Wania Frank, Westgate John N
Department of Physical and Environmental Sciences, University of Toronto Scarborough, 1265 Military Trail, Toronto, Ontario, Canada.
Environ Sci Technol. 2008 Dec 15;42(24):9092-8. doi: 10.1021/es8013198.
The preferential accumulation of selected organic pollutants at higher altitude has been observed in a number of mountain regions. It is proposed that this phenomenon is due to differences in the efficiency of precipitation scavenging at various elevations, which, in turn, is due to the temperature dependence of organic vapor partitioning into rain, snow, and aerosols. The occurrence and extent of enrichment with elevation depends on whether the scavenging efficiency of a chemical is sensitive to temperature within the range encountered along a mountain slope. A multicompartment fate and transport model parameterized for mountain systems suggeststhat substances with equilibrium partitioning coefficients at 25 degrees C between water and air from 10(3.5) to 10(5.5) and between atmospheric particles and air from 10(9) to 10(11) are most likely to be subject to mountain cold-trapping. Such substances remain in the atmospheric vapor phase at higher valley temperatures, but are scavenged efficiently at the lower temperatures prevailing at higher altitudes. This implies that substances subject to mountain cold-trapping are approximately 2 orders of magnitude less volatile than substances that experience global cold-trapping. For example, while lighter PCBs get preferentially trapped at higher latitudes, the heavier PCBs are predicted to experience the strongest mountain cold-trapping. These model results agree with the results of field studies, with the exception of those studies that rely on sample media such as plant foliage for which precipitation is not the dominant deposition pathway. It appears that very fast deposition processes are required to trap contaminants along mountain slopes, whereas such processes reduce contaminant transport to remote polar regions.
在许多山区都观察到某些有机污染物在较高海拔处的优先积累现象。有人提出，这种现象是由于不同海拔处降水清除效率的差异所致，而这又归因于有机蒸汽在雨、雪和气溶胶中分配的温度依赖性。富集现象随海拔的出现和程度取决于一种化学物质的清除效率在山坡沿线所遇温度范围内是否对温度敏感。一个针对山区系统参数化的多隔室归宿和传输模型表明，在25摄氏度时水与空气之间的平衡分配系数在10³.⁵至10⁵.⁵之间、大气颗粒物与空气之间的平衡分配系数在10⁹至10¹¹之间的物质最有可能受到山区冷阱效应的影响。这类物质在山谷较高温度下保留在大气气相中，但在较高海拔处普遍存在的较低温度下会被有效清除。这意味着受到山区冷阱效应影响的物质挥发性比经历全球冷阱效应的物质低约2个数量级。例如，较轻的多氯联苯优先在高纬度地区被捕集，而较重的多氯联苯预计会经历最强的山区冷阱效应。这些模型结果与实地研究结果一致，但那些依赖植物叶片等样本介质的研究除外，因为降水并非这些介质的主要沉积途径。看来需要非常快速的沉积过程才能在山坡上捕集污染物，而这类过程会减少污染物向偏远极地地区的传输。
