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1989 - 1994

Atmospheric Transfer of Agrochemicals

E. Pattey, R.L. Desjardins, P. Rochette, T. Zhu, W.G. Royds, D. Dow,
CLBRR, Agriculture and Agri-Food Canada, Ottawa,
G. St-Amour, CFAR, Agriculture and Agri-Food Canada, Ottawa
A. Cessna, L. Kerr,Regina Research Station, Agriculture and Agri-Food Canada
J.I. MacPherson, Flight Research Laboratory, National Research Council, Ottawa

June 30, 1994

Download report (158 KB pdf )

 

1. RATIONALE AND OBJECTIVES

Atmospheric transport is considered to be a significant input pathway of toxic chemicals to the Great Lakes. Confidence in the measurements of vapour fluxes of agrochemicals between soil, air and water surfaces suffers from uncertainties associated with the measuring techniques. A reliable measuring technique was therefore needed to assess the impact of soil and crop management on atmospheric loading of agrochemicals.

Until recently, the only micrometeorological technique used to measure the vapour flux of agrochemicals was the aerodynamic gradient technique. Four to six measuring levels were used to determine the concentration profile. This technique required the estimation of the turbulent exchange coefficient, K, which introduced considerable uncertainties. Since agrochemical analysis is very time consuming, a measuring technique requiring fewer samples was highly desirable. The relaxed eddy-accumulation technique offered this opportunity, in addition to the fact that it was a more direct approach for measuring fluxes. It was also the only technique that could be used for regional flux measurements with an aircraft.

The objectives of this study were:

  1. To develop methods based on the eddy-accumulation technique to quantify the emission and surface deposition rates of agrochemicals and other toxic substances.

  2. To measure the atmosphere-surface exchange of agrochemicals used in the Great Lakes region using two tower-based systems having different trapping devices.

  3. To measure the atmosphere-surface exchange of agrochemicals in the Great Lakes region using an aircraft-based system.

4. STUDY CONCLUSIONS

A - Development of Three New Flux Measuring Systems

Three new systems for measuring the exchange of agrochemicals above land surfaces were developed: two tower-based systems for field-scale measurements and an aircraft-based system for estimation of regional fluxes. These systems are based on the relaxed eddy-accumulation technique. The advantages of this technique are the following:

  • It is based on a sound theoretical basis;

  • In the case of the tower-based system, it requires air sampling at only one level as opposed to 4 to 6 levels with the more traditional aerodynamic gradient technique, thereby considerably reducing the need for laboratory analysis;

  • In the case of the aircraft-based system, it is the only technique that can be used for flux measurements of agrochemicals.

The tower-based system was developed and tested through several field experiments:

 

* Fall 1989.

OBJECTIVE: Test of a REA system prototype and comparison of results from this new system to results from existing techniques.

  1. Herbicide vapour fluxes measured by REA system were lower than those measured by the gradient technique;

  2. Design problems were identified such as slow valve controlling system and deposition of chemicals on tubing walls.

 

* Summer 1991.

OBJECTIVE: Test of the REA technique for the measurement of carbon dioxide fluxes.

  1. The measurements of CO2 fluxes by REA compared well with the eddy-correlation technique. This was an independent validation of the accuracy of REA for the measurement of gas fluxes.

  2. The REA sampling system could be modified to measure the flux of agrochemicals.

 

* Fall 1992.

OBJECTIVE: Tests of a new device for trapping agrochemicals in air samples were carried out.

  1. Use of mini-tubes filled with Tenax-TA resin considerably shortened the time for the determination of chemical concentration in air samples and increased the accuracy of the analysis.

  2. The deposition of chemicals on the tubing walls had a strong impact on flux calculations because of the low flow rates through the mini-tubes (caused by higher resistance to air flow). Improved heating system of inlet tubing walls solved this problem.

The aircraft-based system was developed and tested during several test flights.

 

* Summers 1990 & 1991.

OBJECTIVE: Theoretical validation of the aircraft-based REA.

  1. CO2 fluxes calculated by REA and eddy correlation using fast-response analyzer data were in close agreement.

 

* Summers 1992 & 1993.

