Indicators for persistence and long-range transport potential as derived from multicompartment chemistry-transport modelling

Total environmental and compartmental residence times as a measure for persistence as well as indicators for long-range transport potential (LRTP) have been derived from global geo-referenced modelling and LRTP is characterized in two geographic directions for the first time. A dynamic multicompartment chemistry-transport model (MCTM) was used to study the fate of the insecticides DDT and alpha-hexachlorocyclohexane (alpha-HCH) during the first 2 years upon entry. The indicators for LRTP were defined such as to address the tendencies of substance distributions to migrate ('plume displacement', PD) and to spread into remote areas ('spatial spreading', SS). The indicators deliver values as function of time upon entry. With the aim to address the effect of location of entry on environmental fate, scenarios of emission from the territories of seven countries were studied. It was found that the effect of location of entry on the spatial scale of countries (400-4000 km) is significant for the compartmental distribution and the inter-compartmental mass exchange fluxes (e.g., number of atmospheric cycles, 'hops'). Location of entry introduces uncertainties in the order of a factor of 5 for the total environmental residence time, tau(overall), and a factor of 5-20 for PD and SS. For the 2nd year upon entry into the environment, tau(overall)=317-1527 days are predicted for DDT and 101-463 days for alpha-HCH. The influence of location of entry does affect the substance ranking, i.e. we cannot simply state that DDT is more persistent than alpha-HCH, but for one scenario studied, application in China, the opposite is predicted. Precipitation patterns proved to be significant, besides other climate parameters, for atmospheric residence time. Integration of the location of entry in chemicals risk assessments is therefore recommended. In general, persistence and some indicators for LRTP, pertinent to their definition, refer to the fate of a large fraction, e.g., 63% (=1-1/e) or 90%, but not the total substance burden. The choice of this fraction may have the consequence of a normative step which defines the spatial and temporal extensions of a related chemicals risk assessment and may affect substance ranking.     

Assessment of the environmental persistence and long-range transport of endosulfan

Concentrations of the insecticide endosulfan (α- and β-isomer) and its degradation product endosulfan sulfate in air, seawater and soil are calculated with the global environmental fate model CliMoChem. As model input, physicochemical properties of all three compounds were assembled and a latitudinally and temporally resolved emission inventory was generated. For concentrations in air, model and measurements are in good agreement; a bimodal seasonality with two peaks in spring and fall as it is observed in Arctic air is reproduced by the model. For seawater, the agreement of model and measurements depends on the values of the hydrolysis activation energy of endosulfan used in the model; with relatively high values around 100 kJ/mol, model results match field data well. The results of this assessment of the levels, persistence, and global distribution of endosulfan are also relevant for the evaluation of endosulfan as a Persistent Organic Pollutant under the Stockholm Convention.     

Assessment of long-range transport potential of polychlorinated Naphthalenes based on three-dimensional QSAR models

Environmental Science and Pollution Research - Experimentally determined octanol–air partition coefficients (K OA) for 43 polychlorinated naphthalene (PCN) congeners and experimentally...     

Development of continental scale multimedia contaminant fate models: integrating GIS

The incentives and approaches for modelling chemical fate at a continental scale are discussed and reviewed. It is suggested that a multi-media model consisting of some 20-30 regions, each of which contains typically seven environmental compartments represents a reasonable compromise between the issues of the need for detailed resolution, avoidance of excessive data demands and inherent complexity and transparency. Strategies adopted in compiling the Berkley-Trent (BETR) model for North America are discussed and used to illustrate the issues of selecting appropriate number and nature of segments, treatment of air and water flows and the acquisition of environmental data. It is suggested that GIS software can play a valuable role in gathering and processing such data and in the display and interpretation of the results of the model assessment. The BETR model will be a useful tool for describing the nature of persistence and long-range transport of chemicals of concern in the North American environment.     

A multimedia environmental model of chemical distribution: fate, transport, and uncertainty analysis

