Deposition of Pollutants on Plants and Soils; Principles and Pathways

Summary

There is considerable variation in the “pollution climate” in Europe, i.e. in the mixtures of pollutants found in various regions, and further monitoring of pollution climates is necessary to define relationships between deposition and effects of pollutants. Knowledge of rainfall chemistry has improved in recent years so that maps of rain acidity in Europe can be prepared, but the relative contributions from natural and anthropogenic sulphur are still unclear. Nitrate in rain has increased since the 1850’s and is associated with a greater proportion of the acidity than in the past. Nitrogen deposition is also important because of its role in plant nutrition. Rainfall chemistry is strongly episodic. It is becoming clear that wet deposition in mountainous regions may be larger than previously estimated because of orographic enhancement and droplet scavenging. “Occult deposition”, the deposition of soluble and insoluble materials in wind-driven cloud is also significant but has seldom been measured. Routine monitoring of cloud water chemistry is necessary as a first step, but is not sufficient for reliable occult deposition estimates. Rates of dry deposition are often strongly controlled by surface factors. Further dry deposition studies are needed for nitrogen-containing gases and for particles. Once deposited, pollutants may react with plant surfaces, they may be absorbed, and ion exchange may occur. These processes need to be studied in association with deposition measurement to improve our knowledge of the pathways by which pollutants move through ecosystems.