Mechanistic and kinetic study of formaldehyde production in the atmospheric oxidation of dimethyl sulfide

Abstract

Tunable diode laser spectroscopic detection of formaldehyde (H₂CO) and HCl coupled with laser flash photolysis of Cl₂CO–CH₃SCH₃–O₂–N₂ mixtures, in both the presence and absence of NO, has been utilized to conduct a mechanistic and kinetic investigation of the atmospheric oxidation of the CH₃SCH₂ radical, a product of dimethyl sulfide (DMS, CH₃SCH₃) reactions with OH and NO₃ in the atmosphere. The temperature dependence of the CH₃SCH₂O₂ + NO rate coefficient (k₂) and the 298 K rate coefficient for the CH₃SCH₂O₂ self reaction (k₄) have been measured. The Arrhenius expression k₂ = 4.9 × 10⁻¹² exp(263/T) cm³ molecule⁻¹ s⁻¹ adequately summarizes our CH₃SCH₂O₂ + NO kinetic data over the temperature range 261–400 K. Contributions from side reactions, which are not completely quantifiable, limit the accuracy of the k₄ (298 K) determination; our results indicate that the true value for this rate coefficient is within the range (1.2 ± 0.5) × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹. In both reactions CH₃SCH₂O₂ is converted to H₂CO with unit yield (at T = 298 K). Our results demonstrate that the lifetime of CH₃SCH₂O, a proposed precursor to H₂CO, is less than 30 µs at 261 K and 10 Torr total pressure.