Fate in the Environment and Long-Range Atmospheric Transport of the Organophosphorus Insecticide, Chlorpyrifos and Its Oxon - Reviews of Environmental Contamination and Toxicology - page 44

decreased to 0.002 μg CPY m −2 s −1 (7.2 μg CPY m −2 h −1 ) by 24 h following
application. Total loss of CPY mass in the 12-24 h after application ranged from
15.8 to 16.5%, but diurnal variability is expected. CPYO was also observed to evap-
orate but at a lesser rate of 0.0164 μg m −2 s −1 for the 3-8 h period after application,
which corresponds to 0.85% of the CPY applied. These results confirm that some of
the CPY was transformed to CPYO on the surface and/or in the atmosphere imme-
diately above the surface and subsequently entered the atmosphere. The average
initial flux was approximately 1,500 μg CPY m −2 h −1 and decreased by a factor of
approximately 200-7.2 μg CPY m −2 h −1 . In an earlier study, the eddy correlation
micro-meteorological technique was used to estimate evaporation fluxes for several
pesticides including CPY in the days following application in California (Woodrow
and Seiber 1997 ). For CPY, a flux of 92.3 μg m −2 h −1 was calculated following appli-
cation of 1.5 kg CPY ha −1 , which is equivalent to 0.15 g CPY m −2 . Fluxes of other
pesticides were directly correlated with vapor pressure (P, Pa) and inversely propor-
tional to K OC (L kg −1 ) as well as solubility in water (S, mg L −1 ). The parameter
described by

P
KS

O ( ) is essentially an air/soil partition coefficient analogous to an
air/water partition coefficient, thus this correlation has a sound theoretical basis.
This flux of 92.3 μg CPY m −2 h −1 from a site containing 0.15 gm −2 corresponds to a
loss of a fraction of 92.3 × 10 −6 /0.15 or 615 × 10 −6 per hour, which is equivalent to
0.0615% per hour or 1.4% per day. The total flux from an area of 1 ha or 10 4 m 2 is
thus predicted to be approximately 0.92 g CPY h −1 , with a possible error judged to
be a factor of 3.
In summary, it is suggested that, in the 12 h following application of the liquid
formulation to the surface, approximately 10-20% of the applied material volatil-
izes, but variability is expected diurnally, with temperature, rainfall and soil mois-
ture content. Sorption then “immobilizes” the CPY and subsequent volatilization is
slower, with a rate of approximately 1% per day that decreases steadily to perhaps
0.1% per day in the subsequent weeks. During these periods on the surface and in
the atmosphere, there is direct photolysis of CPY to CPYO. A detailed characteriza-
tion of the initial 12 h period is given by Rotondaro and Havens ( 2012 ), while
studies by Woodrow et al. (Woodrow and Seiber 1997 ; Woodrow et al. 2001 ) char-
acterized average volatilization during the day or 2 following application. In the
context of modeling volatilization losses, the simplest approach is to determine the
total applied quantity and area treated, assume an immediate volatilization loss of
10-20% followed by a period of slower volatilization at an approximate initial rate
of 1% per day decreasing with a half-life of approximately 3 d to 0.1% after 10 d.
Rain and temperature will affect these rates. For illustrative modeling purposes, it
was assumed that a typical rate of application is 1.5 kg CPY ha −1 , which corre-
sponds to 0.15 g CPY m −2 (Woodrow and Seiber 1997 ) to an illustrative area of
1.0 ha (10 4 m 2 ).

Concentrations in air . Of primary interest here are concentrations of CPY in the
atmosphere following application. A maximum concentration is dictated by the
saturation vapor pressure of solid CPY of 0.0023 Pa, which corresponds to