Fate in the Environment and Long-Range Atmospheric Transport of the Organophosphorus Insecticide, Chlorpyrifos and Its Oxon - Reviews of Environmental Contamination and Toxicology - page 55

Table 8 Estimated Z values of CPY at 25 °C used in fugacity calculations

Value
(mol m −3 Pa) Environmental phase Formula Comment

Air 1/RT Z A = 4.03 × 10 −4 R is 8.314

Water 1/H Z W = 0.90 H = 1.11

Octanol and lipids Z O = K OW /H Z O = 90,000 K OW = 10 5

Organic carbon Z OC = K OC Z W ρoc Z OC = 7,730 K OC = 8,500 ρ OC = 1.01

(density kg L −1 )

Soils solids of 2%
OC

Z S = ρsZ OC f OC Z S = 371 ρs 2.4 kg L −1 , f OC = 0.02

Sediment solids of
10% OC

Z S = ρsZ OC f OC Z S = 1,855 ρs = 2.4 kg L −1 , f OC = 0.10

Aerosol particles Z P = 0.1 Z O Z P = 9,000 Assumes 10% octanol equivalent

Snow Z N Z N = 15 Z W Z N = 13.5 Assuming factor of 15 lesser

Henry's Law constant at 0 °C

Biota of 100% lipid
equivalent

Z B = Z O =90,000 i.e., 100% octanol

Biota of 10% lipid
equivalent

Z B = 0.1 Z O =9,010 i.e., 10% octanol, 90% water

and greater concentrations in terrestrial and aquatic systems, does not apparently
apply to transport of CPY into the Sierra Nevada mountains.
The relationship between CPY and CPYO and their transport in the atmosphere is
summarized as follows: Shortly after application, a fraction of the applied CPY vola-
tilizes to the atmosphere where it is dispersed by atmospheric turbulence to lower
concentrations estimated to be of the order of 100 ng m −3 at a distance of 1 km. It is
also subject to transformation to CPYO, which is also subject to dispersion and trans-
port for moderate or long distances. Some CPY will be transported from the plume
back to neighboring soils and vegetation by direct gas absorption; however, the result-
ing concentrations in soils and vegetation will be small and many orders of magnitude
less than those in the application area. The vapor pressure and K OW of CPYO are
smaller than those of CPY and its solubility in water is greater, thus it has a smaller
K AW . As a result, it is subject to faster deposition and there will be enhanced partition-
ing into water droplets in the air. CPYO is also subject to some gaseous deposition but
it is likely to be further degraded in other compartments such as water and moist solid
surfaces. Once in water, hydrolysis is rapid (Table 7 ). This process also explains the
very infrequent detection of CPYO in surface waters (Williams et al. 2014 ). During
heavy rainfall immediately following application, local deposition will be maximized.
The rates could be estimated but will be speculative and will be difficult to confirm
because most locally deposited CPY will result from spray drift and it will be difficult
to discriminate between gaseous deposition and spray drift.

Interpretation of measured concentrations of chlorpyrifos in media by use of
fugacity. There is an incentive to exploit all the available measured concentrations
of CPY for all sampled media, rather than just air. This is feasible by converting all
concentrations of CPY to the “common currency” of fugacity as outlined in Tables 8
and 9 (Mackay 2001 ). Fugacity is the escaping tendency for chemicals to move