Physical Transformations of Trace Substances in the Atmosphere

Perhaps because the unpolluted atmosphere can appear to be perfectly free of turbidity, it is not immediately obvious that it is a mixture of solid, gaseous, and liquid phases - even in the absence of clouds. Particles in the aerosol state constitute only a miniscule portion of the mass of the atmosphere - perhaps 10 or 10 ° in im-polluted cases. However, the condensed phases are important intermediates in the cycles of numerous elements, notably ammonia-N, suT 

Because the particles in the accumulation mode are very small (most of them have diameters less than 1 pm when dry), they have very small fall speeds (a 1 /im sphere of unit density has a fall speed of about 10 cm/s). Thus, they are only removed in any quantity by the formation of clouds with subsequent precipitation. 

This brief description leads to Fig. 7-13 which depicts the physical transformations of trace substances that occur in the atmosphere. These physical transformations can be compared to the respective chemical transformations within the context of the individual elemental cycles (e.g., sulfur). This comparison suggests that the overall lifetime of some species in the atmosphere can be governed by the chemical reaction rates, while others are governed by these physical processes. 

They are also absolutely necessary participants in the hydrologic cycle (see Sections 10.7 and 10.11). 

Aerosols are solid or liquid particles, suspended in the liquid state, that have stability to gravitational separation over a period of observation. Slow coagulation is implied. 

This brief description leads to Fig. 10-14, which depicts the physical transformations of trace substances that occur in the atmosphere. These physical transformations can be compared to the respective chemical transformations within the context of the 

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Fig. 7-13 Physical transformations of trace substances in the atmosphere. Each box represents a physically and chemically definable entity. The transformations are given in F, (from the ith to the /th box). Q, represents sources contributing to the mass or burden, M,> in the ith box. Rd, and Rw, are dry and wet removals from M,. The dashed box represents what may be called the fine-particle aerosol and could be a single box instead of the set of four sub-boxes (i = 1,2,3,4). The physical transformations are as follows ...

The aim of this book is, first of all, to present the atmospheric cycle of the trace constituents. We will discuss in more detail the trace substances (Chapter 3) with relatively short residence time (<10 yr). The study of these compounds is particularly interesting since their sources and sinks as well as their concentrations are very variable in space and time. They undergo several physical and chemical transformations in the atmosphere. Among these transformations the processes leading to the formation of aerosol particles have unique importance. The aerosol particles control the optical properties of the air, the formation of clouds and precipitation and, together with some gases, the radiation and heat balance of the Earth-atmosphere system. Because of their importance the physical and chemical characteristics of aerosol particles will be summarized in a separate chapter (see Chapter 4). 

The chemical composition of air depends on the natural and man-made sources of the constituents (their distribution and source strength in time and space) as well the physical (e. g. radiation, temperature, humidity, wind) and chemical conditions (other trace species) which determine transportation and transformation. Thus, atmospheric chemistry is not a pure chemistry and also includes other disciplines which are important for describing the interaction between atmosphere and other surrounding reservoirs (biosphere, hydrosphere, etc.). Measurements of chemical and physical parameters in air will always contain a geographical component, i. e., the particularities of the locality. That is why the terms chemical weather and chemical climate have been introduced. For example, diurnal variation of the concentration of a substance may occur for different reasons. Therefore general conclusions or transfer of results to other sites should be done with care. On the other hand, it is a basic task in atmospheric chemistry not only to present local results of chemical composition and its variation in time, but also to find general relationships between pollutants and their behavior under different conditions. 

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