Air Monitoring (2012)/Page 2 of 5 

Chlorpyrifos / 059101 / MRID 48998801 OPPTS 835.8100 / DACO 5.9a

SUMMARY: 

Rates of volatilization of chlorpyrifos were estimated from a treated potato field in June of 2002. 

Rates were estimated using several different methods, including the aerodynamic profile method. 

Volatilization rates, predicted using the aerodynamic profile method, were similar or higher than 

the rates predicted using the other methods, so the remaining discussion focuses on the flux rates 

developed using the aerodynamic profile method. 

The trial field was situated on the experimental farm Oostwaardhoeve near Slootdorp in the 

Wieringermeer Polder (Province of North-Holland), The Netherlands. The soil was a loamy sea 

deposit (27% clay, 4.9% organic matter, pH-KCl) 7.4). A potato crop (variety Agria) was grown 

on ridges 0.75m apart. The potato plants were on average 0.56 m high, and soil cover by the 

plants was estimated visually to be on average 80% (range of 70-90%). The experimental field 

(Figure 1) was surrounded by other arable fields with crops having heights less than or equal to 

that of the potato crop. The nearest wind obstacles were a substantial distance from the field: a 

row of trees 800 m to the west of the field center and some farm buildings with trees 1000 m to 

the southwest. The insecticide chlorpyrifos was sprayed as a mixture with fenopropimorph onto 

the potato crop on 25 June 2002. The application rate of chlorpyrifos was calculated from the 

product of spray volume and spray concentration to be 679 g/ha (0.61 lb a.i./A). Spraying started 

at the west side of the field (upwind of the air sampling spot) at 11:30 h and spraying of the 

upwind part of the field was completed at 12:00 h (taken as zero time). After this, the part of the 

field downwind of the sampling spot was sprayed. 

Figure 1. Field Layout 
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Chlorpyrifos / 059101 / MRID 48998801 OPPTS 835.8100 / DACO 5.9a

Air concentration samples were collected at 1.0, 1.6, and 1.9 m above the top of ridges on the 

soil surface at the center of a potato field (Figure 1). This allowed for a minimum fetch distance 

of 100 m and a fetch distance of 180 m in the predominantly westerly wind direction. Wind 

speed and temperature data were collected at various heights using equipment positioned at the 

center of the field. 

Flux using the aerodynamic profile method is determined using the Thornthwaite-Holzman 

equation (Majewski, et al., 1990) shown in Equation 1. 

Equation 1 

2

2

[ln ]

( )( )

















  

bottom

top

m p z

z

k c u Flux

 

where k is the von Karman’s constant (dimensionless ~0.4), Δc is the vertical gradient pesticide 

residue concentration in air between heights ztop and zbottom , Δū is the vertical gradient wind 

speed between heights ztop and zbottom, and  m and  p are the momentum and vapor stability 

correction terms respectively. Linear regression equations, relating concentration, wind speed, 

and temperature to the logarithm of the sample height, are developed using measured values 

obtained from the field study. The adjusted values of the concentration, temperature, and wind 

speed from these regressions are incorporated into Equation 1 to arrive at Equation 2 which is 

ultimately used to compute the flux. 

Equation 2 2

2

ln

(0.4) ( )( )

















   

bottom

top

m p

ztop zbottom ztop zbottom

z

z

c c u u

Flux

 

The  m and  p stability correction terms are determined according to the following conditions 

based on the calculation of the Richardson number, Ri: 

 2

2

273.16

2

(9.8) ( )( )

ztop zbottom

ztop zbottom

top bottom ztop zbottom

i

u u

T T

z z T T

R

  





























 

  

if Ri >0  0.33  m  116Ri and 0.4 0.885(1 34 )  p   Ri

if Ri <0   0.33 1 16   m   Ri and 0.4 0.885(1 22 )

  p   Ri

The results of the study and study author estimated flux rates are provided for June 25 and 26 in 

Table 1. The linear regression of concentration versus the logarithm of the sample height 

resulted in R2

 values between 0.95 and 1.00. Ztop and zbottom heights of 1.02 and 1.60 m, 

respectively, were used in the estimation of the flux. These heights are between the minimum 
 Air Monitoring (2012)/Page 4 of 5 

Chlorpyrifos / 059101 / MRID 48998801 OPPTS 835.8100 / DACO 5.9a

and maximum heights used in the collection of air concentrations, so the regression equation 

should be appropriate. 

