Air pollutants with lifetimes of a few weeks or longer, such as ozone, fine particulate matter, mercury, and many persistent organic pollutants, are ubiquitous in the atmosphere, and may be transported around the globe under the influence of global atmospheric circulation. The composition of the lower atmosphere forms the background upon which regional air pollution builds, and may thus have a substantial impact on air quality in many parts of the world. The source of some of these pollutants is natural, associated with vegetation, soils, fires, lightning, and descent of air from the stratosphere, while others arise from human activity associated with industry, power generation, transport, residential sources, and agriculture. While some pollutants are emitted directly, others such as ozone may be formed through photochemical processes in the atmosphere from precursors emitted elsewhere. These longer-lived pollutants are transported and dispersed around the globe, reaching even the cleanest and most remote of locations, and this contributes to poor air quality and environmental damage in unpopulated polar and oceanic regions. Atmospheric concentrations of many longer-lived pollutants are continuing to build up in the atmosphere associated with increased anthropogenic emissions, and this is likely to influence future air quality around the globe. Climate change is already altering background air pollutant concentrations, but the effects differ for different pollutants and in different regions, as these changes influence emissions, formation, and removal processes.