Nitrogen is an essential element for life on Earth. While harmless in its inert form (N2), in reactive forms (Nr) it is deleterious as it accumulates and cycles in the air, soil and water.
Reactive nitrogen (Nr) :
– Ammonia (NH3)
– Ammonium (NH4+)
– Nitrogen oxides (NOx)
– Nitrous oxide (N2O)
– Nitric acid (HNO3)
– Nitrous acid (HONO)
– Organic forms
Emissions of atmospheric Nr have increased fivefold since preindustrial times due to enhanced agriculture, industry, transport and domestic production. It is believed that we now have reached a point where the nitrogen biochemical flow has exceeded its planetary boundary for a safe operating zone. This goes together with a cascade of impacts on human health and ecosystems, which is estimated to cost the EU between 70 and 320 billion € per year. Defining and applying better management strategies for nitrogen throughout its lifecycle is on top of the environmental policy agendas, in particular for the protection of sensitive ecosystems and for air pollution control. The balance between food security and mitigation of nitrogen emissions down to farm level is, for example, a cornerstone of the EU Green Deal.
“The current modification of the nitrogen cycle, mainly due to fertiliser use in agriculture, is far greater in magnitude than the modification of the global carbon cycle as a result of GHG emissions”EU SOER 2020
The European environment – state and outlook
According to the European Nitrogen Assessment program, the key threats of excess reactive nitrogen are:
Reactive nitrogen emissions cause Reactive nitrogen emissions cause formation of secondary particulate matter (PM). In Europe for example, reactive nitrogen alone contributes up to 30-70 % of the PM by mass. It accounts for 3-4∙105 premature deaths per year and leads to a reduction of life expectancy of 6 to 12 months across most of Central Europe. NOx also has a range of impacts on air quality and human health, directly or indirectly through its role in the formation of tropospheric ozone (O3).
A key uncertainty in the human-induced perturbations of the carbon cycle is the role of reactive nitrogen deposition on land carbon storage. Reactive nitrogen also drives the formation of the short-term climate forcers like O3 and secondary PM, and affects the chemical lifetime of several climate forcers. While direct positive forcing on climate occurs through N2O, both NO2 and NH3 contribute indirectly and dominate the emission-based view of climate forcing. The uncertainties on all processes and feedbacks of reactive nitrogen are large and preclude a robust assessment of their future role on climate.
Soil and water quality
Through the application of excess nitrogen fertilizers, soil quality is affected, leading to lower agricultural production and in natural areas to eutrophication and acidification. Nitrogen deposition causes similarly eutrophication and acidification in fresh waters, which can lead to toxic algal blooms and coastal dead zones.
Sensitive fauna and flora are gradually out-competed in areas with excess nitrogen deposition, with dramatic impacts on biodiversity. In Europe 62 % of the ecosystems are exposed to levels of nitrogen beyond which they can tolerate. The projections for the next decade show no or weak improvement, with 58 % of the Natura 2000 areas that will remain at risk in 2030.
To better understand and address these impacts, there is a critical need to quantify the global nitrogen cycle and monitor its perturbations on all scales, down to the urban or agricultural source. As was exposed with the nitrogen crisis in the Netherlands, more scientific data is needed in particular with regard to the local sources, which will be key to develop effective policies to maintain the quality of the Natura 2000 areas throughout Europe.
While global monitoring of the longer-lived nitrogen compounds could be carried out to some extent by representative ground-based networks or airplane campaigns, this is not possible for the short-lived components NH3 and NO2.
→ The Nitrosat Earth Explorer will be the first dedicated satellite mission to simultaneously identify the emission contributions of NH3 and NO2 from farming activities, industrial complexes, transport, fires and urban areas.
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