Table 1. Data and Flux Results 

Date Time Δu (m/s) ΔT (K) Δc (μg/m3) Flux (mg/m2-h) 

June 25 12:35 – 13:56 0.512 0.535 4.88 6.72 

 14:01 – 15:34 0.558 0.520 5.52 7.80 

 15:42 – 17:26 0.586 0.437 2.14 2.96 

 19:20 – 20:44 0.393 0.077 0.764 0.605 

June 26 09:54 – 11:32 0.741 0.267 0.381 0.567 

 11:42 – 13:37 0.837 0.398 0.167 0.286 

REVIEWER’S COMMENTS:

1 The ratio of upwind fetch to maximum measuring height above the evaporating surface is 

recommended to be at least 100:1 to allow measurement of the steady-state pesticide flux 

in the internal boundary layer above the field (Majewski, 1999). As the maximum sample 

height was 1.9 m and raw meteorological data were not assessed to confirm the fetch 

distance ratio of 100:1 was maintained, there is some uncertainty surrounding the flux 

rates estimated in this paper. 

2 Only three heights were used to collect air samples. Usually, EPA requires that a 

minimum of 5 sample heights be used to provide a more robust sample set with which to 

derive the regression between concentration and the logarithm of sample height. 

3 While chlorpyrifos is not currently registered for use on potatoes in the United States, 

potatoes are thought to be reasonably representative, for the purposes of extrapolation of 

field volatility data, of other field crops treated with chlorpyrifos and the results of this 

study provide useful information on the potential volatility of chlorpyrifos under field 

conditions. 

4 While Dursban EC [the currently marketed equivalent is Dursban 4E (EPA Reg. 

No.62719-220)] may be an end-use emulsifiable concentrate used on a range of crops it 

may not represent the preferred formulation by applicators. 

5 The maximum single application rate for many chlorpyrifos uses is higher than the 0.61 

lb a.i./A used in this study. In addition, these rates are substantially lower than current 

maximum single applications for grapes (33.3 lb a.i./A; 33.3 lb a.i./A per year) permitted 

on currently labeled chlorpyrifos products. In addition to grapes, several other uses also 

have maximum single application rates substantially higher than 0.61 lb a.i./A including 

citrus (6 lb a.i./A; 10.5 lb a.i./A per year), mint (2.0 lb a.i./A; 6 lb a.i./A per season), and 

turf (4.0 lb a.i./A; no yearly application restriction). 

Field volatility studies are typically conducted similar to terrestrial field dissipation 

studies (OCSPP 835.6100); therefore, the maximum single application rate of all uses is 

normally recommended. To address this uncertainty, additional studies may be necessary 
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Chlorpyrifos / 059101 / MRID 48998801 OPPTS 835.8100 / DACO 5.9a

to evaluate flux rates under different cropping conditions and application rates. 

6 In addition to temperature, environmental factors such as humidity and rainfall may 

influence the amount of chlorpyrifos and chlorpyrifos-oxon observed in air samples. It is 

uncertain how the results obtained from this study, conducted in the Netherlands, 

compare to the results that may be obtained for sites within United States. To address this 

uncertainty, additional studies may be necessary to evaluate flux rates under different 

environmental conditions in different regions of the country. 

7 The study did not measure the concentration of chlorpyrifos-oxon, a chlorpyrifos 

transformation product of toxicological concern. It is unclear how much conversion may 

have occurred and impact it would have on the estimated concentration of chlorpyrifos. 

CONCLUSION:

The data and the flux rate estimates derived from the data appear to be scientifically sound. This 

study is classified as supplemental. 

REFERENCES:

Majewski, M.S., Glotfelty, D.E., Kyaw Tha Paw U, Seiber, J.N. 1990. A field comparison of 

several methods for measuring pesticide evaporation rates from soil. Environmental Science and 

Technology, 24:1490-1497. 

Majewski, M. S. Micrometeorological methods for measuring the post application volatilization 

of pesticides. Water, Air, Soil Pollut. 1999, 115, 83-113.