MEASURED AND COMPUTED CONCENTRATIONS OF 1,3-DICHLOROPROPENE AND METHYL ISOTHIOCYANATE IN AIR IN A REGION WITH INTENSIVE USE OF SOIL FUMIGANTS

A sampling programme was set up to measure 1,3-dichloropropene and methyl isothiocyanate in air in a region with intensive agricultural use of these soil fumigants. In two consecutive autumns, 6-hour air samples were taken at two locations in that region with charcoal tubes using automatic samplers. Most (81%) of the 6-hour concentrations of 1,3-dichloropropene measured in both years were below 3.2 mu g m(-3) and a few percent could not be measured with a detection limit of around 0.3 mu g m(-3). Only 4% of the 6-hour concentrations exceeded 10 mu g m(-3), almost all of which were measured at a location where a field just upwind of the measuring site had been treated. For methyl isothiocyanate, 73% of the 6-hour concentrations of both years could not be measured with a detection limit in the two years of 1 and 2 mu g m(-3), respectively. A small fraction (3%) of the concentrations were in the range of 3.2 to 10 mu g m(-3) and only 1% exceeded 10 mu g m(-3). The rates of emission of 1,3-dichloropropene and methyl isothiocyanate into air were estimated for,weeks with many applications in the region studied. Using the PAL model, the concentration of fumigant in air at a receptor site was computed for representative fumigations at different upwind positions. The computed concentrations in air ranged up to 9.9 mu g m(-3) for 1,3-dichloropropene and up to 2.5 mu g m(-3) for methyl isothiocyanate.