This paper presented a framework for analysis of chemical concentration in the environment and evaluation of variance propagation within the model. This framework was illustrated through a case study of selected organic compounds of benzo[alpha]pyrene (BAP) and hexachlorobenzene (HCB) in the Great Lakes region. A multimedia environmental fate model was applied to perform stochastic simulations of chemical concentrations in various media. Both uncertainty in chemical properties and variability in hydrometeorological parameters were included in the Monte Carlo simulation, resulting in a distribution of concentrations in each medium. Parameters of compartmental dimensions, densities, emissions, and background concentrations were assumed to be constant in this study. The predicted concentrations in air, surface water and sediment were compared to reported data for validation purpose. Based on rank correlations, a sensitivity analysis was conducted to determine the influence of individual input parameters on the output variance for concentration in each environmental medium and for the basin-wide total mass inventory. Results of model validation indicated that the model predictions were in reasonable agreement with spatial distribution patterns, among the five lake basins, of reported data in the literature. For the chemical and environmental parameters given in this study, parameters associated to air-ground partitioning (such as moisture in surface soil, vapor pressure, and deposition velocity) and chemical distribution in soil solid (such as organic carbon partition coefficient and organic carbon content in root-zone soil) were targeted to reduce the uncertainty in basin-wide mass inventory. This results of sensitivity analysis in this study also indicated that the model sensitivity to an input parameter might be affected by the magnitudes of input parameters defined by the parameter settings in the simulation scenario. Therefore, uncertainty and sensitivity analyses for environmental fate models was suggested to be conducted after the model output was validated based on an appropriate input parameter settings.     

Identification of substances with potential for long-range transport as possible substances of very high concern

According to the European legislation, REACH, organic compounds are considered as substances of very high concern (SVHC) if they are persistent, bioaccumulative and toxic (PBT). A substance's long-range transport potential (LRTP) may also pose a risk to remote regions. This is, however, not yet explicitly included. For identification of compounds, which are not PBT according to REACH criteria, but show LRTP, we investigated 22,438 compounds from the Canadian Domestic Substance List (CDSL). The CDSL was searched for organic, neutral compounds. Substance properties were estimated with EPI Suite v4.00. Next, the substance list was edited in two ways: (1) The half-life criterion in air for LRTP as defined in the Stockholm Convention was applied. (2) For all compounds, indicators for persistence and LRTP were calculated with the multimedia model ELPOS v2.2. Applying the half-life criterion, we identified 594 substances, which are prone to LRT but are not PBT (non-PBT-L substances). In contrast, investigations with ELPOS lead to a shorter list of 188 substances, which are non-PBT-L substances. Finally, the list was compared with potential Arctic contaminants identified in previous literature. Our results show that there is a large number of organic chemicals which would not be considered as SVHC since they are not, at the same time, persistent, bioaccumulative and toxic according to REACH criteria. Nevertheless, they show LRTP according to different screening approaches and thus a potential hazard to remote regions.     

Expanding box models for the long-range transport of chemically reacting airborne material

If a power station plume significantly perturbs the levels of chemically active species in the atmosphere, then the rates of chemical reactions become non-uniform across the plume. This results in different effective plume widths for the different chemical species, which in turn influence the reaction rates. Here coupled equations are derived which for a reaction involving a single oxidant accurately model the total amount of a species in a plume and the associated plume width. The conventional box model slightly underestimates the amount of oxidized material produced. It is therefore suggested that the sensitivity of plume models to assumptions regarding lateral mixing should be tested, using the system of coupled equations derived in the paper.     

Chemical fate,latitudinal distribution and long-range transport of cyclic volatile methylsiloxanes in the global environment: A modeling assessment

Cyclic volatile methylsiloxanes (cVMS) such as octamethycyclotetrasiloxane (D4), decamethycyclopentasiloxane (D5), and dodecamethylcyclohexasiloxane (D6) are widely used as intermediates in the synthesis of high-molecular weight silicone polymers or as ingredients in the formulation of personal care products. The global environmental fate, latitudinal distribution, and long range transport of those cVMS were analyzed by two multimedia chemical fate models using the best available physicochemical properties as inputs and known persistent organic pollutants (POPs) and highly persistent volatile organic chemicals (“fliers”) as reference. The global transport and accumulation characteristics of cVMS differ from those of typical POPs in three significant ways. First, a large fraction of the released cVMS tends to become airborne and is removed from the global environment by degradation in air, whereas known POPs have a tendency to be distributed and persistent in all media. Secondly, although cVMS can travel a substantial distance in the atmosphere, they have little potential for deposition to surface media in remote regions. This contrasts with a deposition potential of known POPs that exceeds that of cVMS by 4–5 orders of magnitude. Thirdly, cVMS have short global residence times with the majority of the global mass removed within 3 months of the end of release. Global residence times of POPs on the other hand are in years. The persistent fliers resemble the cVMS with respect to the first two attributes, but their global residence times are more like those of the POPs.     

