Accumulation of persistent organochlorine compounds in mountains of western Canada

Persistent, semi-volatile organochlorine compounds, including toxic industrial pollutants and agricultural pesticides, are found everywhere on Earth, including in pristine polar and near-polar locations1,2,3,4. Higher than expected occurrences of these compounds in remote regions are the result of long-range transport in the atmosphere, precipitation and ‘cold condensation’ — the progressive volatilization in relatively warm locations and subsequent condensation in cooler environments3,4 which leads to enhanced concentrations at high latitudes. The upper reaches of high mountains are similar to high-latitude regions in that they too are characterized by relatively low average temperatures, but the accumulation of organochlorine compounds as a function of altitude has not yet been documented. Here we repororganochlorine deposition in snow from mountain ranges in western Canada that show a 10- to 100-fold increase between 770 and 3,100 m altitude. In the case of less-volatile compounds, the observed increase by a factor of 10 is simply due to a 10-fold increase in snowfall over the altitude range of the sampling sites. In the case of the more-volatile organochlorines, cold-condensation effects further enhance the concentration of these compounds with increasing altitude. These findings demonstrate that temperate-zone mountain regions, which tend to receive high levels of precipitation while being close to pollutant sources, are particularly susceptible to the accumulation of semivolatile organochlorine compounds.