These problems may be difficult to remedy in that a wide range of sources affect air concentrations and deposition thousands of kilometres from the source.
For example, Sweden can hardly influence the amount of acidification or eutrophication through sulphur and nitrogen which falls on the country when only a small part of it comes from own emission sources. Most derive from outside of the country. Similarly, most of Sweden’s sulphur and nitrogen is exported to countries outside of Sweden.
Substances are transported over large regions
The reason these substances are transported from one country to another is because of the average amount of time they spend in the atmosphere, which can be from a few days to up to a week. They follow therefore the winds and weather systems throughout this time. The average amount of time spent in the atmosphere is nevertheless limited, which means that these substances are not dispersed across the entire planet (as are carbon dioxide and freons).
Regions that are connected and that are usually studied from an Air Quality point of view include Europe, North America, East Asia, southern Asia and Southeast Asia. In Europe and North America, most of the regional air pollutants are reducing while they are increasing in Asia and other parts of the world.
SMHI's dispersion model MATCH was originally designed to study the regional transport of acidifying substances in Europe. It is still used for this purpose both in Europe and other parts of the world, for example Asia.