Fate in the Environment and Long-Range Atmospheric Transport of the Organophosphorus Insecticide, Chlorpyrifos and Its Oxon - Reviews of Environmental Contamination and Toxicology - page 42

partition coefficient (K OA ) can be used to determine partitioning from air to aerosol
particles. Its value is not used directly, but is estimated from the ratio K OW /K AW ;
however, its relatively low value proves to be less important because monitoring
data confirm that CPY does not partition appreciably to aerosol particles in the envi-
ronment (Yao et al. 2008 ) or indoors (Weschler and Nazaroff 2010 ).
From the perspective of LRT, the single most important parameter determining
concentrations at any given location and the distance that a chemical can be trans-
ported, is transformation half-life in the atmosphere. Results of a study of the
atmospheric chemistry of CPY and CPYO at the EUPHORE experimental facility
in Spain have been reported showing that the principal process that transforms
CPYintheatmosphereisreactionwith•OHradicals,althoughtherearealsocon-
tributions from direct photolysis and reactions with ozone and nitrate radicals
(Muñoz et al. 2012 ). In that study, a second-order rate constant for transformation
of 9.1 × 10 −11 cm 3 molecules −1 s −1 was determined. Combining that second order
rate constant with a concentration of 1.5 × 10 6 •OH molecules cm −3 gives a first
order rate constant of 13.6 × 10 −5 s −1 which corresponds to a half-life of 1.4 h. Half-
lives of CPY, thus depend directly on the assumed concentration of •OH. For
CPYO, the corresponding rate constant is less certain (0.8-2.4 × 10 −11 cm 3 mole-
cules −1 s −1 ) and was estimated to be a factor of approximately 5.5 slower.
Experimental results indicated a 10-30% yield of CPYO from transformation of
CPY, which is judged to be relatively small, given the absence of significant yields
of other transformation products.
In their assessment of LRT, Muir et al. ( 2004 ) used the AOPWIN, structure activ-
ity (SAR) program to predict a second-order rate constant for CPY of 9.17 × 10 −11 cm 3
molecules −1 s −1 , a value almost identical to that estimated by Muñoz et al. ( 2012 ).
Muiretal.usedamoreconservativeconcentrationof•OHthatistenfoldless,which
yielded an estimated half-life of 14 h (Muir et al. 2004 ). The lesser concentration of
•OH was selected to account for concentrations of •OH likely to occur in more
remote regions and at higher latitudes, for example in Canada. Global concentra-
tionsof•OHhavebeencompiledandaconcentrationof0.9×10 6 •OHmolecules
cm -3 was reported for April in the Central Valley of California and increasing to
1.46 × 10 6 in July and decreasing to 0.63 × 10 6 in October (Spivakovsky et al. 2000 ).
AtthelatitudeofIowa,USA,concentrationsof•OHinsummerwereapproximately
80-85% of the concentrations observed in California. In the assessment of LRT
reported here, atmospheric half-lives of 3 and 12 h were selected as being reason-
able and conservative daily averages for CPY and CPYO, respectively. The actual
half-lives of CPY could be a factor of two shorter, especially during midsummer
daylight hours and polluted conditions when concentrations of •OH are greater.
Monitoring data suggest that CPYO might have a shorter half-life. Half-lives, based
on experimental data for CPY-methyl (CPY-methyl), have been reported to be in the
rangeof3.5hforreactionsbetweenCPY-methyland•OH,15hfordirectphotoly-
sis, >8 d for reactions with ozone (O 3 ) and a half-life of 20 d for transformation of
CPY-methyl through reactions with nitrate radicals (Munoz et al. 2011 ). Given the
structural similarity between CPY and CPY-methyl, it is likely that similar propor-
tions apply to both substances for reactions in the atmosphere, but not necessarily in
other media such as rainwater and surface water where rates are pH-dependent.