Fate in the Environment and Long-Range Atmospheric Transport of the Organophosphorus Insecticide, Chlorpyrifos and Its Oxon - Reviews of Environmental Contamination and Toxicology - page 57

but it is likely to be smaller because of dilution in carrier fluids or granules.
Incorporation of 0.15 g m −2 into solid phases of soils to a depth 2.5 cm or 0.025 m
gives a bulk soil concentration of 0.017 mol m −3 and the corresponding fugacity is
46,000 nPa, a factor of 43 less than that of the applied chemical and is attributable
to sorption and dilution.
A concentration of 100 ng m −3 in air close to a site of release corresponds to
0.286 × 10 −9 mol m −3 and the fugacity would be 710 nPa. This is a factor of 64 less
than the fugacity of the chemical in soil and is from dilution that occurs during
evaporation. The total decrease in fugacities of CPY from the point of application
is, thus, approximately 64×43 or 2,750. Most measured concentrations of CPY
were in the range 0.01-1.0 ng m −3 , which corresponds to a range of fugacities of
0.07-7 nPa, a factor of 100-10,000-foldless than that of the initial concentrations of
100 ng m −3 . Therefore, CPY undergoes high dilution in the hundreds of km down-
wind of the source.
The concentration of CPY in rain of approximately 0.4 ng CPY L −1 or 400 ng m −3
that was reported by Mast et al. ( 2012 ) corresponds to approximately
1.1 × 10 −9 mol m −3 and a fugacity of 1.32 nPa. The corresponding equilibrium con-
centration in air is 0.18 ng m −3 which is typical of concentrations in air in the Sierra
Nevada. Fugacities of CPY in air and rain thus appear to be of a similar order of
magnitude, which lends support to the use of fugacity as a method of combining and
comparing measured concentrations among media.
Conversion of concentrations of CPY in snow to fugacities is more problematic
because the Z value for snow is uncertain. This is because the low temperatures and
the variable sorption to ice surfaces as distinct from partitioning to liquid water.
There might also be greater deposition of aerosols in snow at lower temperatures.
Concentrations of CPY in snow were reported to be approximately tenfold greater
in snow than in rain (Mast et al. 2012 ). This result is consistent with the greater Z
value, which is due to the lesser Henry's Constant and vapor pressure of CPY. The
enthalpy of vaporization, which has been reported to be 73 kJ mol −1 for CPY (Goel
et al. 2007 ) corresponds to a 15-fold decrease in vapor pressure from 25 to 0 °C. The
value of Z for snow appears to be a factor of 10-20-fold greater than that of water.
For this reason, rates of deposition of CPY associated with snow are expected to be
greater than those in rain from a similar atmospheric concentration. Snow concen-
trates and integrates CPY more than does rain and can be useful for monitoring the
presence of CPY, but using this information quantitatively is problematic because of
uncertainties in translating concentrations of CPY in air to those in snow, especially
for more intense snow-fall events when extensive scavenging of chemicals from the
atmosphere occurs.
Concentrations in biota such as zooplankton, tadpoles, lichen, and pine needles
can also be converted to fugacities by assuming a content of lipid, or more correctly
an equivalent content of octanol. If data are reported on a lipid weight basis, conver-
sion to fugacity involves division by the Z value of lipid or octanol. The average
CPY lipid-based concentration in tadpoles from the Sierra Nevada in 2008-2009
has been reported to be 22.2 ng CPY g −1 (Mast et al. 2012 ). The corresponding
fugacity of CPY is 0.7 nPa, which is similar in magnitude to the fugacities of air and rain.