Eutrophication -- 2.4 Transformation

All chemical components which constitute nutrients in the ecosystem are reactive in one way or the other in the atmosphere. The chemistry is taking place on very different time scales and is highly complex. It is crucial to include the most imporant chemical transformations in the pathway description if the contributions of nutrients are to be described as accurately as possible.

In the chemistry of air pollution one distinguishes between primary and secondary air pollutants. Roughly it can be said that primary pollutants are directly emitted from the sources and in the air they transform by chemical reactions into secondary pollutants which are subsequently deposited. In the case of nitrogen, the primary pollutants are NH₃, NO and NO₂ and the secondary pollutants are NO₂ and all the other nitrogen containing components that are produced from the primary pollutants in the air. Most important reaction products for the nutrient deposition are components containing nitrate (NO₃^-) or ammonium (NH₄⁺) in particulate phase.

In Figure 5 the pathways for emission and deposition of NH₃ and NH₄⁺ are shown. NH₃ is emitted in agricultural processes and is then present in the atmosphere in gaseous form. NH₃ quickly dry deposits close to the source, however some of it is also transformed to NH₄⁺ in particulate form. Particulates have a long lifetime in the atmosphere (≈one week) and can therefore be transported across long distances. The most important removal pathway for NH₄⁺ is wet deposition.

LiseFig5

Figure: Pathway for ammonia and ammonium in the atmosphere from emission sources to final deposition. Ammonia is deposited close to the source, whereas ammonium can be transported across long distances before subsequent deposition occurs.

In Figure 6 the corresponding pathways for emission and deposition of NO_x and NO₃^- are shown. NO_x is emitted in combustion processes and is then present in the atmosphere in the form of gaseous NO and NO₂. NO is slowly transforming into NO₂ by reacting with oxygen in the atmosphere (this is why NO₂ is both a primary and a secondary component), and some of the NO₂ dry deposits close to the source. Some of the NO₂ and NO transforms into nitric acid (HNO₃) which subsequently transforms into NO₃^- in particulate form. As for ammonium the lifetime of particulate nitrate can be quite long and the most important removal pathway is also wet deposition.

LiseFig6

Figure: Pathway for nitrogen oxides and nitrate in the atmopshere from emission sources to final deposition. Nitrogen dioxide and nitric acid are deposited close to the source, whereas nitrate can be transported across long distances before subsequent deposition occurs.

There are many chemical components in the atmosphere, the total number has been estimated to be around 3000. The majority is reactive and the approximate number of different reactions taking place between these approximately 3000 species is ca 20,000. Reaction rates typically depend on temperature, pressure, sun angle and concentration of the chemical components. Due to this complexity of the chemical atmosphere, it is not straightforward to predict what will happen in terms of chemical reactions with the emitted nitrogen before it deposits again somewhere else.