Reducing the discrepancies between the Aerodynamic Gradient Method and other micrometeorological approaches for measuring fumigant emissions

Authors: Anderson, Raymond - Ray; Yates, Scott; ASHWORTH, DANIEL - University Of California - Cooperative Extension Service; Jenkins, Dennise; Zhang, Qiaoping

Submitted to: Science of the Total Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/8/2019
Publication Date: 6/11/2019
Publication URL: https://handle.nal.usda.gov/10113/6465987
Citation: Anderson, R.G., Yates, S.R., Ashworth, D.J., Jenkins, D.L., Zhang, Q. 2019. Reducing the discrepancies between the Aerodynamic Gradient Method and other micrometeorological approaches for measuring fumigant emissions. Science of the Total Environment. 687:392-400. https://doi.org/10.1016/j.scitotenv.2019.06.132.
DOI: https://doi.org/10.1016/j.scitotenv.2019.06.132

Interpretive Summary: Gaseous emissions of fumigants and pesticides contribute to smog formation and can pose a hazard to human health through direct exposure. Reliable observations of pesticide and fumigant emissions are needed to estimate total environmental exposure and to evaluate techniques to reduce emissions. One longstanding atmospheric technique, the Aerodynamic Gradient Method, has been frequently used to measure emissions. However, this method has been found to measure significantly higher rates of pesticide emissions than other measurement techniques. Depending upon the level of risk tolerance, this measurement discrepancy could result in significantly different assessments of risks from fumigant emissions. To resolve this issue, we used a newer micrometeorological measurement technique, eddy covariance, to test alternate transport functions for the Aerodynamic Gradient Method. We found that more recent transport functions (Högström) agreed with independent observations from the eddy covariance method. When we tested the Aerodynamic Method with the Högström functions, the fumigant emissions measurements agreed better with other observational approaches. We evaluated how the Högström functions could be retroactively applied to previous studies that used the Aerodynamic Method to improve fumigant emissions observations. The results are of interest to farmers and pesticide applicators who may face limits as a result of previous, higher fumigant emission estimates.

Technical Abstract: Observations of fumigant and pesticide emissions are needed for multiple public health and environmental protection mandates. The aerodynamic gradient method (ADM) is commonly used to measure fumigant and pesticide emissions. However, the ADM may over estimated emissions compared to other micrometeorological and modelling approaches, increasing uncertainty over the true flux estimate. Different studies with ADM have also used multiple differing transport functions that relate concentration gradients to emissions. Therefore, we tested different and more recent transport functions to try to correct the observed higher values with ADM using observations from two sites in California, USA. We evaluated different transport functions against eddy covariance observations and found that using the functions developed by Högström (1996) corrected the ADM values to be most in line with other observational methods. For the Fresno experiment, cumulative emission masses from the ADM- Högström functions were within 7% of other approaches while the Pruitt function was more than 15% higher. Applying the Högström functions to a series of previous fumigation experiments in California saw reductions in the ADM observations of more than 25% for cumulative mass emissions. The results indicate that the Högström functions should be used for future ADM experiments in the absence of more robust transport factors for local meteorological conditions. The results also illustrate how previous ADM observations could be corrected to reduce uncertainty in flux emissions estimates.