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Trends in acidic pollution

Anne Bourin (IMT Lille Douai) takes the floor

Are we seeing the full benefits of reduced air pollution in atmospheric deposition levels for sulphur and nitrogen in France?

Summary

Air pollution is one of the main ways human activities modify global environmental conditions. In particular, the acid rain crisis, which roused the spectre of a generalized forest decline in the 1980s, brought to light the effect of long-distance pollutants from burning fossil fuels and from agricultural practices. The crisis pushed the international political community to put in place legislation to reduce the atmospheric emissions causing the damage (sulphur oxides, nitric oxides, ammonia) and to monitor their impacts on the environment.

In France, several complementary structures were put in place to monitor atmospheric deposition. Two networks, MERA (between 8 and 12 sites) and CATAENAT (27 sites included in the RENECOFOR network), whose sites are located away from the main sources of emissions, both measure background pollution for two European programmes (EMEP and ICP Forests). Though the methodologies used in these two networks respond to different objectives and do not provide comparable results in absolute terms, they do make it possible to independently detect relative tendencies and usefully complement each other when we wish to assess the efficiency of air pollution reduction programmes.

Between 1993 and 2015, both networks recorded a significant decrease in the direct acidity of atmospheric deposits, as reflected by an increase in pH (see Figure). Indeed, pH increased by +0.53% per year in the MERA network and by +0.34% per year in the CATAENAT network. This change is mostly due to decreased concentrations of sulphur in the form of sulphates (-3,12 % per year et -2,74 % per year in MERA and CATAENAT) and to a more moderate decline in nitrogen in the form of nitrates (-1,53 % per year et -1,21 % per year in MERA and CATAENAT). A decrease in nitrogen concentrations in the form of ammonium (-1,95 % per year et -1,86 % per year in MERA and CATAENAT) also indirectly contributed to reduced acidity in atmospheric deposits. On the other hand, a simultaneous reduction in calcium concentrations caused a non-negligible drop in nutrient content for the poorest soils. These trends were confirmed not only for open-air sites but also under forest canopy cover at the 14 CATAENAT sites.

Trends over time at the national scale for average annual pH and sulphur (sulphate), nitrogen (nitrates, ammonium) and calcium concentrations in atmospheric deposits at the MERA sites (in red, rainfall only) and the CATAENAT sites (blue, total deposits outside forest cover).
Trends over time at the national scale for average annual pH and sulphur (sulphate), nitrogen (nitrates, ammonium) and calcium concentrations in atmospheric deposits at the MERA sites (in red, rainfall only) and the CATAENAT sites (blue, total deposits outside forest cover). © Aude Bourin / IMT Lille Douai

The enforcement of new regulations designed to decrease sulphur oxide pollutants had a clear positive effect on sulphates in atmospheric deposits.


The situation for nitrogen deposition, however, is rather different; trends for nitrogen do not seem to have followed the reported decrease in emissions. The strong estimated decrease in nitrogen oxide emissions has not been entirely reflected in the atmospheric deposits of nitrates.

Meanwhile, ammonia emissions have remained stable, yet a decrease in ammonium deposits was detected.

What is more, spatial modelling based on emissions inventories does not reflect field observations for geographical variations in atmospheric nitrogen deposited in the form of ammonium. This discrepancy underlines the need for more research to better understand and more accurately simulate transport and transformation mechanisms of pollutants in the atmosphere. Therefore, continued monitoring of atmospheric deposits is necessary to evaluate the real impact of air pollutants on the environment and to define a strategy to control polluting emissions.

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