Transport and Deposition
The sulfur compounds are converted to sulfuric acid and NOx to nitric acid, and ammonia reacts with these acids to form ammonium salts. Both precursors (SO2, NOx, and NH3) and products (sulfuric acid, nitric acid, and ammonium salts) can be transported over long distances (up to 1500 km), depending on meteorological conditions, speed of conversion, and removal by deposition processes. The gases emitted, SO2, NOx, and NH3, behave differently in the atmosphere. The residence time of NH3 is relatively short because it is emitted at low level (near the ground), it converts quickly to NH4", and the dry deposition rate is fairly high. NH3 is transported 100-500 km. The horizontal and vertical concentration gradients are steep, so concentrations and depositions can vary significantly over small distances.
This means that a large proportion of the NH3 emitted in The Netherlands is also deposited within this country. Once converted into NH4, which has a much lower rate of deposition, the transport distances are much greater (up to 1500 km). SO2 is mainly emitted into the atmosphere by high sources and can therefore be transported over long distances, despite its relatively high deposition rate. Some NOx is also emitted by low sources (traffic). However, because of its low deposition rate and relatively slow conversion rate into rapidly deposited gases (HNO3 and HNO2), NOx is transported over relatively long distances before it disappears from the atmosphere. SO2 is quickly converted to sulfuric acid (H2SO4) after deposition, both in water and in soil. NOx and NH3 and their subsequent products contribute to the eutrophication and also the acidification of the environment as a result of conversion to nitric acid (HNO3) in the air (NOx) or in the soil (NH3).
Since the three primary gases (SO2, NOx, and NH3) can react and be in equilibrium with each other and with the different reaction products in the atmosphere, there is a strong and complex mutual relationship. If, for example, there were no NH3 in the atmosphere, SO2 would be converted less quickly to SO2~. The environment, however, would also be ‘‘more acid’’ so that the deposition rate of acidifying compounds would be reduced (poor solubility of these compounds in acid waters). The net impact is difficult to determine, but it is certainly true that if the emission of one of the compounds increases or decreases relative to that of the others, this will also influence the transport distances and deposition rates of the other compounds. This is only partly taken into account in the scenario calculations because such links have not been fully incorporated into the models.
The base cations can neutralize the acidifying deposition and, after deposition to the soil, they act as a buffer both in terms of neutralization and in terms of uptake by plants and the prevention of nutrient deficiencies. This applies especially to the Mg2 +, Ca2 +, and K+ compounds. The degree of deposition of base cations is therefore important in determining the critical loads that an ecosystem can bear or the exceedances thereof. Thus, accurate loads are also required for base cations on a local scale. These estimates are not available for The Netherlands nor for Europe.
The nature and size of the load of acidifying substances on the surface depend on the characteristics of the emission sources (height and point or diffuse source), the distance from the source, physical and chemical processes in the atmosphere, and the receptor type (land use, roughness, moisture status, degree of stomatal opening of vegetation, snow cover, etc.). When gases and/or particles are deposited or absorbed directly from the air, we speak of dry deposition. When they reach the surface dissolved in rain or another form of precipitation, we refer to wet deposition. If this occurs in mist or fog, it is cloud/fog deposition. The total deposition is the sum of the dry, wet, and cloud/fog deposition. Throughfall is washed from the vegetation by rain falling on the soil beneath the forest canopy. It is a measure of the load on the vegetation’s surface, whereas the total deposition indicates the load on the whole system (soil + vegetation). Net throughfall is the difference between throughfall and wet deposition in the open field; it is a measure of the dry deposition plus cloud/fog deposition when no exchange takes place with the forest canopy (uptake or discharge of substances through leaves, microorganisms, etc.).
To understand the impact of emissions on the effects caused by acid deposition and eutrophication, the whole chain of emissions, transport, atmospheric conversion, dry and wet deposition, and the effects caused by the total deposition loads, including the role of groundwater and soils, must be understood. Wet and dry deposition are not only the processes by which pollutants are transported to the earth surface and humans and ecosystems are exposed to acid deposition and eutrophication but also play an essential role in cleaning of the atmosphere. If removal by wet and dry deposition stopped, the earth’s atmosphere would be unsuitable to sustain life in a relatively short period—on the order of a few months.
Since acidifying deposition involves different substances, it is necessary to give these substances a single denominator in order to indicate the total load of acidifying substances. For this purpose, the total load is expressed as potential acid, calculated as follows:
2SOx + NOy + NHx(mol H+ha/year),
where SOx are oxidized sulfur compounds, NOy are oxidized nitrogen compounds, and NHx are reduced nitrogen compounds. The concept of potential acid is used because NH3 is considered to be a potentially acidifying substance. In the atmosphere, NH3 acts as a base, which leads to the neutralization of acids such as HNO3 and H2SO4. However, the NH4 formed in the soil can be converted to NO3 so that acid is still produced via bacterial conversion (nitrification)
NH+ + 2O2 - NO3 + H2O + 2H+.
Two moles of acid are finally formed via this process: one originating from the neutralized acid and one originating from NH3. On balance, as for 1 mol NOy 1mol NH3 acts maximally to acidify 1 mol H + acid. One mole of the bivalent SO4~ can lead to the formation of 2 mol of H +. The actual acidification depends on the degree to which NO3 and SO4~ leach out of the soil. Only when this occurs completely is the actual acidification equal to the potential acidification.