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

• Main Authors: S. Kutsuna, L. Chen, K. Ohno, N. Negishi, K. Takeuchi, T. Ibusuki, K. Tokuhashi, A. Sekiya
• Format: Article
• Language: English
• Published: Copernicus Publications 2003-01-01
• Series: Atmospheric Chemistry and Physics
• Online Access: http://www.atmos-chem-phys.net/3/1063/2003/acp-3-1063-2003.pdf
• ISSN: 1680-7316; 1680-7324

Methyl chloroform (1,1,1-trichloroethane, CH₃CCl₃) 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; CH₃CCl₃ was converted quantitatively to CH₂=CCl₂. The activities of the clay minerals were compared via their pseudo-first-order reaction rate constants (<i>k</i>₁). A positive correlation was observed between the <i>k</i>₁ value and the specific surface area (<i>S</i>) of clay minerals, where the <i>S</i> value was determined by means of the general Brunauer-Emmett-Teller (BET) equation. The <i>k</i>₁ value was anti-correlated with the value of <i>n</i>, which was a parameter of the general BET equation and related to the average pore size of the clay minerals, and correlated with the water content that can be removed 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 troposphere did not accelerate the decomposition of CH₃CCl₃, but it induced heterogeneous photodecomposition of CH₂=CCl₂. The temperature dependence of <i>k</i>₁, the adsorption equilibrium coefficient of CH₃CCl₃ and CH₂=CCl₂, and the surface reaction rate constant of CH₃CCl₃ were determined for an illite sample. The <i>k</i>₁ value increased with increasing temperature. The amount of CH₃CCl₃ adsorbed on the illite during the reaction was proportional to the partial pressure of CH₃CCl₃. The reaction was sensitive to relative humidity and the <i>k</i>₁ value decreased with increasing relative humidity. However, the reaction was found to proceed at a relative humidity of 22% at 313 K, although the <i>k</i>₁ value was about one-twentieth of the value in non-humidified 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 CH₃CCl₃ on the ground surface in arid regions is worth taking into consideration when inferring the tropospheric lifetime of CH₃CCl₃ and global OH concentration from the global budget concentration of CH₃CCl₃.