Three long-range transport models compared to the ETEX experiment: a performance study

For operational or research purposes (dispersion computations of radioactive effluents during nuclear emergency situations, simulations of chemical pollution in the vicinity of thermal power plants), different models of passive dispersion in the atmosphere have been developed at the Environment Department of EDF’s R and D Division. This report presents the comparison of the performances of three such models: DIFTRA (lagrangian puff model, with operational goal), DIFEUL (three dimensional eulerian) and DIFPAR (Monte Carlo particle model) for the simulation of the first ETEX release, an international tracer campaign during which a passive tracer cloud has been followed over Europe. The results obtained in this study give model vs. experience differences of the same order as the model vs. experience differences observed during an international model comparison experiment using data of the Chernobyl release, the ATMES exercise. In addition to the standard statistical scores used in the evaluation of the performances of the transport models two asymmetric scores (in contradistinction with the Figure of Merit in Space) are proposed: “efficiency” and “power”. Their aim is to separate the two manners in which a model may be wrong: by predicting presence of pollutant while none is measured or conversely predicting absence when pollutant is actually detected.     

Estimating half-lives of pesticides in/on vegetation for use in multimedia fate and exposure models

Degradation half-lives in/on vegetation are needed in environmental risk assessment of pesticides, but these data are often not available for most active ingredients. To address this, we first correlated experimental soil degradation half-life data of 41 pesticides obtained from the reviewed literature with the corresponding experimental half-lives on plant surface. Degradation half-lives in soil were found to be four times slower compared with half-lives on plant surfaces. In a second step, we explored measured plant surface half-lives directly with those in vegetation. The results were validated by comparing computed values with results obtained from an experimental set-up. The uptake and dissipation of alpha-cypermethrin (insecticide) and bromopropylate (acaricide) was studied by detecting pesticide residues in whole and peeled tomato fruits using gas chromatography. Half-lives within vegetation were found to be four times faster compared with plant surface half-lives. Using this experimental based approach, it is concluded that the estimation of degradation half-lives of pesticides in/on vegetation to be used as input data in environmental mass balance models can be directly correlated from the more abundant ready experimental degradation half-life data for soil.     

Evaluation and comparison of multimedia mass balance models of chemical fate: application of EUSES and ChemCAN to 68 chemicals in Japan

The European Union System for Evaluation of Substances (EUSES) and the ChemCAN chemical fate model are applied to describe the fate of 68 chemicals on two spatial scales in Japan. Emission information on the chemicals has been obtained from Japan's Pollutant Release and Transfer Registry and available monitoring data gathered from government reports. Environmental concentrations calculated by the two models for the four primary environmental media of air, water, soil and sediment agree within a factor of 3 for over 70% of the data, and within a factor of 10 for over 87% of the data. Reasons for certain large discrepancies are discussed. Concentrations calculated by the models are generally consistent with the lower range of concentrations that are observed in the environment. Agreement between modeled and observed concentrations is considerably improved by including an estimate of the advective input of chemicals in air from outside Japan. The agreement between the EUSES and ChemCAN models suggests that results of individual chemical assessments are not likely to be significantly affected by the choice of chemical fate model. Primary sources of discrepancy between modeled and observed concentrations are believed to be uncertainties in emission rates, degradation half-lives, and the lack of data on advective inflow of contaminants in air.     

Atmospheric fate and transport of dioxins: local impacts

We conducted model simulations of the atmospheric fate and transport of PCDD/F to assess the fraction of emitted PCDD/F that would deposit within 100 km from the source. We considered eight major categories of PCDD/F emission sources and six different locations, to cover a wide range of source characteristics, PCDD/F congener profiles and particle size distributions, meteorological conditions and terrain configurations. These results suggest that for sources that have tall stacks and/or high plume rise (e.g., copper smelters, cement kilns, sinter plants), only a small fraction of PCDD/F emissions is deposited locally (typically, less than 10% within 100 km). Other source categories such as municipal solid waste incinerators, medical waste incinerators and diesel trucks lead to a greater fraction of PCDD/F being deposited locally; nevertheless, the majority of their PCDD/F emissions tends to be transported beyond 100 km. Although local impacts may need to be addressed for these latter source categories, it appears that the long-range potential impacts of PCDD/F need to be addressed for all source categories. Sensitivity studies were conducted to investigate the effect of various key model inputs on simulation results. These studies suggest that an advanced atmospheric dispersion model should be used for cases where PCDD/F emissions may present some local concerns because the results are very sensitive to the treatment of dispersion. Also, it is essential to obtain accurate characterizations of the particle size distribution of particulate PCDD/F because the dry deposition flux is very sensitive to the particle size distribution.     

