Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 3 2 2003 10.5194/acpd-3-1843-2003 http://www.atmos-chem-phys-discuss.net/3/1843/2003/ http://www.atmos-chem-phys-discuss.net/3/1843/2003/acpd-3-1843-2003.html http://www.atmos-chem-phys-discuss.net/3/1843/2003/acpd-3-1843-2003.pdf 1843 1891 2003-04-01 Laboratory study on heterogeneous decomposition of methyl chloroform on various standard aluminosilica clay minerals as a potential tropospheric sink S. Kutsuna L. Chen O. Ohno N. Negishi K. Takeuchi T. Ibusuki K. Tokuhashi A. Sekiya National Institute of Advanced Industrial Science and Technology (AIST), 16-1 Onogawa, Tsukuba, Ibaraki 305-8569, Japan 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>, a parameter of the general BET equation, 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 constants of CH₃CC₃ and CH₂=CCl₂, and a surface reaction rate constant 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 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 CH₃CC₃ on the ground surface in arid regions is worth taking into consideration when inferring the tropospheric lifetime of CH₃CC₃ and global OH concentration from the global budget concentration of CH₃CCl₃.