Air Pollution Export from and Import to North America: Experimental Evidence

Abstract

North America (Canada and the USA) contains only 5% of the world’s population but accounts for 15–23% of the world’s emissions in terms of trace gases important for the radiative processes of the atmosphere, ozone formation and acid rain. As a result, North America has a strong impact on global atmospheric chemistry. The bulk of North American trace gas and particulate matter emissions originate in the heavily populated eastern region, especially in the USA. The emission regions, coupled with the formation of deep convective systems and mid-latitude cyclones determines the export pathways of North American pollution plumes. Deep convection can rapidly loft surface pollutants to the upper troposphere over relatively small spatial areas. In contrast mid-latitude cyclones loft pollutants via slantwise ascent over several days, but can potentially vent the boundary layer of a region as large as the eastern seaboard in just one or two days. The warm conveyor belt airstream of mid-latitude cyclones is the mechanism most strongly associated with rapid intercontinental transport of pollution; this and other transport pathways are discussed in terms of transport from Asia to North America and from North America to Europe. The impacts of North American emissions on tropospheric chemistry, and the implications of emissions from other continents on North American air quality are also discussed.