OBJECTIVE: Test of an aircraft-based REA system prototype.

  1. Measurements of low background concentration of agrochemicals in the lower troposphere indicated that very high sampling flow rates were needed for aircraft-based REA system.

  2. A high-volume sampler (646 L min-1) was able to collect measurable quantities of atrazine and metolachlor at 150 m above agricultural regions during flights lasting approximately one hour.

B - Measurement of Agrochemical Vapour Flux under Field Conditions.

Vapour fluxes of herbicides were measured during several field experiments:

* Fall 1989.

OBJECTIVE: To measure the vapour fluxes of triallate (3.37 kg a.i./ha) and trifluralin (2.66 kg a.i./ha) applied at high rates and not incorporated in the soil.

  1. Highest vapour fluxes occurred just after spraying and after rainfall. Smaller peaks coincided with dew evaporation.

  2. After 24 hours, 11 and 15% of the applied triallate and trifluralin were volatilized, and after four days, the volatilization losses accounted for 30% of the applied chemicals.

  3. The measurements were used to evaluate the results obtained by the air-soil exchange model of Scholtz and Voldner (1992, 1993).

* Fall 1992

OBJECTIVE: To measure the vapour fluxes of triallate (1.70 kg a.i./ha) and trifluralin (1.15 kg a.i./ha) applied at recommended rates but not incorporated in the soil.

  1. After 24 hours, 10 and 9% of the applied triallate and trifluralin were volatilized, and after four days, the volatilization losses accounted for 21 and 13% of the applied chemicals.

  2. The measurements were compared to the results obtained by the air-soil exchange model of Scholtz et al., (1994) and they agreed reasonably well.

* Spring and summer 1993.

OBJECTIVE: To measure the vapour fluxes of two herbicides used in the Great Lakes region: metolachlor and metribuzin incorporated in the soil, for two separate periods (in June and July).

  1. Vapour fluxes were small but persisted long after the application. However, the cumulative losses represented less than 1% of the applied amount.

  2. Difficulties were encountered for detecting metribuzin concentrations.

  3. The capability of our tower-based systems to measure very small fluxes was successfully tested in this experiment.

 

C- Estimation of Regional Atmosphere-surface Exchange of Agrochemicals in the Great Lakes Region

* Summer 1993.

OBJECTIVE: To measure the emission or deposition of atrazine and metolachlor over the Great Lakes region.

  1. The results from a return flight between Ottawa and London, Ontario demonstrated that the aircraft-based sampling system can be used to quantify regional fluxes of agrochemicals. Metolachlor and atrazine, the two herbicides used in largest quantities in Ontario, were found in all samples except one. The concentration of atrazine was higher than that of metalochlor and the concentrations of both chemicals were higher over the more intensive agricultural areas. An upward flux was observed over intensive agricultural areas and a downward flux over less intensive areas (Zhu et al., 1994).

  2. A test flight was also done over Lake Ontario but because of the smaller vertical wind over the lake that day (more than 82% of the time) the sampler was in the deadband region, where no sample is collected. Therefore, less than 1 m3 of upward and downward moving air was collected by each PUF plug and no chemicals were detected in this relatively small volume of air. In the future the deadband region should be considerably smaller over a lake.

  3. These aircraft-based results are based on a very limited data set. More measurements of this type are required to evaluate the technique more fully.

5. NEW TECHNOLOGIES AND BENEFITS

Many new technologies have been developed. Three new systems for measuring the air surface exchange of agrochemicals were developed: two tower-based systems for field-scale measurements and an aircraft-based system for estimation of regional fluxes. These systems are based on the relaxed eddy-accumulation technique. A new thermal desorption unit, developed with industry, was shown to simplify the analysis of agrochemicals and improve the accuracy of the measurements considerably.

These new technologies provide a better knowledge of the magnitude of the exchange of agrochemicals between agricultural lands and the atmosphere. Previously accurate measurements of such vapour fluxes under field conditions were lacking. The new measuring systems developed in this project provide the scientific community with a much needed tool for quantifying the volatilization of agrochemicals under field conditions. Volatilization subroutines in models simulating the fate of agrochemicals in the environment can now be validated.