The effect of export to the deep sea on the long-range transport potential of persistent organic pollutants

BACKGROUND: Export to the deep sea has been found to be a relevant pathway for highly hydrophobic chemicals. The objective of this study is to investigate the influence of this process on the potential for long-range transport (LRT) of such chemicals. METHODS: The spatial range as a measure of potential for LRT is calculated for seven PCB congeners with the multimedia fate and transport model ChemRange. Spatial ranges for cases with and without deep sea export are compared. RESULTS AND DISCUSSION: Export to the deep sea leads to increased transfer from the air to the surface ocean and, thereby, to lower spatial ranges for PCB congeners whose net deposition rate constant is similar to or greater than the atmospheric degradation rate constant. This is fulfilled for the PCB congeners 101, 153, 180, and 194. The spatial ranges of the congeners 8, 28, and 52, in contrast, are not affected by deep sea export. With export to the deep sea included in the model, the spatial ranges of the heavier congener are similar to those of the lighter ones, while the intermediate congeners 101 and 153 have the highest potential for long-range transport. CONCLUSIONS: Transfer to the deep ocean affects the mass balance and the potential for LRT of highly hydrophobic chemicals and should be included in multimedia fate models containing a compartment for ocean water.     

Local and distant residence times of contaminants in multi-compartment models. Part II: application to assessing environmental mobility and long-range atmospheric transport

In Part I, the concepts of inherent, local and distant residence times (DRTs) were reviewed as metrics of the extent to which chemical discharges or emissions in one region or box are transported to distant regions. In this second part, the concepts are applied to geographically relevant systems to illustrate their applicability to the assessment of chemicals for long-range transport potential (LRTP). It is shown that the relative ranking of chemicals as characterized by the DRT method is similar to that of the characteristic travel distance concept. A DRT source-receptor matrix is developed that can express the chemical-specific potential of source regions to contaminate a specific receptor region of concern such as the Arctic. The matrix can be modified to identify for a specific source region the likely destinations of emissions as well as to assess the relative vulnerability of regions in the global environment to contaminants of concern.     

The effect of export to the deep sea on the long-range transport potential of persistent organic pollutants

Background  

Export to the deep sea has been found to be a relevant pathway for highly hydrophobic chemicals. The objective of this study is to investigate the influence of this process on the potential for long-range transport (LRT) of such chemicals.     

The effect of variable sub-grid deposition factors on the results of the lagrangian long-range transport model of EMEP

The EMEP/MSC-W routine model for long-range atmospheric transport of sulphur and nitrogen includes a correction for the unresolved sub-grid-scale deposition in emission grid-squares. A constant fraction of the emissions is assumed to be directly deposited inside the first grid-square. Experiments have been performed to estimate the effects of using factors which vary with emission height and meteorological conditions. Results indicate that the constant local deposition factor used for sulphur dioxide in the routine model, is an overestimate, in particular for high-level emission sources. The change in annual deposition caused by the new local deposition factor for sulphur dioxide, is most clearly seen as a decreased deposition in the grid-squares with the largest emissions such as due for the former German Democratic Republic. The amount of this decrease strongly depends on the source height distribution. With the presently available emission data, a decrease of maximum 36% can be expected in individual grid squares. In grid-squares dominated by transboundary deposition, the increase is typically lower than 10%.     

The fate and persistence of trifluoroacetic and chloroacetic acids in pond waters

The environmental fate of trichloro-, dichloro-, and monochloroacetic acids, and trifluoroacetic acid was investigated using field aquatic microcosms and laboratory sediment-water systems. Trifluoroacetic acid was extremely persistent and showed no degradation during a one-year field study, though it appeared to undergo transient partitioning within an unknown pond phase as the temperature of the surroundings was reduced. Of the three chloroacetic acids, trichloro had the longest residence time (induction and decay) (approximately 40 d), dichloro the shortest (approximately 4 d), and monochloro an intermediate residence time (approximately 14 d). Laboratory studies suggest that the biodegradation of trichloro-, dichloro-, and monochloroacetic acids leads primarily to the formation of chloride and oxalic, glyoxalic, and glycolic acids, respectively.     

Satellite detection of long-range pollution transport and sulfate aerosol hazes

While not designed for such a task, meteorological satellites now play a growing role in our understanding of long range transport of secondary pollutants. This paper reports on a demonstration project showing that currently available synchronous satellite data can detect the aerial extent and motion of large-scale “hazy” air masses associated with sulfate and ozone episodes. An interactive computer graphics system is utilized showing that digital satellite data can obtain precise measurements of upward scattered solar radiation which is correlated to aerosol optical thickness and therefore to sulfate concentrations. Measurements over Lake Michigan for instance, reveal over-water image brightness enhanced fully 60–70% as visibility estimates of fb_scatincreasedfrom 1.8 × 10⁴m⁻¹to 5.7 × 10⁻⁴m⁻¹.Digital satellite data is shown to have great promise in mapping sulfate haze areas, especially over water.