Air Monitoring (2012)/Page 2 of 5 Chlorpyrifos / 059101 / MRID 48998801 OPPTS 835.8100 / DACO 5.9a

SUMMARY :
Rates of volatilization of chlorpyrifos were esti mated from a treated potato field in June of 2002. Rates were estimated using several different me thods, 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-Holla nd), The Netherlands. The soil was a loamy sea deposit (27% clay, 4.9% organic ma tter, pH-KCl) 7.4). A potato crop (variety Agria) was grown on ridges 0.75m apart. The pot ato plants were on average 0.56 m high, and soil cover by the plants was estimated visually to be on averag e 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 near est wind obstacles were a substant ial distance from the field: a row of trees 800 m to the west of the field cente r 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 wa s 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 comple ted 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 LayoutAir Monitoring (2012)/Page 3 of 5 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 cente r 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 we sterly 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 de termined using the Thornthwaite-Holzman equation (Majewski, et al., 1990) shown in Equation 1

.

Equation 1

2 2

] [ln ) )( (
         



bottom top
p m z z u c k Flux  

where k is the von Karman’s constant (dimensionless ~0.4), Δ 

### c is the vertical gradient pesticide residue concentration in air between heights z top and z bottom , Δū is the vertical gradient wind speed between heights z top and z bottom , and

m 

###  and p  are the momentum and vapor stability correction terms respectively. Linear regressi on equations, relating con centration, wind speed, and temperature to the logarithm of the sample height, are developed using measured values obtained from the field study. Th e adjusted values of the concen tration, temperature, and wind speed from these regressions are incorporated in to Equation 1 to arrive at Equation 2 which is ultimately used to compute the flux.

Equation 2 2 2

ln ) )( ( ) 4

. 0 (

          



bottom top
p m zbottom ztop zbottom ztop

z z u u c c Flux  

The

m 

###  and p  stability correction terms are determin ed according to the following conditions based on the calculation of the Richardson number, R i :


#   2 2

16

. 273 2 ) )( ( ) 8

. 9 (

zbottom ztop zbottom ztop zbottom ztop bottom top i

u u T T T T z z R

         
           



if R i >0 

##   33

. 0 

### 16 1 i m 

### R    and 4

. 0 

### ) 34 1 ( 885

. 0 i p 

### R   

if R i <0 

##   33

. 0 

### 16 1 



###   i m 

### R  and 4

. 0 

### ) 22 1 ( 885

. 0 



###   i p 

### R 

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 con centration versus the logarithm of the sample height resulted in R 2 values between 0.95 and 1.00. Z top and z bottom heights of 1.02 and 1.60 m, respectively, were used in the estimation of th e flux. These heights are between the minimumAir 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 Δ u (m/s) Δ T (K) Δ c ( μ g/m 3 ) Flux (mg/m 2 -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 measuri ng height above the evaporating surface is recommended to be at least 100:1 to allow m easurement of the steady-state pesticide flux in the internal boundary layer above the field (Majewski, 199 9). As the maximum sample height was 1.9 m and raw mete orological data were not a ssessed to confirm the fetch distance ratio of 100:1 was maintained, ther e 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 provi de a more robust sample set with which to derive the regression between concentrati on and the logarithm of sample height.

3 While chlorpyrifos is not currently register ed for use on potatoes in the United States, potatoes are thought to be reason ably representative, for the pu rposes of extrapolation of field volatility data, of other fi eld crops treated with chlorpyrifos and the results of this study provide useful information on the potenti al volatility of chlo rpyrifos under field conditions.

4 While Dursban EC [the currently marketed equivalent is Durs ban 4E (EPA Reg. No.62719-220)] may be an end-us e 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 ch lorpyrifos uses is hi gher 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 gr apes, 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), mi nt (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 t ypically conducted similar to terrestrial fi eld dissipation studies (OCSPP 835.6100); therefor e, the maximum single application rate of all uses is normally recommended. To address this uncer tainty, additional studies may be necessaryAir Monitoring (2012)/Page 5 of 5 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 chlorpyr ifos and chlorpyrifos-oxon observ ed 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 fo r sites within United States. To address this uncertainty, additional studies may be necessa ry to evaluate flux rates under different environmental conditions in different regions of the country.

7 The study did not measure the concentrati on 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 th e 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 evaporat ion rates from soil. Environmental Science and Technology, 24:1490-1497.

Majewski, M. S. Micrometeorol ogical methods for measuring the post application volatilization of pesticides. Water, Air, Soil Pollut. 1999, 115, 83-113.