Woo 900 800 700 600 500 400 300 200 100

Wet deposition (D. ordinate at right) and concentration (C: ordinate at left) of sulfate ions in precipitation as a function of precipitation quantity (P)

On average, the data show an inverse relation between the concentration (C) of different components and the precipitation quantity (P) during the sampling time. This fact is illustrated by curve C of Fig. 45, constructed on the basis of sulfate ion analyses of monthly precipitation samples collected in Hungary (Kozak and Meszaros, 1971). The relation between C and P was calculated separately for the summer and winter half-years. However, results for the two half-years do not differ significantly. A more detailed analysis of the data showed that the minimum concentration values in a given precipitation quantity interval are independent of P, which means that the character of the curve is determined by the relation between maximum concentrations and precipitation amount. This means that with increasing precipitation amount the scatter of the data decreases. This phenomenon was first demonstrated by Georgii and Weber (see Junge, 1963). Junge (1963)

3 Concerning details of chemical analyses the interested reader is referred to the original papers cited.

speculates that the greater data scatter in the case of smaller precipitation amounts is caused by differences in wash-out and evaporation below the cloud base. On the other hand, the composition of precipitation water in the case of greater precipitation quantity is determined by rain-out processes taking place inside the clouds.

Precipitation amount is normally reported in millimeters. One millimeter of precipitation means that 1 liter of water fell on 1 m2 of horizontal surface. Thus, if we multiply the concentration, expressed in mg 1 ~1, by the precipitation quantity in mm, we obtain the mass of trace constituents in mg deposited on 1 m2 of the ground during the sampling time. The value of this parameter, termed the wet deposition, increases linearly with increasing precipitation quantity (see curve D in Fig. 45).

The study of the chemical composition of precipitation is of interest for the evaluation of many scientific and practical problems. Thus, a knowledge of the chemical composition makes it possible to evaluate the degree of air pollution, the sink terms in the atmospheric cycle of aerosol particles and water soluble gases, the corrosiveness of atmospheric waters and the effect of precipitation on the material balance of soils, waters and vegetation. For these reasons, many precipitation analyses have been done, mostly in the last three decades. In this book only those works are mentioned in which an extensive network were (or are) operated, as in the American, Swedish and Soviet programs.

We have seen that the trace constituents of precipitation are principally soluble acids and salts. Furthermore, it has been pointed out that, under tropospheric background conditions, water-soluble components are predominant in the particulate matter. For this reason we shall restrict our discussion to the ions found in precipitation water, including hydrogen ion.

5.42 Americas measure meats

The first network to collect precipitation samples for subsequent chemical analyses all over the United States of America was organized by Junge and Werby (1958). More recently researchers at the National Center for Atmospheric Research4 have published a compilation of data obtained between 1960 and 1966 by means of a network consisting of 33 stations (Lodge et ai, 1968). At the stations, situated mostly in locally non-polluted rural environments, automatic rain gauges were used.

The aim of this program has been:

(a) to determine the spatial distribution of the composition over the country;

(b) to study the trend of concentrations during a longer time;

(c) to reveal the source distribution of trace constituents on the basis of precipitation chemistry data

* The National Center for Atmospheric Research is sponsored by the National Science Foundation, U.S.A.

The spatial distribution of the concentration of sulfate ions is reproduced in Fig. 46. One can see that the concentrations are higher over industrialized northeast territories as well as over arid areas of west and mid-west. It is speculated

Fig. 46

Average concent ration of sulfate in precipitation over the U.S.A. according to Lodge et at. (1968). Values are expressed in ppmm. (By courtesy of the National Center for Atmospheric Research)

Fig. 46

Average concent ration of sulfate in precipitation over the U.S.A. according to Lodge et at. (1968). Values are expressed in ppmm. (By courtesy of the National Center for Atmospheric Research)

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