Executive summary

Nature of the problem

• Reactive nitrogen (Nr) is of fundamental importance in biological and chemical processes in the atmosphere–biosphere system, altering the Earth's climate balance in many ways. These include the direct and indirect emissions of nitrous oxide (N2O), atmospheric Nr deposition and tropospheric ozone formation (O3), both of which alter the biospheric CO2 sink, Nr supply effects on CH4 emissions, and the formation of secondary atmospheric aerosols resulting from the emissions of nitrogen oxides (NOx) and ammonia (NH3).

• Human production and release of Nr into the environment is thus expected to have been an important driver of European greenhouse balance. Until now, no assessment has been made of how much of an effect European Nr emissions are having on net warming or cooling.

Approaches

• This chapter summarizes current knowledge of the role of Nr for global warming. Particular attention is given to the consequences of atmospheric Nr emissions. The chapter draws on inventory data and review of the literature to assess the contribution of anthropogenic atmospheric Nr emissons to the overall change in radiative forcing (between 1750 and 2005) that can be attributed to activities in Europe.

• The use of Nr fertilizers has major additional effects on climate balance by allowing increased crop and feed production and larger populations of livestock and humans, but these indirect effects are not assessed here.

Executive summary

Nature of the problem

• Increased public and institutional awareness of both the benefits and threats of nitrogen has the potential to greatly increase the efficacy of nitrogen policy.

• Insufficient recognition of the financial, behavioural and cultural barriers to achieving an optimal nitrogen future risks policy antagonisms and failure.

• Here we examine some of the key societal levers for and barriers to achieving an optimal nitrogen future in Europe, drawing lessons from the more-developed societal and policy challenge of climate change mitigation.

Key findings/state of knowledge

• There is currently a very low level of public and media awareness of nitrogen impacts and policies. However, awareness is high regarding the threats and benefits of ‘carbon’ to society (e.g. energy use and enhanced climate change).

• Many national climate change mitigation policies now overtly recognize the importance of societal choice, and are increasingly utilizing behavioural change strategies to achieve greenhouse gas emission reduction targets.

• In achieving an optimal nitrogen future, lessons can and should be learned from existing climate change-focused communication and behavioural science (e.g. use of a ‘segmented strategy’ to reach disparate groups of stakeholders).

• Key sectors where societal choice has the potential to greatly influence nitrogen use efficiency include food production, consumption and waste.

Executive summary

Nature of the problem

• Although cities take only 1.5%–2% of the Earth's land surface, due to their dense population, settlement structure, transportation networks, energy use and altered surface characteristics, they dramatically change the regional and global nitrogen cycle. Cities import and concentrate Nr in the form of food and fuel, and then disperse it as air and water pollution to other ecosystems covering much larger areas.

Approaches

• A mass-balance approach was used in order to quantify the fluxes of reactive nitrogen (Nr) in and out of cities.

• Cities can be characterised either as a source of Nr (i.e. emitting large amounts as liquid or solid household waste, automobile exhaust, air pollution from power plants) or a sink of Nr (through importing more food, fossil fuels, etc., and having fewer emissions to the air and water).

• Paris metropolitan area is used as a case study, which represents an evolving European capital with much available data.

Key findings/state of knowledge

• The Paris Metropolitan Area changed from being a sink in the eighteenth and nineteenth centuries to a source of Nr today. Major changes in the city functioning occurred before 1950, but especially recent decades have been characterised by an unprecedented amplification of those changes.

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Executive summary

Nature of the problem

• Reactive nitrogen has both positive and negative effects on ecosystems and human health. Reactive nitrogen is formed through the use of fossil fuels releasing large amounts of nitrogen oxides into the atmosphere and through the production of ammonia by the Haber–Bosch process and using it in agriculture to increase our food, feed and fuel production. While the use of nitrogen as a fertilizer and chemical product has brought enormous benefits, losses of fertilizer nitrogen and combustion nitrogen to the environment lead to many side effects on human health, ecosystem health, biodiversity and climate.

Approaches

• The European nitrogen problem is placed in a global perspective, showing the European nitrogen fixation, transport and environmental impacts compared with different regions of the globe.

Key findings/state of knowledge

• Humans, largely through agriculture, but also through burning of fossil fuels, have had a huge impact on the nitrogen budget of the Earth. Europe is one of the leading producers of reactive nitrogen, but it is also the first region in the world where the issue was recognized and in some parts of Europe the reactive nitrogen losses to the environment started to decrease. Europe is a nitrogen hotspot in the world with high nitrogen export through rivers to the coast, NOx and particulate matter concentrations and 10% of the global N2O emissions.

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Executive summary

Nature of the problem

• The two main groups of atmospheric reactive nitrogen compounds (reduced and oxidized nitrogen) have different fates due to differences in governing processes.

• Abatement strategies need to take into account these differences when assessing the impact on the sensitive ecosystems.

Approaches

• The chapter outlines the governing physical and chemical processes for the two main groups of reactive nitrogen compounds.

• The chapter is divided into sections concerning: emissions, transformation, aerosol processes, dry deposition and wet deposition.

Key findings/state of knowledge

• Reactive nitrogen compounds consist of reduced nitrogen (ammonia and its reaction product ammonium), oxidized nitrogen (nitrogen oxides) and organic nitrogen compounds.

• Nitrogen oxides have little impact close to the sources since they are emitted as nitrogen monoxide and nitrogen dioxide with low dry deposition rates. These compounds need to be converted into nitric acid (about 5% per hour) before deposition is efficient.

• Ammonia has a high impact near the sources due to high dry deposition rates. Ammonia may therefore have significant impact on ecosystems in areas with intense agricultural activity leading to high emissions of ammonia.

• Both ammonia and gaseous nitrogen oxides lead to formation of aerosol phase compounds (ammonium and nitrate) which are transported over long distances (up to more than 1000 km).

• Very little is known either quantitatively or qualitatively about organic nitrogen compounds, other than that they can contribute a significant fraction of wet-deposited N, and are present in gaseous and particulate forms in the atmosphere.