Laboratory study on heterogeneous decomposition of methyl chloroform on various standard aluminosilica clay minerals as a potential tropospheric sink

[摘要] Methyl chloroform (1,1,1-trichloroethane,CH₃CCl₃) was found to decompose heterogeneously on seven types of standard clay minerals (23 materials) in dry airat 313 K in the laboratory. All reactions proceeded through the elimination ofHCl; CH₃CCl₃was converted quantitatively to CH₂=CCl₂. The activitiesof the clay minerals were compared via their pseudo-first-order reaction rate constants(k₁). A positive correlation was observed between the k₁value and the specific surface area (S) of clay minerals, where theS value was determined by means of the general Brunauer-Emmett-Teller (BET) equation. Thek₁ value was anti-correlated with the value of n, which was a parameter of the general BET equation and related to theaverage pore size of the clay minerals, and correlated with the water content that canbe removed easily from the clay minerals. The reaction required no special pretreatmentof clay minerals, such as heating at high temperatures; hence, the reaction can beexpected to occur in the environment. Photoillumination by wavelengths present in thetroposphere did not accelerate the decomposition of CH₃CCl₃, but it inducedheterogeneous photodecomposition of CH₂=CCl₂. The temperature dependenceof k₁, the adsorption equilibrium coefficient of CH₃CCl₃and CH₂=CCl₂, and the surface reaction rate constant ofCH₃CCl₃ were determined for an illite sample. The k₁value increased with increasing temperature. The amount of CH₃CCl₃ adsorbedon the illite during the reaction was proportional to the partial pressure ofCH₃CCl₃. The reaction was sensitive to relative humidity and thek₁ value decreased with increasing relative humidity. However, the reaction was found to proceed at a relative humidity of22% at 313 K, although the k₁ value was about one-twentieth of the value in non-humidified air. The conditionsrequired for the reaction may be present in major desert regions of the world. A simple estimationindicates that the possible heterogeneous decomposition of CH₃CCl₃ on the ground surface in arid regionsis worth taking into consideration when inferring the tropospheric lifetime ofCH₃CCl₃ and global OH concentration from the global budget concentration ofCH₃CCl₃.

[学科分类] 大气科学