1989 - 1994
Atmospheric Transfer of Agrochemicals
E. Pattey, R.L. Desjardins, P. Rochette, T. Zhu,
W.G. Royds, D. Dow,
June 30, 1994
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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:
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:
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.
* Summer 1991.
OBJECTIVE: Test of the REA technique for the measurement of carbon dioxide fluxes.
* Fall 1992.
OBJECTIVE: Tests of a new device for trapping agrochemicals in air samples were carried out.
The aircraft-based system was developed and tested during several test flights.
* Summers 1990 & 1991.
OBJECTIVE: Theoretical validation of the aircraft-based REA.
* Summers 1992 & 1993.
OBJECTIVE: Test of an aircraft-based REA system prototype.
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.
* 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.
* 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).
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.
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:
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
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