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Long-Range Transport of Air Pollution

Long-range air pollution, sometimes referred to as transboundary air pollution, was recognized in the 1960s when scientistsdemonstrated the relationship between sulphur emissions in Europe and the acidification of Scandinavian lakes (UNECE 2005).

Many studies have since confirmed that nitrogen, particulates, acidifying gases, heavy metals, and organic pollutants travel in the atmosphere for thousands of kilometres within and across national borders, interacting with each other and forming secondary pollutants before being deposited. One of the defining early activities of European environmental regulation, under the United Nations Economic Commission for Europe (UNECE), was action on the sulphur emissions that contribute to acid rain (Box 4). Since then, there have been other national and regional efforts to address long-range pollution – for instance, the Association of Southeast Asian Nation’s Agreement on Transboundary Haze Pollution, signed in 2002, the Malé Declaration on Control and Prevention of Air Pollution in South Asia, and the US EPA’s Clean Air Interstate Rule issued in early 2005. Studies indicate that sulphur-related acidification also affects parts of Asia (Figure 6).

Sulphur dioxide emissions in the region are projected to increase substantially (Figure 7). Data availability to monitor the trends is improving, as the Acid Deposition Monitoring Network East Asia (EANET), formed in 1998 by 12 countries, has a monitoring programme in place since 2003. As anthropogenic emissions of nitrogen increased (Figure 8), it also emerged that the deposition of nitrogen compounds, including ammonia, causes problems such as eutrophication of freshwaters and marine and terrestrial ecosystems (see GEO Year Book 2003). Eutrophication is the consequence of excess input of nutrients into ecosystems. Common outcomes are excessive algal growth (or algal blooms) insurface waters, oxygen depletion, enhanced growth and dominance of some species that grow better with higher nitrogen conditions and reduced growth of others, resulting in ecosystem imbalance and a decline in species richness. In Europe and North America, forests are growing faster now than earlier this century, and at least part of that increase has been attributed to increased atmospheric nitrogen inputs (Matson and others 2002). Other energy related air pollutants that pose long-range problems include mercury from coal burning and persistent organic pollutants (POPs), including dioxins, which are produced when biomass, waste containing chlorine and wood treated with pentachlorophenol (PCP) are burnt. POPs, which do not degrade easily, can travel long distances through air and water. They accumulate in the bodies of humans and animals, interfering with the hormone system and neurological development. Recently there has been growing concern over the hemispheric-scale transport of pollutants, including black carbon particles (Box 5), and tropospheric ozone (Figure 9).

Box 3: Conversion to natural gas in Santiago
de Chile

A shift to natural gas in Santiago, Chile, resulted in a drastic decrease in particulate (PM10) and SO2 emissions from industrial sources that had previously relied on fuel oil and wood. PM10 concentrations fell from an annual average of 42 µg/m3 in 1997, the year the change took place, to 34 µg/m3 in 2002 (CONAMA 2004). However, this gain may be reversed by uncertainty in the supply of natural gas. The shift to natural gas was facilitated by the building of a pipeline in 1997 to bring natural gas from Argentina, and by convenient pricing and stronger emission regulations in Chile. Similar polices to keep prices of natural gas low in Argentina, however, increased domestic demand. Since production levels remained constant, supplies to Chile were restricted in 2004. Industries moved back to burning fuel oil during the periods of interrupted gas supply. With the future of natural gas availability uncertain, some large industries in Chile are considering switching to clean coal technologies. Although these technologies are capable of complying with environmental regulations, emission levels will be higher than with natural gas.
Box 5: The Atmospheric Brown Cloud
Since 1995 an international group of over 250 scientists has been collecting data over the Indian Ocean as part of the Indian Ocean Experiment (INDOEX). In spring 1999, scientists discovered a brownish pollution haze layer caused by aerosol (particulate) air pollution, or Atmospheric Brown Cloud (ABC) over the Indian Ocean. The formation of the haze layer is a seasonal phenomenon facilitated by an extended dry season, which prevents the removal of pollution from the atmosphere through rainfall. Since this discovery, the impacts of aerosol pollution around the world have received considerable attention (Ramanathan and others 2002). Aerosols reduce the amount of sunlight that can reach the Earth’s surface, with significant impacts on terrestrial and marine ecosystems. Preliminary assessments of the impact of the ABC, commissioned by UNEP in 2001, identified potential direct and indirect impacts, including regional and global climate change and impacts on ecosystems, the water cycle, agriculture and human health (UNEP and C4 2002). The ‘dimming’ effect of aerosols on sunlight causes ‘global cooling’ – and therefore reduces the impact of global warming. Exactly how much aerosols counteract global warming is still being investigated, but this finding implies that studies on global warming that have not taken the aerosol cooling into account have underestimated the extent of global warming.
The ‘benefit’ of this global cooling will progressively decline as the influence of the short-lived aerosols is gradually overwhelmed by the countervailing influence of the long-lived greenhouse gases. Based on the scientific review of the initial assessment, UNEP has established an international scientific team to assess the impacts of ABC on health, agriculture, water budgets, and climate change. Aerosol observatory stations have been established and capacity building activities such as training have been initiated.

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