Publications

In 1974, Junge derived an empirical relationship between the variability of concentrations of volatile trace gases in air at remote locations and their atmospheric residence time. Here, the Junge relationship is adapted to incorporate the deposition and revolatilization of sernivolatile chemicals and applied to interpret nearly a decade of data on polychlorinated biphenyl (PCB) concentrations in air. A multimedia fate model, which accounts for deposition and revolatilization, is used to estimate the characteristic travel distance (CTD) for PCBs, where CTD serves as a measure of the effective atmospheric lifetime for sernivolatile organic chemicals. Data are taken from sites of the Integrated Atmospheric Deposition Network in the North American Great Lakes and the Alert monitoring station in the Arctic,which is operated by the Canadian Northern Contaminants Program. Five factors that may introduce variability into measured concentrations are defined. By suppressing the effect of three of these factors in the data analysis, we identified variability consistent with the Junge relationship in many of the annual data sets (62%), with the relationship showing statistical significance (p < 0.05) in 23% of these annual data sets. The more remote monitoring sites from the Great Lakes region display the highest number of statistically significant Junge-type relationships between the variability in concentrations in air and estimated long-range transport potential in air. At sites in close proximity to areas of high population density, variability in PCB concentrations in air displays patterns that are consistent with primary or secondary temperature-driven volatilization sources. Analysis of variability in long-term monitoring data, using the techniques developed and illustrated here, provides useful insights into the factors that control the behavior of persistent sernivolatile chemicals in the environment.

A global-scale multispecies mass balance model was used to simulate the long-term fate and transport of perfluorocarboxylic acids (PFCAs) with eight to thirteen carbons (C8-C13) and their conjugate bases, the perfluorocarboxylates (PFCs). The main purpose of this study was to assess the relative long-range transport (LRT) potential of each conjugate pair, collectively termed PFC(A)s, considering emissions from direct sources (i.e., manufacturing and use) only. Overall LRT potential (atmospheric + oceanic) varied as a function of chain length and depended on assumptions regarding pK(a) and mode of entry. Atmospheric transport makes a relatively higher contribution to overall LRT potential for PFC(A)s with longer chain length, which reflects the increasing trend in the air-water partition coefficient (K-AW) of the neutral PFCA species with chain length. Model scenarios using estimated direct emissions of the C8, C9, and C11 PFC(A)s indicate that the mass fluxes to the Arctic marine environment associated with oceanic transport are in excess of mass fluxes from indirect sources (i.e., atmospheric transport of precursor substances such as fluorotelomer alcohols and subsequent degradation to PFCAs). Modeled concentrations of C8 and C9 in the abiotic environment are broadly consistent with available monitoring data in surface ocean waters. Furthermore, the modeled concentration ratios of C8 to C9 are reconcilable with the homologue pattern frequently observed in biota, assuming a positive correlation between bioaccumulation potential and chain length. Modeled concentration ratios of C11 to C10 are more difficult to reconcile with monitoring data in both source and remote regions. Our model results for C11 and C10 therefore imply that either (i) indirect sources are dominant or (ii) estimates of direct emission are not accurate for these homologues.

Diel (24-h) variations of concentrations of polychlorinated biphenyls (PCBs) in air are reported at two sites in Zurich, Switzerland, a city surrounded by hills. One site was located in the valley near the city center and the other site was on a hill called Uetliberg, 411 m higher and about 5 km distant from the city center site. Air samples were collected simultaneously at both sites over 4-h time periods for 3 consecutive days under stable meteorological conditions during a high pressure system in August 2007. PCB concentrations at the city site were markedly higher than those at Uetliberg, indicating that the city site is influenced by urban sources of PCBs. Concentrations measured at both sites show a clear diel cycle but have opposite phases: in the city concentrations were lower during the day and higher at night, while at Uetliberg concentrations were higher during the day and lower at night. These observations are explained and interpreted using a multimedia mass balance model that includes a stable night-time inversion layer that formed over the city but below the hilltop site. At Uetliberg the concentration of PCBs is consistent with background levels and the diel concentration pattern can be explained by temperature-mediated air-surface exchange and the influence of nearby woodland canopies. The diel pattern and concentrations in the city are attributable to volatilization from urban sources. We have developed a new method to estimate the strength of the urban PCB source using the model scenarios for the two sampling sites. Emission estimates derived from this method are in good agreement with earlier estimates derived from PCB production, consumption, and usage estimates. This study demonstrates the potential for estimating the strength of diffuse, regional sources of persistent organic pollutants (POPS) using a combined measurement and modeling approach. Such studies can provide important information to derive regional and national POPS emission inventories as required under the Stockholm Convention, and to quantify the effectiveness of actions to reduce POP emissions.

