Development of a black carbon-inclusive multi-media model: application for PAHs in Stockholm.

A multi-media model was developed for predicting the fate of organic chemicals in the Greater Stockholm Area, Sweden, and applied to selected polycyclic aromatic hydrocarbons (PAHs). Although urban models have been previously developed, this model is novel in that it includes sorption to pyrogenically-derived particles, commonly termed "black carbon" (BC), within the model structure. To examine the influence of BC sorption on environmental fate of PAHs, two versions of the model were generated and run: one in which sorption to BC was included and one in which BC sorption was excluded. The inclusion of BC sorption did not cause any significant variations to air levels, but it did cause an average 20-30% increase in sediment concentrations related to increased sediment solids partitioning. The model also predicted reduced advective losses out of the model domain, as well as chemical potential to diffuse from sediments, whilst total chemical inventory increased. In all cases, the lighter PAHs were more affected by BC inclusion than their heavier counterparts. We advocate the addition of sorption to BC in future multi-media fate and exposure models, which as well as influencing fate will also alter (lower) chemical availability and, thus, wildlife exposure to hydrophobic chemicals. A quantification of the latter was derived with the help of the soot-inclusive model version, which estimated a lowering of dissolved water concentrations between five and >200 times for the different PAHs of this study.

Comparison and analysis of different approaches for estimating the human exposure to phthalate esters.

The human exposure estimates for dibutyl (DBP) and bis(2-ethylhexyl) phthalate (DEHP) made by two models EUSES and ACC-Human, and by an estimation approach which utilized measured concentrations in exposure media, were compared. The approach which utilized the latest monitoring data for important exposure media, yielded median daily intakes for adult humans for DBP and DEHP of 2.7 and 5.6 microg/kg body weight per day, respectively, which were in the same range as previous estimates based on back-calculation from urinary metabolites. EUSES estimated average daily intakes of DBP and DEHP for humans that were between 8 and 13 times lower. ACC-Human does not estimate average daily intakes, but ACC-Human-estimated human milk concentrations/fugacities were more than a thousand times lower than measured concentrations/fugacities in human milk. It was concluded that the two models underestimate human exposure to phthalate esters because they consider only a few key pathways that are known to be important for other, more persistent, hydrophobic organic compounds. Further, it was shown that there are differences between the two models on the methodology for estimating concentrations in exposure media such as vegetation, milk, beef and fish. ACC-Human uses a mechanistic approach for estimating transfer through aquatic and terrestrial food chains that are known to be important for human exposure to persistent, hydrophobic organics and can, unlike EUSES, account for food chain metabolism. It proved difficult, however, to obtain organism metabolism rates needed as model inputs to ACC-Human. If exposure estimates of phthalate esters are needed, it is recommended to use an estimation approach based on high quality monitoring data as presented here and/or back-calculate daily intake from concentrations of metabolites in human urine samples from the general population.

Continental scale passive air sampling of persistent organic pollutants using rapidly equilibrating thin films (POGs).

A novel design of rapidly equilibrating passive air sampler was deployed at 38 sites across 19 European countries to investigate short-term spatial variability of persistent organic pollutants (POPs). Devices were sealed in airtight containers to eliminate the possibility of contamination during transit and couriered to recipients with deployment instructions. Exposure times of 7days permitted the use of back trajectory analysis to further understand the factors responsible for influencing the large-scale spatial distribution of PCBs, PBDEs, PCNs, PAHs, lindane and HCB. Following sampler harvest, devices were sealed and returned for analysis. Comparison of sequestered levels showed that PAHs exhibited the greatest spatial variability (by a factor of 30) with higher levels often associated with greater population density. In contrast, HCB values were much more uniform, reflecting its well mixed distribution in the atmosphere. Spatial variation was strongly influenced by air mass origin, with lower levels being observed at most sites impacted by maritime air masses.

The role of soil organic carbon in the global cycling of persistent organic pollutants (POPs): interpreting and modelling field data.

