Determination of Methyl Isothiocyanate in Air Downwind of Fields Treated with Metam-Sodium by Subsurface Drip Irrigation

Abstract

Air concentrations of methyl isothiocyanate (MITC) were determined near two fields treated with metam-sodium (MS) by subsurface drip irrigation. The two study fields showed measurable airborne MITC residues during application of MS and for periods up to 48 h postapplication. Using a Gaussian plume dispersion model, flux values were estimated for all of the sampling periods. On the basis of the flux estimates, the amount of MITC that volatilized within the 48 h period was about 1.4% of the applied material. Compared to other studies, MITC residues in air measured during application by subsurface drip irrigation were up to four orders-of-magnitude lower than those previously published for applications involving delivery through surface irrigation water. Our measured concentrations of MITC in field air were at levels below current regulatory guidance and thresholds for adverse human health effects.

Correlation To Estimate Emission Rates for Soil-Applied Fumigants

“…For the correlation method to be of value, it is important to use reliable chemical property and emission rate data. For example, a number of different sources, such as reference works, industry and university reports, and published peer-reviewed papers, were consulted to obtain the data summarized in Table − . If there were several values for a particular property, as determined by different investigators under a given set of conditions, the value selected was the one that occurred most often in the scientific literature, or, short of that, was determined by investigators whose work has been recognized as the most reliable by other investigators.…”

Contributions of Pesticide Residue Chemistry to Improving Food and Environmental Safety: Past and Present Accomplishments and Future Challenges

“…Volatilization and downwind air movement have been well documented for fumigants  volatile compounds by nature that are used to sterilize the soil before planting, to kill insects in stored grains, and to kill termites and other insects in structures. Examples of fumigants that have been studied in this regard are methyl bromide, methyl isothiocyanate (formed from Metam Sodium in soil or water), chloropicrin, and 1,3-dichloropropane. The release rate (flux) of these chemicals from soil to air generally follows their vapor pressures, that is, highest release rate for methyl bromide, lowest for MITC  all determined by residue chemistry using the aerodynamic flux method − and various trapping techniques and postsampling analysis procedures…”

Infrared Emission Following Photolysis of Methylisothiocyanate and Methylthiocyanate

“…A fumigant is a broad-spectrum pesticide used to enhance the yield and quality of agricultural produce. When metam sodium is applied to fields, up to 60% of the MITC escapes into the atmosphere, although the air concentration varies by application method . Alvarez and Moore found that, at 308 nm, MITC photolyzes to produce methyl isocyanide (CH 3 NC) and sulfur atoms with a quantum yield of 0.98 ± 0.24 .…”

“…When metam sodium is applied to fields, up to 60% of the MITC escapes into the atmosphere, 2 although the air concentration varies by application method. 3 Alvarez and Moore found that, at 308 nm, MITC photolyzes to produce methyl isocyanide (CH 3 NC) and sulfur atoms with a quantum yield of 0.98 ( 0.24. 4 Based on a quantum yield of 1 and the absorbance of MITC at actinic wavelengths (λ g 295 nm), they estimated MITC's environmental lifetime as 41 h. 4 The final reaction products of MITC photolysis are of interest environmentally because CH 3 NC is not photochemically active in the troposphere, but as a carbene, it should react with other atmospheric species.…”