Modern agricultural practices, especially intensive farming methods and widespread fertiliser use, are mainly responsible for ammonia emissions, which have more than doubled since pre-industrial times. Fossil fuel combustion and burning of vegetation, including forest fires, are other major sources of ammonia pollution. The EU1,2 has set a target that by 2020 ammonia emissions are to be reduced by 27 per cent compared with the year 2000.
Ammonia can react in the atmosphere with other emissions, such as sulfur dioxide and nitrogen oxides, and form fine particles containing ammonium ions. Particulate pollution worsens air quality and can lead to health problems, including heart and respiratory illnesses, especially in susceptible individuals. When ammonia and ammonium particles are deposited on land or in water, they can act like a fertiliser and interfere with the normal functioning of ecosystems. For example, enrichment of coastal waters with nitrogen, from ammonia deposits and agricultural run-off, can encourage harmful algal blooms, reduce oxygen levels and cause loss of biodiversity.
Uncertainties over the amount of global ammonia emissions can arise through lack of widespread ground-based air quality monitoring and insufficient data on fertiliser use and animal production. In addition, ammonia emissions are short-lived in the atmosphere, making ground measurements more difficult.
European researchers used observations from space to overcome these problems. Using an infrared sensor aboard a satellite launched to monitor the environment and climate, the researchers developed a method to measure ammonia concentrations over a period of one year (2008), collecting over one million measurements a day. By comparing these measurements with recent models of the atmosphere, the researchers could locate ammonia hotspots and discrepancies between the satellite-measured ammonia levels and global ammonia inventories estimated from models.
In all, a total of 28 hotspots for ammonia emissions were identified around the world. Some high levels of ammonia were explained by fires in the mid-latitudes in South Siberia and Inner Mongolia, South America and East, West and South Africa. Almost all the other hotspots were identified above agricultural regions of North America, Europe and Asia.
In general, good agreement was found between measured and modelled ammonia distributions. However, some differences were detected, which suggest that ammonia emissions have been underestimated in current global inventories in the northern hemisphere, especially above 30 degrees north. This was found, for example, in North America for the San Joaquin Valley, California and in Europe, for the Po Valley in Italy and Ebro Valley in Spain.
Particularly large differences between measured and modelled data were found in Central Asia, for example, the agricultural region irrigated by rivers in the basin surrounding the Aral Sea, where cotton and wheat are intensively farmed using large quantities of inorganic fertilisers. Surrounding mountains can cause weather to move slowly and concentrate ammonia emissions in the valleys. Large ammonia levels were also found over non-mountainous regions: over the Nile Delta, and parts of Western Europe including Belgium, the Netherlands and Germany.