Deposition

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This article covers general atmospheric deposition. For other meanings of the word deposition, see Deposition (disambiguation)

Under atmospheric deposition material flows from the are earth's atmosphere to the earth's surface understood, that is, the discharge and deposition of dissolved, particle-bound or gaseous substances in the air on surfaces (acceptors) of biotic or abiotic systems. Biotic acceptors are the aboveground parts of plants, especially the leaves and needles. Abiotic acceptors can be the soil, surface water, snow cover or structures (roofs, streets, etc.).

Deposition principles

Atmospheric deposition is a complex process consisting of many individual mechanisms. Larger particles and drops follow gravity and sediment. Smaller floating particles are deposited on surfaces with the turbulence of the air currents (impaction). Gases are dissolved on moist surfaces or adsorbed on dry surfaces.

Dry atmospheric deposition is the discharge of substances through solid particles and small liquid particles ( mist and cloud droplets ) as well as gases from the atmosphere including their deposition on acceptor surfaces. The process of discharging and depositing substances through small liquid particles (droplets) is also referred to separately as moist atmospheric deposition . Wet atmospheric deposition is the discharge of dissolved and undissolved substances (adhering to particles) through aqueous precipitation such as rain , snow and hail .

A special feature of the wet deposition is the interception deposition . This is the substance load that reaches the ground with the precipitation that drips off the plants (in the case of trees: canopy water). Above-ground parts of plants, especially leaves and needles in forests, represent an important acceptor surface even in dry periods or during foggy events (combing-out effect, fog eaves ). In the event of precipitation, the substances previously deposited dry and in the meantime partly metabolized can be washed off. As a result, the wet deposition rates on the ground are generally significantly higher in forests, especially in coniferous forests that are not bare of winter, compared to the field.

Global differences

The discharge of substances from the atmosphere by deposition is not a completely irreversible process. Part of the deposited amount of substance can evaporate back into the atmosphere ( re-emission ). Because the reemission rate is higher in warmer climates than in cold regions and the microbiological activity also increases with temperature, long-lived re-emitted substances accumulate in the polar regions over time ( grasshopper effect ).

Measurement of deposition rates

The deposition is measured with deposition collectors , which are basically open-topped pots or funnels with a collecting vessel. To measure the total deposition, the collecting unit is open to the atmosphere throughout the entire collection period ( bulk collector ). For the separate determination of wet and dry deposition, the collectors have a sensor to detect precipitation events, which controls the automatic opening and closing of a shutter above the collecting unit ( wet-only collector or dry-only collector ).

The deposition rate can be calculated from the collection area, amount of precipitation and substance concentration . This is the amount of substance deposited per unit of time and area. Usual units are (milli-, micro-) grams per square meter and day or (kilo-) grams per hectare and year.

Substantial amounts of substance from the atmosphere are carried onto and into the soil via the deposition. This also includes air pollutants . For certain pollutants (e.g. POP ), deposition is the only cause of their ubiquitous distribution in soils. The measurement, knowledge and reduction of the deposition rates is therefore an important part of environmental protection , especially soil protection and forest science .