Source characteristics

Natural sources of atmospheric components are principally distinguished into bio-genic and abiogenic (or geogenic). They may relate to biological processes in the soil (e. g., bacteria), in vegetation (plants), in the oceans (substances produced by algae) and non-biological processes in the earth's crust (volcanoes or gas/oil seepage), or in the atmosphere (lightning), but also simply in dispersion of particles by wind-surface interaction. These emissions often depend strongly on climate, soil or water characteristics, and thereby show a strong temporal variation in seasonality, diurnal cycle or both. Soils acts as source as well as sink; the direction of the flux is changing with the compensation point. Thus, soil biological activities (as found for CoS by Conrad and Meuser 2000) may depend from the ambient concentration, stressing the necessity to measure fluxes as function of concentration to obtain reliable source or sink data for atmospheric budgets.

Thus, natural sources often have a distinctly different character than anthropogenic sources, which are comparably constant with respect to seasonality. In addition, the source strength as well as the spatial distribution of natural sources may differ substantially from year to year. An extreme example is volcanic emission.

Abiogenic sources can be distinguished as physical processes (dispersed soil dust and sea spray), geochemical processes (gaseous emanations, volcanic eruptions), and atmospheric chemical processes (lightning causing nitrogen oxides). Biogenic processes in wild and agricultural plants are the same in principle. The main difference lies in the input of chemicals (mainly fertilizers) onto farmland with the purpose to control and to accelerate the plant growth. Cultivated soils produce soil dust by wind erosion: is that a contribution by anthropogenic or natural sources? Nonetheless, desert dust is globally the dominant source in this category. Changes in land use over hundreds or even thousands of years (for example, deforestation) have changed the quantity and the mix of VOC emissions from soils and vegetation. Biomass burning (wildfires) is nowadays almost all attributed to human activity, whether intentional or not; the only natural cause is lightning strikes, which are of minor importance globally.

We see that it becomes more and more difficult to distinguish clearly between natural and anthropogenic emissions. This book will not draw a sharp line - it is more important to understand different sources and the controlling parameters of the emissions. With our understanding that humans are part of the biosphere and have changed it in an evolutionary way into the noosphere, we have to consider the (climate) system as a whole. The only difference between natural and man-made emissions lies in the fact that man-made emissions are unbalanced in nature or, in other words, are in an open-loop. As a somewhat extravagant example, we could assert that it does not matter how one reduces the concentration of chlorine in the upper troposphere; whether by cleansing volcanic plumes of chlorine (if there were such a facility) or by banning its use. The effects are always based on the sum of molecules in the atmosphere. However, some sources or substances used without anticipating any environmental problem, due to small volume and/or chemical virtual harmlessness, such as halogenated and resistant organic compounds, and some heavy metals, have resulted in serious problems and required counteractions.

An emission (and deposition) represents a flux (mass per time). Concerning the source characteristics, we separate emissions from

1. point sources (volcanoes, fumaroles),

2. specific sources (plants, animals), and

3. diffuse sources (surfaces such as soils, lakes, landscapes etc.).

The flux can be related to an area (a standard area such as ha and m-2) and is then termed a specific emission. The matter state is

4. gaseous (volatile elements, molecules) or

5. solid (particles, see Chapter 4.3.5).

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