Cyclooctane tropospheric degradation initiated by reaction with Cl atoms

Within the non-methane hydrocarbons, alkanes constitute the largest fraction of the anthropogenic emissions of volatile organic compounds. For the case of cyclic alkanes, tropospheric degradation is expected to be initiated mainly by OH reactions in the gas phase. Nevertheless, Cl atom reaction rate constants are generally one order of magnitude larger than those of OH. In the present work, the reaction of cyclooctane with Cl atoms has been studied within the temperature range of 279–333 K. The kinetic study has been carried out using the fast flow tube technique coupled to mass spectrometry detection. The reaction has been studied under low pressure conditions, p=1 Torr, with helium as the carrier gas. The measured room temperature rate constant is very high, k=(2.63±0.54)×10−10 cm3molecule−1s−1, around 20 times larger than that for the corresponding OH reaction. We also report the results of the rate coefficients obtained at different temperatures: k = (3.5±1.2)×10−10 exp[(−79±110)/T] cm3 molecule−1 s−1 within the range of 279–333 K. This reaction shows an activation energy value close to zero. Quantitative formation of HCl has been observed, confirming the mechanism through H-atom abstraction. The reactivity of cyclic alkanes towards Cl atoms is clearly dependent on the number of CH2 groups in the molecule, as is shown by the increase in the rate constant when the length of the organic chain increases. This increase is very high for the small cyclic alkanes and it seems that the reactions are approaching the collision-controlled limit for cyclohexane and cyclooctane. These results show that gas-phase reaction with Cl in marine or coastal areas is an efficient sink (competing with the gas phase, OH initiated degradation) for the Earth’s emissions of cyclooctane, with a Cl-based lifetime ranging from 11 to 2000 hours, depending on the location and time of day. Cl and OH fast reactions with cyclooctane are expected to define the lifetime of cyclooctane emissions to the atmosphere. The degradation of cyclooctane occurs in a short period of time and consequently (under conditions of low atmospheric mass transport), close to the emission sources enabling a significant contribution to local effects, like the formation of photochemical smog.