Pesticide Residues in the Atmosphere - Pesticides: Evaluation of Environmental Pollution - page 212

the volatilization of the pesticides used. They concluded that, with all other factors being
constant, when the temperature is higher, more volatilization occurs. On the other hand,
an increase in the soil moisture implies an increase in the gas-liquid distribution of the
pesticide and its diffusive transport. Finally, these authors pointed out that for pesticides
with vapor pressures in the range from 5 × 10 −3 to 5 × 10 −2 Pa, volatilization can represent
up to 22.6% of the total fate in the environment. This shows its important influence in the
air quality of the surrounding agricultural areas.
Despite the fact that, initially, vapor pressure could be considered the best parameter to
assess the volatilization of a compound, this factor is a measurement of the volatilization
of the pure compound in its condensed state. For this reason, Henry's law constant may
be a better indicator as it is a measure of the volatilization tendency of a pesticide from
dilute aqueous solutions (Unsworth et al. 1999). Taking into account that water is always
present in soils and on plant surfaces, even a pesticide with a low vapor pressure can have
an appreciable Henry's law constant and thus be subject to volatilization. Unsworth et al.
(1999 and references therein) pointed out that cultivation practices and formulations can
affect the extent of volatilization. In the same sense, soil characteristics also influence vola-
tilization. For example, dry soils reduce volatilization due to their higher adsorption of the
compounds when compared with moist surfaces. Van den Berg et al. (1999) also point out
that temperature and soil moisture conditions have an important influence on the volatil-
ization rate of pesticides at the soil surfaces. In a chamber study simulating field condi-
tions, Wolters et al. (2004) observed that the cumulative volatilization of parathion-methyl
ranged from 2.4% under dry conditions to nearly 33% under moist conditions.
7.3.3 Volatilization from Plants
Volatilization from plants usually exceeds volatilization from soils. Most losses occur
within the first few days after the application and follow a diurnal pattern, with the larg-
est losses at midday, when temperature and solar irradiation are higher. The main factors
influencing pesticide volatilization from crops and plants are physicochemical properties,
pesticide persistence on plant surfaces, and meteorological conditions during and some
days after the application (Van den Berg et al. 1999). Several studies showed the relation-
ship between the vapor pressure and the volatilization rate, mostly during the first 24 h
after the application (Woodrow and Seiber 1997; Smit et al. 1998). The information and data
on the postapplication volatilization of pesticides are mainly focused on the initial periods
after the application, and little is written about emissions further from the time of applica-
tion. Ramaprasad et al. (2004) pointed out that compounds with vapor pressures higher
than 9.75 × 10 −5 mmHg could sustain a total loss of 58% in the first 30 days after the appli-
cation, although this data is only an estimation as other degradation processes can also
participate in the loss of the pesticides (such as runoff, plant uptake, and soil adsorption).
Pesticide emission into the air depends on other factors as well, for example, molecular
interaction forces in the deposit and adsorption by plant foliage. The adsorption on the leaf
surface can be described by the octanol-water partitioning coefficient (K ow ). On the other
hand, pesticide persistence on leaves can suffer different kinds of dissipation processes
such as wash-off due to rain or photodegradation on the leaf surface.
Wolters et al. (2004) studied the volatilization of parathion-methyl, quinoxyfen, and fen-
propimorph from plants in a wind tunnel during a 10-day period. The highest volatilization,
29.2%, was observed for parathion-methyl. Quinoxyfen showed a volatilization of 15%, and
fenpropimorph had the lowest volatilization, with only 6%. In another experiment under
simulated field conditions in a wind tunnel, volatilization rates up to 50% were measured