A mass balance model for mercury based on the fugacity concept is applied to Lake Superior, Lake Michigan, Onondaga Lake and Little Rock Lake to evaluate model performance, analyze cycling of three mercury species groups (elemental, divalent and methyl mercury), and identify important processes that determine the source-to-concentration relationship of the three mercury species groups in these lakes. This model application to four disparate ecosystems is an extension of previous applications of fugacity-based models describing mercury cycling. The model performs satisfactorily following site-specific parameterization, and provides an estimate of minimum rates of species interconversion that compare well with literature. Volatilization and sediment burial are the main processes removing mercury from the lakes, and uncertainty analyses indicate that air-water exchange of elemental mercury and water-sediment exchange of divalent mercury attached to particles are influential in governing mercury concentrations in water. Any new model application or field campaign to quantify mercury cycling in a lake should consider these processes as important. (C) 2008 Elsevier Ltd. All rights reserved.

We present the software implementation of The OECD P-OV & LRTP Screening Tool (The Tool) that is used to assess the environmental hazard of organic chemicals using metrics of overall persistence (P-OV) and long-range transport potential (LRTP). The Tool is designed to support decision making for chemical management and includes features that are recommended by the Organization for Economic Cooperation and Development (OECD) expert group on multimedia modeling. The Tool is useful for screening the environmental hazard potential of non-ionizing organic chemicals whose environmental partitioning can be described by absorptive capacities of environmental media estimated from partitioning between air, water and octanol in the laboratory. The software includes data storage functionality, and a user interface that is designed to facilitate simple data input and straightforward interpretation of the model results. The effect of uncertainties in input properties describing chemicals can be assessed with a Monte Carol analysis. The software is evaluated and illustrated by comparing results from The Tool with those from other models and by evaluating four substances that are candidates for regulation or ban under the Stockholm convention on Persistent Organic Pollutants. (C) 2008 Elsevier Ltd. All rights reserved.

Soil and marine aerosol suspension are two physical mass transfer processes that are not usually included in models describing fate and transport of environmental pollutants. Here, we review the literature on soil and marine aerosol suspension and estimate aerosol suspension mass transfer velocities for inclusion in multimedia models, as a global average and on a 1 x 1 scale. The yearly, global average mass transfer velocity for soil aerosol suspension is estimated to be 6 x 10(-10) m h(-1), approximately an order of magnitude smaller than marine aerosol suspension, which is estimated to be 8 x 10(-9) m h(-1). Monthly averages of these velocities can be as high as 10(-7) m h(-1) and 10(-5) m h(-1) for soil and marine aerosol suspension, respectively, depending on location. We use a unit-world multimedia model to analyze the relevance of these two suspension processes as a mechanism that enhances long-range atmospheric transport of pollutants. This is done by monitoring a metric of long-range transport potential, phi-one thousand (phi(1000)), that denotes the fraction of modeled emissions to air, water or soil in a source region that reaches a distance of 1000 km in air. We find that when the yearly, globally averaged mass transfer velocity is used, marine aerosol suspension increases phi(1000) only fractionally for both emissions to air and water. However, enrichment of substances in marine aerosols, or speciation between ionic and neutral forms in ocean water may increase the influence of this surface-to-air transfer process. Soil aerosol suspension can be the dominant process for soil-to-air transfer in an emission-to-soil scenario for certain substances that have a high affinity to soil. When a suspension mass transfer velocity near the maximum limit is used, soil suspension remains important if the emissions are made to soil, and marine aerosol suspension becomes important regardless of if emissions are made to air or water compartments, We recommend that multimedia models designed to assess the environmental fate and long-range transport behavior of substances with a range of chemical properties include both aerosol suspension processes, using the mass transfer velocities estimated here. (C) 2009 Elsevier Ltd. All rights reserved.

We used the multimedia chemical fate model BETR Global to evaluate changes in the global distribution of two polychlorinated biphenyls, PCB 28 and PCB 153, under the influence of climate change. This was achieved by defining two climate scenarios based on results from a general circulation model, one scenario representing the last twenty years of the 20th century (20CE scenario) and another representing the global climate under the assumption of strong future greenhouse gas emissions (A2 scenario), The two climate scenarios are defined by four groups of environmental parameters: (1) temperature in the planetary boundary layer and the free atmosphere, (2) wind speeds and directions in the atmosphere, (3) current velocities and directions in the surface mixed layer of the oceans, and (4) rate and geographical pattern of precipitation. As a fifth parameter in our scenarios, we consider the effect of temperature on primary volatilization emissions of PCBs. Comparison of dynamic model results using environmental parameters from the 20CE scenario against historical long-term monitoring data of concentrations of PCB 28 and PCB 153 in air from 16 different sites shows satisfactory agreement between modeled and measured PCBs concentrations. The 20CE scenario and A2 scenario were compared using steady-state calculations and assuming the same source characteristics of PCBs. Temperature differences between the two scenarios is the dominant factor that determines the difference in PCB concentrations in air. The higher temperatures in the A2 scenario drive increased primary and secondary volatilization emissions of PCBs, and enhance transport from temperate regions to the Arctic. The largest relative increase in concentrations of both PCB congeners in air under the A2 scenario occurs in the high Arctic and the remote Pacific Ocean. Generally, higher wind speeds under the A2 scenario result in more efficient intercontinental transport of PCB 28 and PCB 153 compared to the 20CE scenario. Our modeling indicates that in a future impacted by climate change,we can expect increased volatilization emissions and increased mobility of persistent organic pollutants with properties similar to those of PCBs.