Soil is an important global reservoir for persistent organic pollutants (POPs). The interaction between air (which often receives the majority of emissions) and soil plays a key role in the long term environmental cycling and fate of these chemicals. Soil surveys have been carried out to try and estimate regional and global distribution/inventory of POPs. A correlation between soil POPs concentration and soil organic carbon (SOC) has been observed in background soils [Meijer et al., 2003. Global distribution and budget of PCBs and HCB in background surface soils: implications for sources and environmental processes. Environ. Sci. Technol., 37, 667], provoking discussion about whether POPs will approach steady-state (or equilibrium) between air and SOC, on a global scale. This manuscript investigates this relationship and in particular how soil concentrations can be influenced by factors such as temperature, SOC content and physicochemical properties. A simple two box model designed to investigate parameters that are likely to affect air-soil exchange revealed that more volatile chemicals such as HCB are likely to achieve steady-state conditions between air and soil relatively quickly whilst relatively involatile chemicals, such as heavy PCBs, may take considerably longer and other compounds (e.g. OCDD) may never achieve it. These model calculations provide an insight into which fate processes (e.g. volatilisation or degradation) may control a chemicals fate in the terrestrial environment. A different modelling exercise was used to explore the complex interaction of environmental parameters, representative of 'real world' conditions to study their potential influence on POPs cycling at the European scale. Results from the model suggested that compound degradation rates in soil (linked to SOC content), temperature, vegetation cover and ecosystem C turnover are all likely to significantly influence POP air-soil exchange and fate.

Understanding levels and trends of BDE-47 in the UK and North America: an assessment of principal reservoirs and source inputs.

Despite increasing interest in the occurrence of Polybrominated diphenyl ethers (PBDEs) in the environment, there has been only limited effort expended to identify principal source input reservoirs and pathways into the environment. Taking a single congener BDE-47, an important component of the penta commercial product, we have estimated principal contemporary reservoirs and emissions in two regions with very different historical use patterns. In North America, production and use of the penta-product has been rising steadily for the last 20 years, whilst in the UK, use has been restricted over the last 10 years, and most recently, there has been an EU ban on use. Mass balance models have been applied using our contemporary emission estimates to predict environmental levels. Results highlight that contemporary emissions to air can account for contemporary air concentrations but cannot account for measured concentrations in principal reservoirs such as soils and sediments. Future efforts are needed to improve our knowledge of the stocks of PBDEs in use, and emission factors from those stocks, to better understand/predict past and likely future trends.

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.

Seasonal and long-term trends in atmospheric PAH concentrations: evidence and implications.

Atmospheric monitoring data for selected polynuclear aromatic hydrocarbons (PAHs) were compiled from remote, rural and urban locations in the UK, Sweden, Finland and Arctic Canada. The objective was to examine the seasonal and temporal trends, to shed light on the factors which exert a dominant influence over ambient PAH levels. Urban centres in the UK have concentrations 1-2 orders of magnitude higher than in rural Europe and up to 3 orders of magnitude higher than Arctic Canada. Interpretation of the data suggests that proximity to primary sources 'drives' PAH air concentrations. Seasonality, with winter (W) > summer (S), was apparent for most compounds at most sites; high molecular weight compounds (e.g. benzo[a]pyrene) showed this most clearly and consistently. Some low molecular weight compounds (e.g. phenanthrene) sometimes displayed S>W seasonality at some rural locations. Strong W>S seasonality is linked to seasonally-dependent sources which are greater in winter. This implicates inefficient combustion processes, notably the diffusive domestic burning of wood and coal. However, sometimes seasonality can also be strongly influenced by broad changes in meteorology and air mass origin (e.g. in the Canadian Arctic). The datasets examined here suggest a downward trend for many PAHs at some sites, but this is not apparent for all sites and compounds. The inherent noise in ambient air monitoring data makes it difficult to derive unambiguous evidence of underlying declines, to confirm the effectiveness of international source reduction measures.

Modelling the atmospheric fate and seasonality of polycyclic aromatic hydrocarbons in the UK.

