Laboratory study on heterogeneous decomposition of methyl chloroform

Methyl chloroform (1,1,1-trichloroethane, CH 3 CCl 3) was found to decompose heterogeneously on seven types of standard clay minerals (23 materials) in dry air at 313 K in the laboratory. All reactions proceeded through the elimination of HCl; CH 3 CCl 3 was converted quantitatively to CH 2 =CCl 2. The activities of the clay minerals were com-5 pared via their pseudo-first-order reaction rate constants (k 1). A positive correlation was observed between the k 1 value and the specific surface area (S) of clay minerals, where the S value was determined by means of the general Brunauer-Emmett-Teller (BET) equation. The k 1 value was anti-correlated with the value of n, a parameter of the general BET equation, and correlated with the water content that can be removed 10 easily from the clay minerals. The reaction required no special pretreatment of clay minerals, such as heating at high temperatures; hence, the reaction can be expected to occur in the environment. Photoillumination by wavelengths present in the tropo-sphere did not accelerate the decomposition of CH 3 CCl 3 , but it induced heterogeneous photodecomposition of CH 2 =CCl 2. The temperature dependence of k 1 , the adsorption 15 constants of CH 3 CCl 3 and CH 2 =CCl 2 , and a surface reaction rate constant were determined for an illite sample. The k 1 value increased with increasing temperature. The amount of CH 3 CCl 3 adsorbed on the illite during the reaction was proportional to the partial pressure of CH 3 CCl 3. The reaction was sensitive to relative humidity and the k 1 value decreased with increasing relative humidity. However, the reaction was found 20 to proceed at a relative humidity of 22% at 313 K, although the k 1 value was about one-twentieth of the value in dry air. The conditions required for the reaction may be present in major desert regions of the world. A simple estimation indicates that the possible heterogeneous decomposition of CH 3 CCl 3 on the ground surface in arid regions is worth taking into consideration when inferring the tropospheric lifetime of CH 3 CCl 3 25 and global OH concentration from the global budget concentration of CH 3 CCl 3 .