The measurement of the volatilization of agrochemicals under a wide range of conditions has the potential to provide the guidelines for selecting the management practices which minimize atmospheric contamination.

6. IMPLICATIONS FOR THE GREAT LAKES ECOSYSTEM

   A - Volatilization Losses of Agrochemicals

Volatilization losses of agrochemicals from agricultural fields are transported in the atmosphere and deposited elsewhere. The study of the impact of atmospheric transfer of agrochemicals on the Great Lakes ecosystem cannot, therefore, be limited to the Great Lakes basin since the amount of agrochemicals deposited is dependent on the amount that was emitted.

Measuring the losses from all individual sources (fields) is impossible. However, measurements of the volatilization of the most important substances under a wide range of agricultural management practices are needed in order to:

  1. validate simulation models that can later be used for obtaining large area emission estimates;

  2. select management strategies which minimize atmospheric contamination.

The tower-based measuring systems that were developed in this project are the first systems based on the REA approach.

With the new REA measuring systems, the volatilization losses of four herbicides were measured after their field application. The soil texture and structure on which the experiments were carried out are similar to those of the Great Lakes basin. These results were successfully used to validate model estimates of vapour fluxes.

   B - Atmospheric Deposition of Agrochemicals

The atmospheric deposition of agrochemicals into the Great Lakes basin is affected by several factors such as the atmospheric concentration of agrochemicals, the atmospheric conditions, and the surface conditions. These factors are highly variable resulting in a large spatial variability of the deposition rates. Regional estimates of the atmospheric deposition rates can be best obtain using aircraft-based measurement techniques which integrate local variability. REA is the only technique that can be used for flux measurements of agrochemicals using an aircraft-based system because of the lack of fast-response analyzer for most substances and the fact that observations at one level preclude the use of gradient techniques.

In this project, an aircraft-based REA system was developed and measurements obtained for the first time. This new measuring facility represents important progress in our capability for measuring regional exchange of agrochemicals. However, the difficulties encountered in the development process were significant and the resulting delays prevented us from carrying out as many field experiments in the Great Lakes area as had been originally planned.

7. TECHNOLOGY TRANSFER POTENTIAL

The relaxed eddy-accumulation technique is now well known within the micrometeorological community. The real challenge for anyone who wants to use REA is to build a measuring system that respects the principles of the technique and that performs reliably. The systems that we developed are unique for the measurement of volatile compounds for which no real-time gas analyzer exists. The air samples are trapped in either Tenax-TA resin or polyurethane foam which are later analyzed in the laboratory. After three years of improvements, we have now reached a degree of reliability and automation which, we believe, makes it attractive for commercial development. We have developed programs to automate the measurements of the flux of agrochemicals over a wide range of scales. The analysis system of the mini-tube is now available commercially and we collaborated with the Canadian Centre for Advanced Instrumentation, to improve their mini-tube technology.

8. GAPS/NEEDS FOR FUTURE RESEARCH

  • More measurements of the exchange rates of agrochemicals above agricultural fields are needed i) to develop and validate simulation models of the volatilization of agrochemicals and ii) to make recommendations to reduce the atmospheric contamination by agrochemicals.

  • Measurements of the volatilization of the most frequently used substances under a wide range of agricultural management practices are needed at the scale of a field (mostly for pesticides applied on crop canopies), to make recommendations for remedial actions.

  • A tower-based system should be run in the Great Lakes region to monitor the agrochemical deposition over selected areas during intensive application periods, in order to quantify the sources.

  • Aircraft-based flux measurements should be carried out above and around the Great Lakes in order to quantify the fluxes (emission and deposition) of agrochemicals as a function of land use. There is a great need to have such data to evaluate regional and long-range transport models, which are used to quantify the contribution of agricultural activities to the contamination of the Great Lakes basin.

  • Systems with higher sampling rates and improvement in the agrochemical determination are required for more accurate measurements of background level deposition with both the tower- and aircraft-based systems.

 

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