Comparative assessment of the global fate of α- and β-hexachlorocyclohexane before and after phase-out

Technical hexachlorocyclohexane (HCH) was one of the most widely used pesticides during the 20th century. Although production and use were phased-out during the 1990s, two of its major components, α- and β-HCH, are still ubiquitous in the environment. Here, we have collected and analyzed data on concentrations of α- and β-HCH in the atmosphere and oceans, including spatial and temporal trends and seasonalities. We apply a global fate and transport model to both isomers over the period 1950 to 2050 to rationalize current levels and trends at remote locations with estimated emissions and to forecast into the near future. Our model results indicate that secondary emissions from soils and oceans are currently controlling the observed rates of decline in the atmosphere. β-HCH is declining more slowly than α-HCH due to its higher persistence, and we hypothesize that it will eventually become the predominant isomer of HCH in the environment. The model reproduces over 70% of measured concentrations of α-HCH in air and ocean water within factors of 3 and 5, respectively, and over 70% of measured concentrations of β-HCH within factors of 8 and 20, respectively. The model results are only weakly sensitive to climate change-induced trends in Arctic sea-ice cover and temperature.

BETR global--a geographically-explicit global-scale multimedia contaminant fate model

We present two new software implementations of the BETR Global multimedia contaminant fate model. The model uses steady-state or non-steady-state mass-balance calculations to describe the fate and transport of persistent organic pollutants using a desktop computer. The global environment is described using a database of long-term average monthly conditions on a 15°×15° grid. We demonstrate BETR Global by modeling the global sources, transport, and removal of decamethylcyclopentasiloxane (D5).

Remoteness from emission sources explains the fractionation pattern of polychlorinated biphenyls in the northern hemisphere

The global distillation hypothesis states that fractionation patterns of persistent semivolatile chemicals in the environment are determined by the effect of spatially varying environmental temperature on the temperature-dependent phase partitioning coefficients of chemicals. Here, we use a model experiment and an analysis of monitoring data for polychlorinated biphenyls (PCBs) to explore an alternative hypothesis, the differential removal hypothesis, which proposes that fractionation results from different loss rates from the atmosphere, acting along a gradient of remoteness from emission sources. Model calculations for a range of PCB congeners demonstrate that fractionation occurs with distance from sources, regardless of the temperature gradient. We have assembled two independent data sets of PCB concentrations in European air that show fractionation, and quantified the remoteness of monitoring sites from PCB sources using the remoteness index, RI. Regression analysis of these empirical data against RI and temperature demonstrates that RI determines fractionation patterns. Based on this result, we calculate empirical effective residence times in air for a set of PCB congeners from the relationship between measured concentrations and RI. These empirical effective residence times agree well with values calculated by a multimedia mass balance model. Our conclusion from the model experiment and analysis of monitoring data is that temperature is not a driver of the fractionation of PCBs currently observed in European air, but rather that fractionation reflects differential removal from the atmosphere.

Quantifying remoteness from emission sources of persistent organic pollutants on a global scale

One of the four screening criteria that are assessed when a chemical substance is nominated for international regulation under the Stockholm Convention is potential for long-range transport. Measured levels of a chemical in locations distant from sources can be used as evidence of long-range environmental transport, but until now, there has been no quantitative measure of the distance of a location from likely source areas of chemicals. Here we use a global atmospheric transport model to calculate atmospheric concentrations for a set of volatile tracers that differ in their effective atmospheric residence time. We then derive an empirical relationship to express these concentrations as a function of the atmospheric residence time and a location-specific parameter, the remoteness index, RI. We present maps of RI for two generic emissions scenarios that represent areas for emissions of industrial and technical chemicals and pesticides, respectively. Our results can be used to better interpret spatial patterns of measured and modeled concentrations of chemicals in the global environment and to derive long-range transport potential metrics for specific substances. We thus provide, to our knowledge for the first time, a description of remoteness that is applicable to measurement sites of continental- and global-scale monitoring programmes. Our results can be used to plan future measurement campaigns and extend monitoring networks.