), a three-dimensional Eulerian grid model. The OBM is then used to analyze a dataset obtained from the 1990 Atlanta Ozone Study, an EPA field sampling program conducted during the summer of 1990. Because of limitations and potential flaws in the 1990 Atlanta dataset, the results of this OBM analysis are largely illustrative rather than definitive. Nevertheless, a few important issues are elucidated by the analysis. These include (1) the importance of accounting for biogenic hydrocarbons produced from urban vegetation; (2) the potential flaw in using early-morning VOC-to-NOx ratios to infer whether ozone production is limited by VOC or NOx; (3) the critical need for high-sensitivity nitrogen oxide measurements to quantify the sub-ppbv concentrations of NO during the afternoon hours; and (4) the need to consider a number of individual ozone episodes in studying an urban atmosphere because of the possibility that the degree of VOC- and NOx-limitation may vary from one episode to another.     

Isomeric analysis of BTEXs in the atmosphere using beta-cyclodextrin capillary chromatography coupled with thermal desorption and mass spectrometry

An analytical method capable of determining trace levels of BTEX-aromatics (benzene, toluene, ethylbenzene, m-, p- and o-xylenes) in the atmosphere with as high resolution as possible has been developed. The method is based on the preconcentration of air samples using a multibed tube (Carbopack C, Carbograph 1) at ambient temperature, followed by thermal desorption, and analysis of aromatic species by a beta-cyclodextrin capillary chromatography coupled with mass spectrometry. The resolution achieved was sufficient for individual separation of BTEXs as well as m- and p-xylenes. The BTEX-ratios have been determined in an air tunnel and in on-road, suburban and rural forest atmosphere. The ethylbenzene/m-xylene ratios could provide a deep insight into anthropogenic related NMHC patterns at different locations and under different meteorological conditions and may reflect photochemical processes in the best way.     

Evaluation of the thermal/optical reflectance method for discrimination between char- and soot-EC

Many optical, thermal and chemical methods exist for the measurement of elemental carbon (EC) but are unable or neglect to differentiate between the different forms of EC such as char- or soot-EC. The thermal/optical reflectance (TOR) method applies different temperatures for measuring EC and organic carbon (OC) contents through programmed, progressive heating in a controlled atmosphere, making available eight separate carbon fractions - four OC, one pyrolyzed organic carbon, and three EC. These fractions were defined by temperature protocol, oxidation atmosphere, and laser-light reflectance/transmittance. Stepwise thermal evolutional oxidation of the TOR method makes it possible to distinguish char- from soot-EC. In this study, different EC reference materials, including char and soot, were used for testing it. The thermograms of EC reference materials showed that activation energy is lower for char- than soot-EC. Low-temperature EC1 (550 degrees C in a 98% He/2% O2 atmosphere) is more abundant for char samples. Diesel and n-hexane soot samples exhibit similar EC2 (700 degrees C in a 98% He/2% O2 atmosphere) peaks, while carbon black samples peaks at both EC2 and EC3 (800 degrees C in a 98% He/2% O2 atmosphere). These results supported the use of the TOR method to discriminate between char- and soot-EC.     

A methodology to estimate odors around landfill sites: the use of methane as an odor index and its utility in landfill siting

The estimation of odor production and dispersion from landfill sites is a very complicated task because of the different chemical species that exist in biogas. To site a new landfill, it is necessary to know the distance that odors can travel around the landfill under atmospheric conditions that increase the concentration of pollutants. Although CH4 is an odorless gas, it can be used as an index to determine the dispersion of low-reactivity odorous species around a landfill site. Methane production rates, estimated by biogas production models, were used by an air dispersion model to determine the spatial distribution of CH4 around landfill sites. By utilizing dispersion models under extreme atmospheric conditions, a maximum CH4 concentration around the landfills was determined. Based on the ratio between CH4 and odorous chemical species, the spatial distribution of the concentration of an odorous species was determined for those species with low reactivity in the atmosphere. For odorous species with high reactivity in the atmosphere, a dispersion-reaction model must be used. In this way, the acceptable distance between new landfills and residential areas can be determined. The proposed methodology could be used as a design tool for those who are interested in landfill siting.     

Fate and potential environmental effects of methylenediphenyl diisocyanate and toluene diisocyanate released into the atmosphere

Information from a variety of sources has been collected and summarized to facilitate an overview of the atmospheric fate and potential environmental effects of emissions of methylenediphenyl diisocyanate (MDI) or toluene diisocyanate (TDI) to the atmosphere. Atmospheric emissions of both MDI and TDI are low, both in terms of concentration and mass, because of their low volatility and the need for careful control over all aspects of their lifecycle from manufacture through disposal. Typical emission losses for TDI are 25 g/t of TDI used in slabstock foam production. MDI emission losses are lower, often less than 1 g/t of MDI used. Dispersion modeling predicts that concentrations at the fenceline or beyond are very low for typical releases. Laboratory studies show that TDI (and by analogy MDI) does not react with water in the gas phase at a significant rate. The primary degradation reaction of these aromatic diisocyanates in the atmosphere is expected to be oxidation by OH radicals with an estimated half-life of one day. Laboratory studies also show that this reaction is not expected to result in increased ground-level ozone accumulation.