Pesticides in the Asian countries

In comparison with developed countries, some Asian countries have emerged as large users of pesticides, less so in the case of fungicides and herbicides. Total sale of pesticides in the Asian region has grown from US 5.0 billion in 1983 to almost 

0 billion in 1998 and the amount is projected to reach US 10.3 billion in 2003 (ESCAP, 2000). The decrease in pesticide application at the end of the 1980s has been reversed to constant growth in 1990s (Table 1). 

During dry season, the formation of soil cracks, macro- and micropores is common in the Asian countries with monsoon climate. This process is especially important in 

Drift and volatilization are important factors for large-scale transport from the treated plots. Whereas the range of drift of pesticide in droplets is limited to adjacent ecosystems, volatilization (i.e., vaporization) may lead to a large-scale aerial transport. The physics of spray drift involves Stake s law for the derivation of sedimentation velocities of droplets. 

In recent studies, pesticides such as atrazine have been found in precipitation. Therefore volatilization and subsequent transport in the gaseous phase is an important environmental pathway. Vaporization rates of pesticides deposited on surface of soil and plant leaves depend on the physical-chemical properties of the substance. A useful physicochemical criterion is Henry s constant, Ku, which is defined as the equilibrium air-to-water partial pressure ratio of the substance (see Chapter 7). 

As an example, we can compare the solubility of lindane and 2,4-D pesticides with respect to their major loss pathways. The Kh values of these pesticides are 1.33 x 10 andS.5 x 10 mol L , respectively. In addition, the retardation coefficientor degree of sorption of lindane is much higher than that of 2,4-D. Therefore, 2,4-D is more likely to be leached, whereas lindane is more likely to remain near the soil surface from which it can vaporize. Volatilization is thus the major pathway of lindane and degradation and leaching are the major loss pathways of 2,4-D in the Asian environments. 

The volatilization rate at the surface is further influenced by temperature and by the thickness of the stagnant air layer over the surface. Furthermore, random micromete-orological conditions of the crop stand are also very important (turbulence, thickness of boundary layer, humidity, and wind velocity). Special models are available to calculate the volatilization and vaporization rates of pesticides (Richter, 1999). 

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