Pesticide Residues in the Atmosphere - Pesticides: Evaluation of Environmental Pollution - page 223

where J(pest), k and are the rate constants for the photolysis,

reaction with ozone, hydroxyl radical, and nitrate radical for each pesticide, respectively, and
[O 3] , [OH ] , and [NO ] are the concentration values of these compounds previously specified.
Despite its importance, there are few studies on pesticide photodegradation in air
(Atkinson et al. 1999). More studies on the photodegradation or photolysis of pesticides
in liquid phase are available, as for methanol, hexane, or water, or on soil or leaf surfaces,
as shown in other chapters of this topic (i.e., Barceló et al. 1993; Bavcon Kralja et al. 2007;
Burkhard and Guth 1979; Chukwudebe et al. 1989; Floesser-Mueller and Schwack 2001).
Some examples of the degradation of pesticides in air are the studies on the degradation
of chloropicrin (Vera et al. 2010), chlorpyrifos (Hebert et al. 2000a,b), chlorpyrifos-methyl
(Muñoz et al. 2011), dichlorvos (Feigenbrugel et al. 2006), trifluraline (Le Person et al. 2007;
Hebert et al. 2000a), tertbuthylazine (Palm et al. 1997), chlorotoluron, and isoproturon
(Millet and Zetzsch 1998). In general terms, modern pesticides have lifetimes in air some-
where between 15 min and 2 days, whereas organoclorine pesticides have lifetimes of
more than 2 days. Table 7.4 shows atmospheric lifetimes of selected pesticides in gas phase.
In most of these cases, the values of the constants are estimated using the structure-
activity relationship (SAR) developed by Kwok and Atkinson (1995; EPIweb).
Because degradation products are expected to have considerably longer atmospheric
lifetimes than pesticides, they will undergo LRT and their oxidation products will thus
merit investigation. To date, degradation products are not well determined for a large
number of pesticides; however, there are batches of pesticides for which degradation prod-
ucts are well known. This is the case of 1,1-Dichloro-2,2-bis(4-chlorophenyl)ethene and
2-(2-Chlorophenyl)-2-(4-chlorophenyl)-1,1-dichloroethene (DDEs), as products of the deg-
radation of DDTs (e.g., Daly et al. 2007); phosgene, from the photolysis of chloropicrin (Vera
et al. 2010) or from the photoxidation of dichlorvos (Feigenbrugel et al. 2006); and oxones,
from the organothiophosphate pesticides (Muñoz et al. 2011), among others. Nevertheless,
more studies focused on the degradation products of pesticides in air and their implica-
tions for human health and the environment are advisable.

( pest k ), ( pest ), k ( VOC )

O OH NO

3 3

TABLE 7.4
Lifetime of Selected Pesticides, Considering the Average 12 h Daytime Concentration of OH
Radicals 2 × 10 6 Molecule cm 3

Main Degradation
Process in Gas Phase Pesticide Lifetime References

Chloropicrin 5.4 h Photolysis Vera et al. (2010)

Chlorpyrifos 2 h Photolysis and OH

reaction

Hebert et al. (2000a), Hebert et al.
(2000b)

Chlorpyrifos-methyl 3.5 h OH reaction Muñoz et al. (2011)

Dichlorvos 6 h OH reaction Feigenbrugel et al. (2006)

α-hexachlorocyclohexane 41 days OH reaction Brubaker and Hites (1998)

Lindane 304 days OH reaction Brubaker and Hites (1998)

Malathion 1.8 h OH reaction Estimated from SAR method (Kwok

and Atkinson 1995, EPIweb)

Propachlor 2.4 h OH reaction Estimated from SAR method (Kwok

and Atkinson 1995, EPIweb)

Trifluralin 15 min Photolysis Le Person et al. (2007)

Source: From Prinn, R. G., Huang, J., Weiss, R. F., Cunnold, D. M., Fraser, P. J., Simmonds, P. G., McCulloch, A.,

Harth, C., Salameh, P., O'Doherty, S., Wang, R. H. J., Porter, L., Miller, B. R. 2001. Evidence for substantial
variations of atmospheric hydroxyl radicals in the past two decades. Science 292: 1882-1888.