Intercontinental transport of air pollutants

Throughout the northern hemisphere, intercontinental flows of important air pollutants, such as ozone and fine particles, have a significant impact on environmental quality.

A recent report by the Task Force on Hemispheric Transport of Air Pollution says that coordinated international actions to mitigate the intercontinental flows of certain air pollutants would yield significant environmental and public health benefits. The task force has studied ground level ozone (O₃), fine particles (PM), mercury (Hg) and persistent organic pollutants (POPs).

The task force was created in 2004 by the Convention on Long-range Transboundary Air Pollution (LRTAP Convention), and will present its findings and recommendations to the Convention's Executive Body in Geneva on 13-17 December.

The task force concludes that for each of these pollutants, observed concentrations or deposition at any given location is made up of several different fractions, one of which is related to the intercontinental transport of anthropogenic emissions. Other fractions may originate from natural sources or local and regional man-made sources.

In most cases, abating the man-made local or regional emission sources of these pollutants is the most efficient approach to reduce their impacts. However, without further international cooperation to mitigate their intercontinental flows, many countries will not be able to adequately protect public health and environmental quality. Moreover, cooperation to decrease emissions that contribute to intercontinental transport of air pollution has significant benefits for both source and receptor countries.

Model experiments of hemispheric transport of air pollution have been carried out to derive intercontinental source-receptor relationships for four regions - North America, Europe, South Asia, and East Asia. These regions are responsible for more than three quarters of the anthropogenic emission in the Northern Hemisphere. Specific analyses were also made to quantify the impact in the Arctic of emission changes in these four regions.

Regarding ozone, current annual average surface concentrations in the northern mid-latitudes are about 37 parts per billion (ppb). About 20-25 per cent of this is estimated to originate from the stratosphere, and a similar proportion is formed from natural precursor sources. Man-made sources contribute more than 50 per cent, and over the four continental regions investigated about half of this fraction originates from sources in the region itself and about half is transported from other regions. In heavily polluted environments, the fraction attributable to local and regional emissions sources may be much larger. For ozone, the largest relationship is the impact of North American emissions on European ozone levels, followed by the impact of European emissions on South Asia and East Asia. East Asian emissions have a similar impact on ozone in North American as North American emissions have on East Asia. Arctic ozone levels are mostly influenced by European emissions, followed by North American emissions.

Within a region, the precursor pollutants most strongly influencing annual average ozone levels are nitrogen oxides (NOx), volatile organic compounds (VOCs), methane (CH₄), and carbon monoxide (CO), in decreasing order of importance.

In contrast, the intercontinental contributions to annual ozone concentrations are most strongly influenced by changes in methane, followed by NOx, VOCs, and CO. It was shown that cutting methane emissions in a given region produces roughly the same decrease in intercontinental transport of ozone to other regions as a similar percentage decrease in NOx, VOCs and CO emissions combined.

It is worthwhile noting that apart from damaging human health and vegetation, ozone contributes significantly to climate forcing, both directly as a greenhouse gas and indirectly by damaging plants and inhibiting their uptake of carbon dioxide. Consequently, reducing emissions of methane, which itself is an important greenhouse gas, will result in lower direct forcing from methane as well as lower direct and indirect climate forcing from ozone.

Both primary and secondary PM play a role in intercontinental transport. While the transcontinental contribution to PM concentrations is smaller in relative terms as compared to ozone, its impacts on human health are estimated to be comparable to that of ozone, because the relationship between PM and mortality is stronger. The contributions of the three foreign regions to the mortality in a given home region are estimated to range from three to five per cent. Of the total mortalities associated with emissions from North America and Europe, 15 per cent and 12 per cent, respectively, are estimated to be realised outside of these source regions.

Intercontinental transport of newly released man-made mercury emissions is estimated to account for about 10-30 per cent of total mercury deposition, on an annual globally averaged basis, while natural and re-emitted mercury make up some 35-70 per cent of total mercury deposition, depending on the region. East Asia, which accounted for almost 40 per cent of total global newly released mercury in 2000, is the most dominant source among the four regions, accounting for 10-14 per cent of the mercury deposition found in other regions, followed by contributions from Europe, South Asia, and North America. However, where deposition is highest, the dominant sources are local and regional emissions.

The task force notes that international forums for pursuing further international cooperation to mitigate sources of intercontinental transport exist for POPs and mercury (the Stockholm Convention on POPs and the UNEP's mercury program).

For ozone and PM, however, there are currently no such global arrangements. But there are several regional agreements that address at least some of the sources of ozone and PM. The task force argues that "a global confederation of regional cooperative programmes on air pollution could help develop a better and globally shared understanding of air pollution problems and their solutions at the local, regional and global scale while maintaining autonomy and flexibility for regions to develop policies and programmes appropriate for their circumstances."

Christer Ågren

Source: Hemispheric Transport of Air Pollution 2010: Executive Summary. ECE/EB.AIR/2010/10, October 2010.