Fate in the Environment and Long-Range Atmospheric Transport of the Organophosphorus Insecticide, Chlorpyrifos and Its Oxon - Reviews of Environmental Contamination and Toxicology - page 45

approximately 0.00033 g m −3 or 330,000 ng m −3 in an enclosed ecosystem. It is
inconceivable that this concentration could be achieved in the field because of dilu-
tion in formulations and mass transfer limitations during evaporation. Concentrations
of CPY in air above a potato field in the Netherlands at noon in midsummer ranged
from 14,550 to 7,930 ng m −3 at 1 and 1.9 m above the crop 2 h after application
(Leistra et al. 2005 ). These declined to a range of 2,950 to 1.84 ng m −3 after 8 h and
to 26 to 15 ng m −3 in the 6 d following application. The initial flux was large
(5-9 mg m −2 h −1 ), possibly because of the large surface area of the leaves of this
crop. As CPY is not registered for use on potatoes in the U.S., these data were not
used in this assessment. Similarly high concentrations of CPY in air following an
application of 4.5 kg ha −1 to turf were in the range of 1,000-20,000 ng m −3 (Vaccaro
1993 ). This might be a “worst case” in terms of concentrations and represents ~10%
of the saturation concentration in air, i.e., the vapor pressure/RT, where RT is the
gas constant-absolute temperature group. Immediately after application, concentra-
tions of CPY of approximately 10,000 ng CPY m −3 (~3% of saturation) were mea-
sured at a height of 1.5 m above an alfalfa crop (Rotondaro and Havens 2012 ).
Concentrations then decreased to approximately 100 ng m −3 after the initial more
rapid evaporation. The USEPA conducted a modeling study to assess potential
exposures of bystanders close to the site of application (USEPA 2013 ), but these
values are not directly relevant to larger distances, in which concentrations would
be much smaller because of dilution.
Concentrations of pesticides in air downwind of the site of application can, in
principle, be calculated from an estimated flux by assuming a wind-speed, a mixing
height, an atmospheric stability class and dimensions of the site. This is most rigor-
ously done by using air dispersion models, such as SCREEN3 (Turner 1994 ; USEPA
1995 ). Detailed estimation of near-source concentrations in the atmosphere are
beyond the scope of the simulation utilized here, which was focused on transport
over distances up to 100s of km. Such estimates are nonetheless useful to estimate
the order of magnitude of these “source” concentrations when monitoring data have
been obtained in the vicinity of sources. The SCREEN3 model has been used to
estimate concentrations in air at ground-level (1.5 m) immediately downwind, such
as 10-30 m, from treated crops (Woodrow and Seiber 1997 ). Measured concentra-
tions of five pesticides were of similar magnitude to predicted concentrations
(μg m −3 ) and similar in magnitude to estimated fluxes (μg m −2 s −1 ), a result that is
consistent with the ratio of these two parameters being approximately 1 m s −1 . This
ratio of flux to concentration can be regarded as an effective wind-speed or mass
transfer coefficient into which the evaporated chemical is diluted and is similar to
the actual wind-speed of a few meter per second. Measured and simulated concen-
trations of pesticides in air were in good agreement. Accordingly, using this simple
estimation method, ground-level concentrations in air at the site studied by Woodrow
et al. are expected to be approximately 92 μg m −2 h −1 divided by a typical wind-
speed of 3,600 m h −1 , giving 0.025 μg m −3 , which is 25 ng m −3 . This result is consis-
tent with the above estimate. Volatilization rates of approximately 1,500 μg CPY
m −2 h −1 (Rotondaro and Havens 2012 ) yielded a concentration of approximately
500 ng CPY m −3 . Concentrations would be expected to be less downwind because