Short-term variability of concentrations of polybrominated diphenyl others (PBDEs) and chlordanes in air at a semirural site in England over a 5 day period is reported. Four-hour air samples were collected during a period dominated by a high pressure system that produced stable diel (24-h) patterns of meteorological conditions such as temperature and atmospheric boundary layer height. PBDE and chlordane concentrations showed Clear diel variability with concentrations in the afternoon and evening being 1.9 - 2.7 times higher than in the early morning. The measurements are interpreted using a multimedia mass balance model parametrized with forcing functions representing local temperature, atmospheric boundary layer height, wind speed and hydroxyl radical concentrations. Model results indicate that reversible, temperature-controlled air-surface exchange is the primary driver of the diel concentration pattern observed for chlordanes and PBDE 28. For higher brominated PBDE congeners (47, 99 and 100), the effect of variable atmospheric mixing height in combination with irreversible deposition on aerosol particles is dominant, and explains the diel patterns almost entirely. Higher concentrations of chlordanes and PBDEs in air observed at the end of the study period could be related to likely source areas using back trajectory analysis. This is the first study to clearly document diel variability in concentrations of PBDEs in air over a period of several days. Our model analysis indicates that high daytime and low nighttime concentrations of semivolatile organic chemicals can arise from different underlying driving processes, and are not necessarily evidence of reversible air-surface exchange on a 24-h time scale.

Variability in gas-particle partitioning of semivolatile organic compounds (SOCs) and related atmospheric processes (particle-associated deposition, rain washout and degradation) are investigated on a global scale. Two different sorption approaches (one using the octanol-air partition coefficient, K-OA, and one based on poly-parameter linear free energy relationships, ppLFER) and two different atmospheric box models (unit-world and highly spatially and temporally resolved) are applied. In the unit-world model, the overall deposition and atmospheric fate of SOCs calculated with the K-OA-based sorption approach are similar to the ones calculated with the ppLFER approach. Rain washout dominates the atmospheric removal of polar chemicals in the unit-world model while non-polar chemicals are removed mainly through degradation or particle-associated deposition. In contrast, big differences and a high sensitivity to the selected sorption approach are found in the spatially and temporally resolved model. The highly resolved geographic variability cannot be represented using the KOA-based approach if aerosol components other than OM are of importance for sorption. In particular, aerosols in dry regions (desert) and regions with low OM aerosols (arctic, some oceanic regions) are more appropriately described by the ppLFER approach. With the ppLFER approach, good agreement between modeled deposition fluxes and measurement data are found for higher chlorinated PCBs and TCDD/Fs. In general, we recommend the ppLFER approach for highly resolved environmental fate models. (C) 2007 Elsevier Ltd. All rights reserved.

Products with antimicrobial effect based on silver nanoparticles are increasingly used in Asia, North America and Europe. This study presents an analysis of risk to freshwater ecosystems from silver released from these nanoparticles incorporated into textiles and plastics. The analysis is presented in four stages; (i) silver mass flow analysis and estimation of emissions, (ii) assessment of the fate of silver in a river system and estimation of predicted environmental concentrations (PECS), (iii) critical evaluation of available toxicity data for environmentally relevant forms of silver and estimation of predicted no-effect concentrations (PNECs), and (iv) risk characterization. Our assessment is based on estimated silver use in the year 2010, focusing on the Rhine river as a case study. In 2010, biocidal plastics and textiles are predicted to account for up to 15% of the total silver released into water in the European Union. The majority of silver released into wastewater is incorporated into sewage sludge and may be spread on agricultural fields. The amount of silver reaching natural waters depends on the fraction of wastewater that is effectively treated. Modeled PECS in the Rhine river are in satisfactory agreement with monitoring data from other river systems. Because a complete characterization of the toxicity of environmentally relevant silver species is lacking, only a limited risk assessment is possible at this time. However, our study indicates that PEC/PNEC ratios greater than 1 cannot be ruled out for freshwater ecosystems, in particular sediments. No risk is predicted for microbial communities in sewage treatment plants. (c) 2007 Elsevier B.V. All rights reserved.