Fate in the Environment and Long-Range Atmospheric Transport of the Organophosphorus Insecticide, Chlorpyrifos and Its Oxon - Reviews of Environmental Contamination and Toxicology - page 50

Fig. 3 Mass balance output from the TAPL3 LRT model. A 12 h (conservative) half life is
assumed for atmospheric degradation. The CTD is 244 km. Other parameters are as specified in
Tables 5 and 6 and include the upper range (conservative) of half lives in water, soil and sediment
in Table 6 . The model can be downloaded from www.trentu.ca/cemc
in air. If the half-life is increased by an arbitrary factor of 4-12 h as in Fig. 3 , the
CTD increases to 244 km.
In a similar study, Muir et al. ( 2004 ) estimated the CTD of CPY by using two
models (TAPL3 and ELPOS) and obtained values of 290 and 283 km for conditions
inwhichtheconcentrationof•OHradicalswassmaller,thusyieldingahalf-lifeof
14 h. Introducing intermittent rather than continuous precipitation had a negligible
effect on predicted concentrations. Their longer CTDs are entirely attributable to
their assumed longer half-life, which is a factor of 4.7 greater and is regarded as
very conservative, but might be more appropriate for conditions at higher latitudes
and during winter. Since the CTD is the distance over which the mass of chemical
decreases by a factor of e (2.718), at a distance of two CTDs, the mass would be
reduced by a factor of 7.4 and at three CTDs this factor is 20. Under the conditions
simulated for CPY, 5% of the initial mass would remain in air at a distance of
approximately 180 km if the half-life is assumed to be 3 h. If the half-life is increased
to 12 h, the fraction remaining at that distance increases to 47%.
Decreases in concentration caused by dispersion/dilution . In addition to the
decrease in concentration corresponding to loss of mass, there is a decrease in con-
centration attributable to expansion of the volume of the parcel of air in horizontal