Ternary Ion-Molecule Reactions

As previously mentioned, ternary (or 3-body) ion-molecule reactions are only significant in the Earth s atmosphere below the mesopause ( 80 km) where they play a crucial part in the ion chemistry. In this region of the atmosphere, the major problem to be solved is to determine the ionic reaction paths which convert the primary positive and negative ions to the dominant positively and negatively charged water cluster ions [see reactions (6) and (11)]. Most progress has been made in elucidating the positive ion chemistry, so this will be considered first. 

In the D-region, the primary positive ions are NO+ and 02 (NO+ being dominant) and so the problem reduces to one of finding a quantitative chemical scheme for the conversion of NO+ to H30+(H20)n ions, the steps of which are sufficiently rapid to remove NO+ at a rate commensurate with its known production rate from photoionization of NO. The first proposal was made by Fehsenfeld and Ferguson63) who, with their colleagues, have contributed so much to this area of research. Their scheme, based on their FA data, is summarised by the following reactions, the first step being the direct 3-body hydration of NO+  

The ternary reactions (32) and (33) sequentially build up the NO+ hydrates until N0+(H20)3 is produced, which then undergoes a binary reaction to give the hydrated hydronium ions. Further increases in the size of the hydrates is considered to be via direct clustering  

In the atmosphere, M is an abundant neutral i.e. either N2 or 02. Whilst the authenticity of this chemical scheme is not in doubt, since it is based on a considerable amount of data from several groups64,66-69 model calculations have shown that it is not able to quantitatively explain the rate of loss of NO+ in the ionosphere, the major problem being that reaction (32) leading to the first NO+ hydrate, is not sufficiently rapid. The details of this problem have been discussed in several reviews8,164-166 . 

Alternative, more rapid paths for the production of NO+ H20 have been proposed167-170 involving as a first step the ternary association of NO+ with the abundant atmospheric C02 and N2 

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