This paper presents the results from an exercise in atmospheric contaminant fate modelling, which had three main objectives: (1) to investigate the balance between estimated national atmospheric emissions of six selected PAHs and observed ambient measurements for the UK, as a means of testing the current emission estimates; (2) to investigate the potential influence of seasonally dependent environmental fate processes on the observed seasonality of air concentrations; and (3) after undertaking the first two objectives, to make inferences about the likely magnitude of seasonal differences in sources. When addressing objective 1 with annually averaged emissions data, it appeared that the UK PAH atmospheric emissions inventory was reasonably reliable for fluorene, fluoranthene, pyrene, benzo[a]pyrene and benzo[ghi]perylene--but not so for phenanthrene. However, more detailed analysis of the seasonality in environmental processes which may influence ambient levels, showed that the directions and/or magnitudes of the predicted seasonality did not coincide with field observations. This indicates either that our understanding of the environmental fate and behaviour of PAHs is still limited, and/or that there are uncertainties in the emissions inventories. It is suggested that better quantification of PAH sources is needed. For 3- and 4-ringed compounds, this should focus on those sources which increase with temperature, such as volatilisation from soil, water, vegetation and urban surfaces, and possible microbially-mediated formation mechanisms. The study also suggests that the contributions of inefficient, diffusive combustion processes (e.g. domestic coal/wood burning) may be underestimated as a source of the toxicologically significant higher molecular weight species in the winter. It is concluded that many signatory countries to the UNECE POPs protocol (which requires them to reduce national PAH emissions to 1990 levels) will experience difficulties in demonstrating compliance, because source inventories for 1990 and contemporary situations are clearly subject to major uncertainties.

Sources, fate and transport of perfluorocarboxylates.

This review describes the sources, fate, and transport of perfluorocarboxylates (PFCAs) in the environment, with a specific focus on perfluorooctanoate (PFO). The global historical industry-wide emissions of total PFCAs from direct (manufacture, use, consumer products) and indirect (PFCA impurities and/or precursors) sources were estimated to be 3200-7300 tonnes. It was estimated that the majority (approximately 80%) of PFCAs have been released to the environment from fluoropolymer manufacture and use. Although indirect sources were estimated to be much less importantthan direct sources, there were larger uncertainties associated with the calculations for indirect sources. The physical-chemical properties of PFO (negligible vapor pressure, high solubility in water, and moderate sorption to solids) suggested that PFO would accumulate in surface waters. Estimated mass inventories of PFO in various environmental compartments confirmed that surface waters, especially oceans, contain the majority of PFO. The only environmental sinks for PFO were identified to be sediment burial and transport to the deep oceans, implying a long environmental residence time. Transport pathways for PFCAs in the environment were reviewed, and it was concluded that, in addition to atmospheric transport/degradation of precursors, atmospheric and ocean water transport of the PFCAs themselves could significantly contribute to their long-range transport. It was estimated that 2-12 tonnes/ year of PFO are transported to the Artic by oceanic transport, which is greater than the amount estimated to result from atmospheric transport/degradation of precursors.

Modeling global-scale fate and transport of perfluorooctanoate emitted from direct sources.

The long-term (1950-2050) global fate of perfluorooctanoate (PFO) is investigated using the global distribution model, GloboPOP. The model is used to test the hypotheses that direct PFO emissions can account for levels observed in the global oceans and that ocean water transport to the Arctic is an important global distribution pathway. The model emission scenarios are derived from historical and projected PFO emissions solely from direct sources. Modeled ocean water concentrations compare favorably to observed PFO concentrations in the world's oceans and thus ocean inventories can be accounted for by direct sources. The model results support the hypothesis that long-range ocean transport of PFO to the Arctic is important and estimate a net PFO influx of approximately 8-23 tons per year flowing into the model's Northern Polar zone in 2005, an amount at least 1 order of magnitude greater than estimated PFO flux to the Arctic from potential indirect sources such as atmospheric transport and degradation of fluorotelomer alcohols. Modeled doubling times of ocean water concentrations in the Arctic between 1975 and 2005 of approximately 7.5-10 years are in good agreement with doubling times of PFO in Arctic biota estimated from monitoring data. The model is further applied to predict future trends in PFO contamination levels using forecasted (2005-2050) direct emissions, including substantial reductions committed to by industry. Modeled ocean water concentrations in zones near to sources decline markedly after 2005, whereas modeled concentrations in the Arctic are predicted to continue to increase until approximately 2030 and show no significant decrease for the remaining 20 years of the model simulation. Since water is the primary exposure medium for Arctic biota, these model results suggest that concentrations in Arctic biota may continue to rise long after direct emissions have been substantially reduced or eliminated.