Storage structures (silos, warehouses, and similar structures) are routinely fumigated for disinfestation of pests. This process includes sealing the structure, adding a fumigant (phosphine, methyl bromide, and other fumigants), maintaining a certain concentration of the fumigant, and aerating the structure after all fumigant has been removed or has dissipated. Phosphine concentrations in the air surrounding large fumigated structures at a tobacco storage facility were measured using an open-path Fourier transform infrared (FTIR) spectrometer system. These measurements provided more comprehensive data than could be obtained with multiple single-point detection devices. Specifically, this novel application of open-path FTIR was investigated to more completely characterize phosphine concentrations over long path lengths using a single FTIR. Phosphine levels were below the detection limit [Ͻ0.07 parts per million (ppm)] approximately 20 meters (m) east of a warehouse. Phosphine was detected at approximately 4 ppm inside a warehouse at the end of fumigation and just prior to aeration. Levels of phosphine directly in front of the doors of another fumigated warehouse, immediately after aeration, ranged from Ͻ0.07 to 8.3 ppm. Six different sampling paths were also used to investigate the levels of phosphine in various areas of the facility. Average phosphine levels in air around sealed warehouses under active fumigation were found to be the below detection limit.
This work involves the novel use of a radio telemetry-based system that continuously monitors phosphine using two different types of electrochemical detectors (ECD/RT). The ECD/RT units were used to monitor phosphine inside and at varying distances from large tobacco storage warehouses. A master controller unit transferred the data to a personal computer that received and displayed the data. Supervisory control and data acquisition software assimilated the data from each ECD/RT unit, displayed and updated it as new transmissions were received, and stored the data in secure databases. Phosphine concentrations outside five warehouses simultaneously under fumigation and at the facility boundaries were <0.3 parts per million (ppm). Phosphine levels ranged from 0 to 580 ppm inside sealed warehouses. A comparison was made between the data collected at an ECD/RT unit approximately 4 m downwind of a sealed warehouse and a colorimetric tube at the same location. The final phosphine concentration from the colorimetric method was 0.05 ppm and the average over the 20-minute collection period for the ECD/RT was 0.13 ppm. This system allows for continuous, remote monitoring around warehouses under fumigation and superior time resolution allowing timely response to fugitive emissions of phosphine.
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