Pesticide Residues in the Atmosphere - Pesticides: Evaluation of Environmental Pollution - page 213

for fenpropimorph and parathion-methyl (Stork et al. 1998). These authors observed that
volatilization started immediately after the application of the pesticides at relatively high
rates and decreased considerably after the first few days, following a diurnal profile. They
also observed that the volatilization rate was strongly dependent on the soil moisture. In
addition, unknown metabolites in the air samples led them to conclude that other pro-
cesses, such as photolysis reactions on plant or soil surfaces, could play an important role in
the dissipation of the pesticides in the environment. Another field study (Leistra et al. 2006),
which included a characterization of the meteorological conditions, estimated the cumula-
tive volatilization of chlorpyrifos from a potato crop during daylight hours to be 65% of the
dosage. This was reduced to 7% for fenpropimorph because other competing processes on
the plant surface reduced its dissipation for volatilization during daylight hours.
In a study carried out in Washington by Ramaprasad et al. (2004), the air concentrations
of methamidophos were higher in the afternoon when the temperatures were higher and
the vapor pressure increased. These authors concluded that volatilization from fields is not
a linear process, even though the most intense volatilization occurs immediately after the
spraying. Nevertheless, postspray volatilization could release important emissions into
the air during the 30 days after the spraying. This fact should be taken into account due to
its associated health risks, mainly because people do not usually think that they are at a
risk of pesticide exposure after the spraying has already been carried out.
The FOCUS working group concluded by saying that plant volatilization is up to three
times higher than soil volatilization under similar conditions. On the other hand, the
many complex interactions that take place on plant surfaces, involving variations in vapor
pressure, climatic conditions, Henry's law constants, and formulation, make volatilization
from plants highly variable. Finally, as these interactions are not fully understood, there
are no mathematical models available to describe the phenomenon (FOCUS 2008).
7.3.4 Volatilization from Water
Vapor pressure, water solubility, and Henry's law constant dominate the volatilization of
pesticides from water. The physical conditions of the water carrying the pesticide, such
as temperature or turbulence, are also important (Bidleman 1999). However, the water-
air exchange of pesticides has not been extensively studied. A few studies on flooded
rice fields were performed in the late 1980s by the California Department of Pesticide
Regulation (e.g., Siebers and McChesney 1988).
7.4 Assessment of the Concentrations of Pesticides in the Atmosphere
Once the pesticide is emitted to the atmosphere, it can be moved by wind, fog, and rain.
Depending on the meteorological conditions as well as the physicochemical properties of
the pesticide, it can be transported to either long distances (if it is persistent) or short dis-
tances and then react with other pollutants present in the atmosphere, that is, OH radicals,
ozone, VOCs, etc.
Nowadays, increasing amount of information is available on the monitoring of pesticides
in the atmosphere. Up to a few years ago, the largest amount of data had been collected about
pesticides in rainwater; however, in the last years, several studies dealing with pesticides in
air—in the gas phase and/or particle phase—have also been published (see Table 7.2 ; e.g.,