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Methane sulfonamide (MSAM), CH3S(O)(2)NH2, was recently detected for the first time in ambient air over the Red Sea and the Gulf of Aden where peak mixing ratios of approximate to 60 pptv were recorded. Prior to this study the rate constant for its reaction with the OH radical and the products thereby formed were unknown, precluding assessment of its role in the atmosphere. We have studied the OH-initiated photo-oxidation of MSAM in air (298 K, 700 Torr total pressure) in a photochemical reactor using in situ detection of MSAM and its products by Fourier transform infrared (FTIR) absorption spectroscopy. The relative rate technique, using three different reference compounds, was used to derive a rate coefficient of (1.4 +/- 0.3) x 10(-13) cm(3) molec.(-1) s(-1). The main end products of the photo-oxidation observed by FTIR were CO2, CO, SO2, and HNO3 with molar yields of (0.73 +/- 0.11), (0.28 +/- 0.04), (0.96 +/- 0.15), and (0.62 +/- 0.09), respectively. N2O and HC(O)OH were also observed in smaller yields of (0.09 +/- 0.02) and (0.03 +/- 0.01). Both the low rate coefficient and the products formed are consistent with hydrogen abstraction from the -CH3 group as the dominant initial step. Based on our results MSAM has an atmospheric lifetime with respect to loss by reaction with OH of about 80 d.