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In the atmosphere DMS will be subject to gravitational settling and wash out by rain as well as reaction with photochemically produced hydroxyl radicals. The calculated (QSAR) half-life time for this indirect photolysis is 84.3 days (Hoechst, 1996). The method of calculation is according to the Technical Guidance Document (EC 1996). In addition, the reactivity of DMS towards OH radicals has been determined experimentally and the rate constant for this reaction is < 5.10-13cm3.molecule-1.sec-1 (Japar, 1990). This rate constant corresponds with an atmospheric lifetime of > 23 days and a DT50of >16 days ([OH]= For the PECair-calculations the DT50of 84 days is chosen. However, it has to be noted that atmospheric hydrolysis may occur faster.



Dimethyl sulphate hydrolyses in water. Reaction rate and type of hydrolysis products depend on pH and temperature. In neutral or acid medium methanol and sulphuric acid are formed, whereas under alkaline conditions the reaction products are methanol and monomethyl sulphate (room temperature) or methanol and sulphate (elevated temperature), respectively. Several estimates for the hydrolysis rate are reported in literature. The half-life at room temperature and pH 7 is about 24 hours; at room temperature and pH 10 it is about 86 seconds. Other observed degradation times are: >70 hours at 8oC, 14 hours at 23oC, and 70 min. at 40oC. In another study a half-life of 1.2 hours is reported for the hydrolysis of DMS under neutral conditions at 25°C.

The EHC document (1985) mentions additional DT50values of 4.5 h (temperature unknown) and 40 minutes (at 20oC), at neutral conditions.