| | | 2.2 Sources |
Nutrients deposited from the atmosphere include phosphorus and nitrogen. The atmospheric input of phosphorus to the marine environment is insignificant compared to the other sources of this nutrient. Input of nitrogen from the atmosphere is, however, potentially significant. An example taken from an international research project shows that on yearly average approximately 24% of the external nitrogen input to the Kattegat Sea (located between Denmark and Sweden) is from the atmosphere [437]. Furthermore if only the summer period is considered, the contribution from the atmosphere can be as high as 40% on average over the entire sea area. This indicates that the atmospheric load may be even higher in areas close to nitrogen sources and thus can contribute a substantial amount of external nitrogen into e.g. coastal systems.
The most important groups of atmospheric nitrogen compounds in terms of nutrient deposition are
The sum of NO and NO2 is denoted NOx and the sources of all the atmospheric nitrogen compounds are the emissions of NH3 and NOx. NH3 is primarily emitted from processes related to agriculture such as livestock production (including the spread of manure) and fertilizer application and production. NH3 emissions are therefore quite high in countries with widespread intense agricultural activities. An example of the NH3 emission density for the European area is presented in Figure 1.
The high emission density in Denmark, The Netherlands and Belgium is caused, in part, by the high density of pig farms in these countries. There is a strong seasonal variation in NH3 emissions, typically related to the spread of manure and fertilizer on fields. The rate of emission is in general higher during the day than during the night, due to the typically higher temperature and wind speed during daytime.
NOx is primarily emitted from combustion processes involving fossil fuels. The dominating component in the release is NO (estimated to contribute around 90-95% of the total emission). The combustion processes include power plants, industrial furnaces as well as road traffic. An example of the NOx emission density for the European area is presented in Figure 2.
The emission density is highest in densely populated areas and the emissions over sea areas arise from ship traffic and oil rigs. Highest emission rate over land is obtained during rush hours, and for the emissions related to heating and power production there is also a seasonal variation with higher emission rates during wintertime.
There are several institutions that gather emission data and transform them into emission inventories describing the spatial distribution of emissions over large areas. Concerning NH3 and NOx important data providers are:
If the emitted nitrogen compounds were to deposit at the location where they are emitted, it would be easy to determine which of the ecosystems are likely to receive high amounts of nutrients from the atmosphere. Unfortunately the processes are more complex than this. NH3 and NOx transforms into other chemical species that have very long lifetimes in the atmosphere and therefore can be transported across rather long distances, in some cases up to several thousand kilometers.
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