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.     

Comparison of biomarker and pathological responses in flounder (Platichthys flesus L.) induced by ingested polycyclic aromatic hydrocarbon (PAH) contamination

This study investigated the response of biomarker measurements and histopathological indicators of polycyclic aromatic hydrocarbon (PAH) exposure in the flounder (Platichthys flesus L.). Flounder were fed food spiked with a mixture of four PAHs at an environmentally relevant range of concentrations for either one or six months. Ethoxyresorufin-O-deethylase (EROD) activity was elevated following 1 month exposure to PAH concentrations up to 50 mgkg(-1) in food. Bile metabolite concentrations were found to increase with PAH concentration, up to 500 mgkg(-1) PAH. By comparison, no DNA adducts were detected and there were no significant histopathological changes observed. After 6 months exposure, EROD levels were not elevated but bile metabolites showed a similar dose dependent relationship as in the 1 month experiment, while DNA adducts were only detected in the highest PAH exposure groups. No significant histopathological changes were observed. The results are discussed with respect to the implications for the use of these methods in environmental monitoring studies.     

Congener specific transfer of PCDD/Fs from air to cows' milk: an evaluation of current modelling approaches

Three different approaches have been used to model the transfer of individual PCDD/F congeners from the air to cows' milk. These are: (1) an 'Equilibrium Partitioning' approach, (2) a 'Deposition Velocity' approach and (3) a 'Scavenging' approach. Air-leaf transfers and livestock feed-milk transfers, the two most critical components of the food chain exposure model, are discussed. A representative database for measured PCDD/Fs in UK air, herbage and milk is presented and the performance of each predicted model concentration against this measured dataset is assessed. Weaknesses and uncertainties associated with modelling the complex transfer processes involved are highlighted.     

The Rossby Centre Regional Atmospheric Climate Model part II: application to the Arctic climate

The Rossby Centre regional climate model (RCA2) has been integrated over the Arctic Ocean as part of the international ARCMIP project. Results have been compared to observations derived from the SHEBA data set. The standard RCA2 model overpredicts cloud cover and downwelling longwave radiation, during the Arctic winter. This error was improved by introducing a new cloud parameterization, which significantly improves the annual cycle of cloud cover. Compensating biases between clear sky downwelling longwave radiation and longwave radiation emitted from cloud base were identified. Modifications have been introduced to the model radiation scheme that more accurately treat solar radiation interaction with ice crystals. This leads to a more realistic representation of cloud-solar radiation interaction. The clear sky portion of the model radiation code transmits too much solar radiation through the atmosphere, producing a positive bias at the top of the frequent boundary layer clouds. A realistic treatment of the temporally evolving albedo, of both sea-ice and snow, appears crucial for an accurate simulation of the net surface energy budget. Likewise, inclusion of a prognostic snow-surface temperature seems necessary, to accurately simulate near-surface thermodynamic processes in the Arctic.     

Spatial distribution of atmospheric PAHs and PCNs along a north-south Atlantic transect

Ship-board air samples collected between The Netherlands and South Africa in January-February 2001 were analysed for polycyclic aromatic hydrocarbons (PAHs) and polychlorinated naphthalenes (PCNs). The highest PAH concentrations occurred in the European samples, and in samples close to West Africa and South Africa. Consistently low PAH concentrations were measured in the southern hemisphere open ocean samples (190-680 pg/m3). The highest PCN concentrations occurred in the European samples, but high values were also detected off the West African coast, and in the sample taken closest to South Africa. Data are presented for diurnal cycles taken in the remote South Atlantic. The day:night ratios of phenanthrene, 1-methylphenanthrene and fluoranthene were typically approximately 1.5-2.5:1. The mechanism(s) causing this observation is/are not understood at present, but dynamic environmental process(es) is/are implicated.     

Temporal and spatial trends of PCB congeners in UK gannet eggs

Gannet (Sula bassana) eggs collected from Ailsa Craig, Western Scotland between 1977-1998 have been analyzed retrospectively for several PCB congeners. Concentrations of a range of congeners were determined in 8-10 eggs analyzed separately for several individual years. All congeners declined in concentrations throughout the time period, but the rates of decline differed for different congeners. Declines were first order and half-lives varied between 5.4 years for PCB-101 to 10.1 years for PCB-180. Egg concentrations reflect the maternal body burden, which itself is controlled by the birds rate of intake (principally via the diet) and losses (via metabolism and other clearance mechanisms). The declining concentrations in eggs, therefore, broadly reflect reductions in prey concentrations (principally herring and mackerel) and--in turn--water column concentrations. Rates of change in PCB concentrations from this study were similar to those noted in fish-eating birds from the Baltic Sea and North American Great Lakes, and ambient air in the UK. This provides indirect evidence that gannet eggs are broadly reflecting regional/global scale clearance/removal mechanisms which are controlling ambient PCB concentrations.     

Evidence of in situ diel vertical migration of a red-tide microplankton species in Ría de Vigo (NW Spain)

The migration capacity of red-tide species in the natural environment was studied at a station in the Ría de Vigo (Rías Bajas, NW Spain) over a 24 h period in September 1991. The Ría de Vigo, where red tides are frequent, normally shows a positive estuarine circulation and is subjected to seasonal upwelling and downwelling phenomena. A marked diel pattern was observed for five species that are capable of causing red tides (Ceratium furca, Scrippsiella trochoidea, Dinophysis acuminata, Mesodinium rubrum, and Eutreptiella sp.). Such diel behaviour could be clearly advantageous in a stratified environment where light and nutrients are often in two separate layers. Active movement enables species such as dinoflagellates and some ciliates to exploit high levels of irradiance at the surface during the day and to take up nutrients in deeper layers at night. Patchy distribution of phytoplankton, shear-induced horizontal dispersion and density variations were considered, but none of them accounted for the vertical changes observed. Vertical migration is thought to be one of the mechanisms that could promote blooms in nutrient-depleted surface layers.