Fate in the Environment and Long-Range Atmospheric Transport of the Organophosphorus Insecticide, Chlorpyrifos and Its Oxon - Reviews of Environmental Contamination and Toxicology - page 56

Table 9 Concentration-fugaci ty conversion factors for CPY
Environmental phase Conversion

Air 1 ng m −3 = 1 × 10 −9 /(350 × 4.03 × 10 −4 ) = 7.1 × 10 −9 Pa = 7.1 nPa

Water and rain 1 ng L −1 = 1 × 10 −9 × 1,000/(350 × 0.9) = 3.2 × 10 −9 Pa = 3.2 nPa

Snow 1 ng g −1 = 1 × 10 −9 × 1,000/(350 × 0.9 × 15) = 0.21 × 10 −9 Pa = 0.21 nPa

Organic carbon 1 ng/g = 1.01 × 10 6 /(350 × 7,727) = 0.37 nPa

Sediment and soil solids
2% OC

1 ng g −1 = 2.4 × 10 6 /(350 × 371) = 18.4 nPa

Sediment and soil solids
10% OC

1 ng g −1 = 2.4 × 10 6 /(350 × 1,854) = 3.7 nPa

Biota concentrations on a
lipid weight basis

1 ng g −1 = 10 6 /(350 × 90,000) = 0.032 nPa

Biota of 10% lipid or
octanol equivalent

1 ng g −1 = 10 6 /(350 × 9,010) = 0.32 nPa

from one environmental compartment to another and has the units of pressure.
At equilibrium, fugacities of a chemical in all compartments are equal. The relative
concentrations in compartments do not change and are defined by the equilibrium
partition coefficients, even though individual molecules are still moving between
compartments. This conversion requires first that all concentrations (C) be con-
verted to units of mol m −3 , which requires that the molar mass and possibly the
phase density are known. This concentration is then divided by the appropriate Z
value for the medium in which CPY is partitioned. Values of Z, which have units of
mol m −3 Pa −1 , are deduced from partition coefficients. This yields the fugacity, f, as
C/Z, of CPY in that medium, thus enabling fugacities in a variety of phases to be
compared directly. Essentially, this analysis leads to a characterization of the equi-
librium status of CPY in the entire ecosystem.
In many cases, phase fugacities in multi-media environments are similar in mag-
nitude, e.g., water, sediments and small fish might exist at comparable fugacities.
An additional advantage of incorporating rain, snow, and terrestrial components in
the model is that concentrations of CPY are generally greater in solid and liquid
media and can be analyzed more accurately. Concentrations are generally more
stable as a function of time. It is with this perspective that considerable effort has
been devoted to measuring concentrations of CPY in rain, snow, terrestrial, and
aquatic systems in regions of interest. Insights into likely differences in fugacity
between air and other media can be obtained by examining ratios of fugacities as
predicted by models such as TAPL3. For example, in Fig. 2 , the fugacity of CPY in
surface water is 12% of that in air, largely because the rate of transformation in
water is fast relative to the rate of deposition from air. Z-values and conversion fac-
tors are given in Table 6 .
Since effects of mixing, transport, and transformation generally cause a decrease
in fugacity of CPY as it travels from source to destination, it is expected that mea-
sured concentrations and fugacities of CPY will display this trend. In this case, the
most convenient units for fugacity are nano Pascals (nPa) i.e., 10 −9 Pa. The fugacity
of liquid CPY as applied is limited by the vapor pressure of 0.002 Pa, (2 × 10 6 nPa),