Global nitrogen fixation contributes 413 Tg of reactive nitrogen (N _r ) to terrestrial and marine ecosystems annually of which anthropogenic activities are responsible for half, 210 Tg N. The majority of the transformations of anthropogenic N _r are on land (240 Tg N yr ⁻¹ ) within soils and vegetation where reduced N _r contributes most of the input through the use of fertilizer nitrogen in agriculture. Leakages from the use of fertilizer N _r contribute to nitrate (NO ₃ ⁻ ) in drainage waters from agricultural land and emissions of trace N _r compounds to the atmosphere. Emissions, mainly of ammonia (NH ₃ ) from land together with combustion related emissions of nitrogen oxides (NO _x ), contribute 100 Tg N yr ⁻¹ to the atmosphere, which are transported between countries and processed within the atmosphere, generating secondary pollutants, including ozone and other photochemical oxidants and aerosols, especially ammonium nitrate (NH ₄ NO ₃ ) and ammonium sulfate (NH ₄ ) ₂ SO ₄ . Leaching and riverine transport of NO ₃ contribute 40–70 Tg N yr ⁻¹ to coastal waters and the open ocean, which together with the 30 Tg input to oceans from atmospheric deposition combine with marine biological nitrogen fixation (140 Tg N yr ⁻¹ ) to double the ocean processing of N _r . Some of the marine N _r is buried in sediments, the remainder being denitrified back to the atmosphere as N ₂ or N ₂ O. The marine processing is of a similar magnitude to that in terrestrial soils and vegetation, but has a larger fraction of natural origin. The lifetime of N _r in the atmosphere, with the exception of N ₂ O, is only a few weeks, while in terrestrial ecosystems, with the exception of peatlands (where it can be 10 ² –10 ³ years), the lifetime is a few decades. In the ocean, the lifetime of N _r is less well known but seems to be longer than in terrestrial ecosystems and may represent an important long-term source of N ₂ O that will respond very slowly to control measures on the sources of N _r from which it is produced.

Why Wait?: Tropospheric ozone can be dangerous to human health, can be harmful to vegetation, and is a major contributor to climate warming. Black carbon also has significant negative effects on health and air quality and causes warming of the atmosphere. Shindell et al. (p. 183 ) present results of an analysis of emissions, atmospheric processes, and impacts for each of these pollutants. Seven measures were identified that, if rapidly implemented, would significantly reduce global warming over the next 50 years, with the potential to prevent millions of deaths worldwide from outdoor air pollution. Furthermore, some crop yields could be improved by decreasing agricultural damage. Most of the measures thus appear to have economic benefits well above the cost of their implementation.

International audienceAir quality transcends all scales with in the atmosphere from the local to the global with handovers and feedbacks at each scale interaction. Air quality has manifold effects on health, ecosystems, heritage and climate. In this review the state of scientific understanding in relation to global and regional air quality is outlined. The review discusses air quality, in terms of emissions, processing and transport of trace gases and aerosols. New insights into the characterization of both natural and anthropogenic emissions are reviewed looking at both natural (e.g. dust and lightning) as well as plant emissions. Trends in anthropogenic emissions both by region and globally are discussed as well as biomass burning emissions. In terms of chemical processing the major air quality elements of ozone, non-methane hydrocarbons, nitrogen oxides and aerosols are covered. A number of topics are presented as a way of integrating the process view into the atmospheric context; these include the atmospheric oxidation efficiency, halogen and HOx chemistry, nighttime chemistry, tropical chemistry, heat waves, megacities, biomass burning and the regional hot spot of the Mediterranean. New findings with respect to the transport of pollutants across the scales are discusse