Environmental Engineering Reference
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physicochemical properties such as equilibrium vapor pressures and Henry's law constants
(Seiber 2002) and environmental conditions (e.g., temperature, wind direction, height
of cloud base, etc.; Tsal and Cohen 1991; Wania et al. 1998). The processes of pesticide
removal from the atmosphere will be different for the different phases. As most pesticides
are considered semivolatile, they could exist in both the gas phase and the particle phase
(Majewski and Capel 1995; Bidleman 1998).
The main route for the removal of pesticides in air is wet and/or dry deposition; how-
ever, the chemical reactions that can occur in the atmosphere must also be considered
because, contrary to what most people think, chemical changes do not always result in the
detoxification of the pesticide compounds.
Moreover, the fact that pesticides can be transported or dispersed at regional or LRT
scale depending on their properties needs to be taken into account as well.
Recent pesticide measurements in Southern Africa and the subtropics (Shunthirasingham
et al. 2010) show behaviors that seem to be influenced mainly by the patterns of use and
not by meteorological factors or historical uses. By contrast, in northern countries (United
States, Canada, Northern Europe), the measured pesticide concentrations are more influ-
enced by meteorological factors than by other factors (i.e., Venier and Hites 2010). This is
due to dispersion processes and the long- and medium-range transport of the pesticides
from the south to the north. In these cases, more seasonality is observed.
7.5.1  Degradation in Air
The gas-phase degradation of pesticides in the atmosphere may be controlled by photolysis
and/or reaction with ozone, OH, and NO 3 radicals (Atkinson 1994; Atkinson et al. 1999;
Woodrow et al. 1983; Finlayson-Pitts and Pitts 1986; Seinfeld 1986).
Photolysis is a chemical process by which molecules are broken down into smaller units
through the absorption of sunlight.
Ozone arrives in the troposphere by diffusion from the stratosphere, and it is also
formed in the troposphere from the interaction of VOCs and NO x in the presence of sun-
light (Logan 1985; Roelofs and Lelieveld 1997). The 24 h average atmospheric concentration
of ozone is 7 × 10 11 molecule cm 3 (Logan 1985).
The OH radical is the most important reactive species in the troposphere; it reacts with all
organic compounds, including pesticides. It is formed during daylight hours mainly by the
photolysis of O 3 in the presence of water, although other sources of OH are also important,
such as the photolysis of nitrous acid (HONO), the photolysis of formaldehyde and other
carbonyls in the presence of NO, and the reactions of alkenes with O 3 (Atkinson 2000). The
OH radical can also be formed during nighttime from the last-mentioned source. The aver-
age 12 h daytime concentration of OH radicals is 2 × 10 6 molecule cm 3 (Prinn et al. 2001).
NO 3 is formed in the troposphere by the reaction of NO 2 with O 3 . Nevertheless, as it
photolyses rapidly, its possible reactions are only important during nighttime. The average
nighttime concentration of NO 3 radicals is 5 × 10 8 molecule cm 3 (Atkinson 1991).
A parameter widely used by atmospheric chemists to parameterize the persistence of a
pollutant in air is atmospheric lifetime (τ). Atmospheric lifetime is the ratio between the
time needed for a pesticide to disappear and the time 1/e of its initial value (e is the base
of natural logarithms 2.718). The lifetime for each pesticide present in the gas phase due to
gaseous removal can be derived from the expression (Equation 7.1)
(
(
) +
(
)
(
)
(
) [
)
]
τ = 1 J pest
1 k
pest [O ] 1 k
+
pest [OH] 1 k
+
pest NO
(7.1)
O
3
OH
NO
3
3
3
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