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ELECTRICAL STRUCTURE OF THE MIDDLE ATMOSPHERE 186 steps of clustering and switching as shown in Figure 13.3. This mechanism, first proposed by Ferguson (1974), yields steady-state ion distributions that are reasonably close to those observed when appropriate reaction rates are used in model calculations (Reid, 1977). Most of the critical reaction rates are still unmeasured at mesospheric temperatures, however. Figure 13.3 Schematic diagram of the principal positive-ion reactions in the mesosphere. The details of the NO+ hydration scheme, enclosed by the broken lines, are not yet established. In the stratosphere, the picture is rather more complicated. Mass spectrometry has recently been developed for use at the high ambient gas pressures of the stratosphere, and measurements of positive-ion composition have been made from rockets and balloons (Arnold et al ., 1978). These experiments showed the existence of the proton hydrates, as in the mesosphere, and also that below about 40 km the proton hydrates are replaced as the dominant species by ions with a core of mass 42 amu. This has been tentatively identified as protonated acetonitrile, H+ (CH3CN) (Arnold et al., 1978)âan identification that is reasonable in view of the high proton affinity of CH3CN and its recent discovery in the troposphere (Becker and Ionescu, 1982). It should be emphasized that our knowledge of stratospheric ion composition is very sketchy. Almost nothing is known of the composition at heights below 30 km or at locations other than continental middle latitudes. Negative Ions Our knowledge of negative-ion composition in the middle atmosphere is in an unsatisfactory state. Laboratory measurements of the negative-ion reactions thought to be the most important ones in the atmosphere have led to the reaction scheme shown in Figure 13.4 (Ferguson et al., 1979). In this scheme, direct attachment of electrons takes place only to O2 and O3; associative detachment reactions occurring chiefly with atomic oxygen quickly destroy most of the resulting and O-ions in regions where O is present. The ions that escape destruction in this way, however, go on to form a wide variety of species whose electron affinity increases as we progress down the chain. In the absence of annihilation by positive ions, the dominant terminal species in the chain would be the nitrate ion, , with the high electron affinity of 3.9 eV (Ferguson et al., 1972). Mass-spectrometer measurements of negative-ion composition are much more difficult to make than the corresponding positive-ion measurements, largely owing to the problem of contamination by electrons. As a result, few measurements have been made in the mesosphere, and these have given somewhat conflicting results (Narcisi et al., 1971; Arnold et al., 1971, 1982). The predicted dominance of such species as and at heights below 80 km appears to be borne out, but many unidentified light ions have been seen in the mesosphere. Above 80 km, there appears to be a layer of heavy (> 100 amu) ions (Arnold et al., 1982) that may be a result of attachment to neutral species of meteoric origin, perhaps forming the very stable silicon species (Viggiano et al., 1982). In the stratosphere, the first mass-spectrometer mea
