Pesticide Residues in the Atmosphere - Pesticides: Evaluation of Environmental Pollution - page 204

When a pesticide is applied, a fraction of the dosage can enter the different environmental
compartments: the soil, the surface waters and groundwater, the target plant, and also the
atmosphere (Bedos et al. 2002). The atmosphere represents the largest environmental com-
partment. At the same time, due to the characteristics and properties of the atmosphere, gas-
phase compounds and particles can either be released directly or undergo different kinds
of atmospheric removal processes. These atmospheric removal processes include enhanced
transport (at the local or large scale), photochemical reactions with other pollutants in the
atmosphere, for example, nitrogen oxides (NO x ) or volatile organic compounds (VOCs), and
physical deposition mechanisms. These processes are affected by the meteorological condi-
tions as well as the physicochemical properties of the pesticides (Foreman et al. 2000).
Pesticides can enter the air by means of spray drift during their application, wind ero-
sion, or volatilization from plants, soils, or surface waters. Some of the volatilization can
take place immediately after the application (Van den Berg et al. 1999). Another part
of the volatilization from the treated surface can occur sometime after the application.
Postapplication emissions represent an important means of input into the troposphere for
days or even weeks after the application. In general terms, pesticides are considered low-
volatile or semivolatile compounds (only fumigants are considered volatile compounds),
and they can therefore be distributed into gas, aqueous, and solid phases. The distribu-
tion between the different phases is related to their physicochemical properties as well
as to other factors, such as the manner in which the pesticide is applied or the meteoro-
logical conditions during the application. The main factor that governs the distribution of
semivolatile compounds between the gas phase and the particle phase is vapor pressure
(Bidleman 1999; Bedos et al. 2002). Vapor pressures higher than 10 −2 Pa imply that the
compounds are mainly observed in the vapor phase, whereas vapor pressures lower than
10 −5 Pa are almost always present in the particle phase. Bearing in mind that most pesti-
cides have vapor pressures in between these values, their presence in the atmosphere is
also partitioned between gas and particle phases.
Once a pesticide enters into the atmosphere, it tends to become well-mixed and dis-
persed throughout the surface boundary layer. Depending on its time of residence in the
atmosphere, a pesticide will be able to travel short or long distances. At the same time,
meteorological factors also play an important role in this issue.
Although the main route of pesticide removal from air is considered to be wet or dry
deposition, depending on the compound (e.g., Asman et al. 2003), other removal mecha-
nisms are also important, for example, the chemical reactions that can occur in the atmo-
sphere as well as other processes that take place because the chemical changes do not
always result in the detoxification of the initial compound. Regarding the gas phase, it is
important to take into consideration the presence of oxidants such as ozone and hydroxyl
radicals and the action of reactive intermediates such as NO x . All these reactions enhance
the photochemical conversion of the pesticides. At the same time, photolysis is also impor-
tant in some cases. Among all these chemical reactions, the main loss process for a large
number of pesticides is the reaction with hydroxyl radicals (OH) (e.g., Atkinson et al. 1999).
7.2 The Atmosphere
The atmosphere is a dynamic complex system, which is in seasonal equilibrium with the
vacuum of outer space, the gravitational and centripetal forces of the Earth, and the energy