Once emitted into the atmosphere, pollutants do not stay there forever; they end up leaving the atmospheric compartment to contaminate soils, water and ecosystems. This is called fallout. Depending on the route of elimination from the atmosphere, there are two types of fallout: dry fallout in the form of gas or dust and wet fallout in the form of rain, snow or fog.
The sulphur and nitrogen compounds present in the air can be transformed into sulphuric (H2SO4) and nitric (HNO3) acids, thus giving the fallout an acidic nature. Acidification of the environment, better known as "acid rain", is a major environmental problem that primarily affects buildings and vegetation, and is responsible for the decline of forests.
In Wallonia, the share of wet deposits is determined from data from the rain network while the share of dry deposits, which is more difficult to understand, is evaluated via modelling.
Origin of pollution
In Wallonia, the main sources of acidifying pollutants are the transport sector (44%), followed by the industry (32%) and agriculture (11%, 2014 data, source AwAC). Emissions of acidifying pollutants decreased by 55% between 2000 and 2014. This decrease is due to several factors: lowering of the sulphur content of diesel and heavy fuel oil, increasing use of natural gas and catalytic converters, placing more efficient boilers on the market, closing certain polluting industries, etc.
The best-known effect of acid fallout is forest dieback, but the consequences of acidification are multiple: acidification of surface water with an influence on the flora and fauna they harbour, modification of soil properties (change in mobility of elements contained in the soil) with effects on the flora, acceleration of building erosion phenomena, etc. In Wallonia, the Ardennes region, where the soil has little limestone capable of neutralising acidity, is particularly fragile.
In addition to the role played in acidification, nitrogen fallout also has an impact on environmental eutrophication. The nitrogen supply via the fallout exceeds the absorption capacity of certain ecosystems and creates an imbalance by favouring certain species with a loss of biodiversity.
The situation in Wallonia
The impacts of the fallout of acidifying and eutrophicating pollutants depend, on the one hand, on the quantities deposited on the soils and vegetation and, on the other hand, on the sensitivity of the ecosystems, which is expressed by means of the critical load, defined as the maximum amount of atmospheric pollutant deposition that an ecosystem can absorb without long-term adverse effects.
Estimates reveal that the acidification problem is under control with 0.07% (2019) of forest areas affected by deposits above the critical load while other semi-natural ecosystems no longer showed exceedances.
The same is not true for the role of nitrogen in eutrophication. 6.32% (2019) of forest areas have exceedances. For other semi-natural ecosystems, in particular for environments that cannot support excess nitrogen (moors, marshes, peat bogs, etc.), the situation remains critical with up to 94.78% of the surface area exceeding the critical load.
The rain is collected using specific equipment: the rainwater is collected via a funnel connected to a bottle. In the absence of rain, the system is closed by a cover which thus protects against dust fallout (dry fallout). A detector controls the opening of this panel during periods of rain.
The collected water is then brought back to the laboratory, where the quantity of water, its conductivity, acidity and its acid-base ion content are determined via analysis using ion chromatography.