Drivers: What are the main processes that drive these intercontinental flows and determine their magnitudes?
The intercontinental transport of air pollution is driven by several processes including atmospheric circulation, the global distribution of emissions, chemical and physical transformations, and interactions with/transformation in other environmental media.
In the mid-latitudes of the Northern Hemisphere, the general circulation is dominated by westerly winds that flow from Asia across the North Pacific Ocean to North America, from North America across the North Atlantic Ocean to Europe, and from Europe into Asia. Long-range transport of pollutants can produce distinct plumes in the mid- and upper troposphere, but to be relevant to air quality in a downwind continent, the pollution must descend to the surface. As the plumes descend they are diluted and can be difficult to distinguish from local pollution, especially in receptor regions with relatively high emissions.
In the tropics, intercontinental transport is generally from east to west, guided by the trade winds throughout the lower and mid-troposphere.
In the Northern Hemisphere polar region, the cold and stable lower troposphere forms a dome over the Arctic, largely isolating the region from low latitude pollution that is emitted into warm air masses that ascend into the mid- and upper troposphere above the Arctic. Pollutant transport into the Arctic lower troposphere occurs preferentially from Europe when the outer regions of the Arctic dome pass over northern Europe, take up fresh emissions and then retreat back into the Arctic.
The magnitude and impact of hemispheric and intercontinental scale transport of air pollutants is initially determined by the global distribution of emissions, and their spatial relation to the major meteorological transport pathways described above.
For example, the intense emission regions along the east coasts of North America and Asia are at the origins of the North Atlantic and North Pacific mid-latitude cyclone storm tracks, which can loft the emissions and transport them to the free troposphere above downwind continents in a matter of days. With Western Europe located at the end of the North Atlantic storm track, its emissions are not lofted to the same extent as those on the east coasts of North America and Asia. Instead, European emissions are exported at relatively low altitudes and have a strong impact on the Arctic.
Chemical and Physical Transformations
Intercontinental pollution transport occurs on timescales of days to weeks, longer than the atmospheric lifetimes of some pollutants, and ample time for the trace gases and PM emitted or produced at the source to undergo removal or chemical transformation. By the time a polluted air mass arrives at a downwind continent, it is likely to have very different chemical properties than it did at the source.
Interactions With and Transformations in Other Environmental Media
Because of the cycling of Hg and POPs among the atmosphere and land, ocean, and vegetative surfaces, models of the atmospheric transport of Hg and POPs must account for the emission or re-emission of Hg and POPs from surfaces and the chemical and physical processes that occur in surface media that permanently remove the substances from circulation in the environment. Air-sea exchange of semi-volatile POPs and Hg allows for multiple cycles through the atmosphere and ocean. Ocean transport is expected to be most important for persistent substances that are highly water-soluble and have low vapour pressure, such as perfluorinated acids, which are a relatively new class of POPs.