Model calculations of the negative ion chemistry in the mesosphere with special emphasis on the chlorine species and the formation of cluster ions

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The chemistry of negative ions in the D-region is as complex as that of positive ions, and many aspects are still not clear. Only a small number of in situ mass spectrometer measurements have been reported up to now and there is a general lack of laboratory data of negative ion reactions, especially for complex and clustered species. The primary negative ion O 2 − is transformed through a chain of reactions leading to relatively stable ions such as NO 3 −, CO 3 − or HCO 3 −. Beyond this generally accepted scheme, it is expected that all of the above long-lived ions will become hydrated or clustered, especially in the night-time ionosphere, to form higher mass terminal ions which have been observed occasionally in rocket flights. Different mechanisms, which are suggested in the literature, as well as results are presented in a complete ion-composition model. The effect of clustering on the negative ion distribution and free electrons, mostly induced through different photodissociation or reactivity of negative ions, is discussed. Special attention is also given to chlorine.