Persistent organic pollutants and mercury in the Himalaya and Tibetan Plateau.

摘要

Persistent organic pollutants and mercury are important contaminants due to their persistence in the environment and potential toxic effects on ecosystems and humans. Concerns related to these contaminants are particularly pertinent in Asia where the use of pesticides and mercury emissions have been increasing dramatically due to changing agricultural practices and rapidly expanding industrialization. Based on studies in European and North American mountain regions, evidence is increasing that alpine regions function as regional convergence zones for selected organic pollutants due to an effect called orographic cold trapping. It is hypothesized that such an effect may be particularly pronounced in the Himalaya because of dramatic elevational temperature and precipitation gradients relative to contaminant source regions in its immediate vicinity, and because of the regional monsoon system that has been shown to deliver particles and inorganic air pollutants to higher altitudes. We report here evidence for the movement of select environmentally relevant chlorinated organic pesticides into the Central Himalaya with strong seasonal differences due to the Indian monsoon. Atmospheric concentrations of these chemicals are positively correlated with altitude in summer up to an elevation of 5000m a.s.l and then decrease at higher elevation. In winter the atmospheric concentrations are negatively correlated with altitude indicating that during this season remote sites are above the boundary layer. Soil concentrations appear to follow the gradient of forestation, with maximum concentrations at 2600m a.s.l. and then declining above that altitude. Mercury in three Tibetan snow pits, well above the boundary layer shows the likelihood of particulate deposition in winter when particulate concentrations are highest. Strong dust storm activity is the largest source of mercury deposition on the plateau, though it is unsure if the mercury is transported on dust long distances or if mercury is partitioning onto particulates nearby to the deposition site. Both these deposition mechanisms are likely occurring. The highest elevations of the Himalaya and Tibetan plateau are protected by the upper level westerlies above the boundary layer and are likely only affected by chemicals that have long atmospheric lifetimes such as hexachlorobenzene and mercury.