Positive Ion Chemistry

The chemistry of positive ions in the middle atmosphere is relatively well understood. Ions such as O.J and NO+, which dominate in the lower thermosphere, are rapidly lost below 80 km in a set of clustering reactions ending with stable proton hydrates of the type H+(H20)n. The hydration order n depends on temperature and water vapor concentration. In the stratosphere, water ligands are partly or totally replaced by other molecules such as methyl cyanide (CH3CN), whose proton affinity is larger than that of water molecules. 

The primary ion is present in large abundances because its destruction rate by dissociative recombination 

The equations for the ionic constituents of the E-region can now be written. Neglecting minor reactions and eliminating the density of N, we can write 

The equation of electrical neutrality must also be considered in ionospheric models. In the E-region, we may write  

Gas phase metal ions have been observed at mid-latitudes by several investigators and appear to be related to the layers of metallic atoms present in the upper mesosphere and lower thermosphere. Meteoric ablation is believed to be the dominant source of these metals (Plane, 1991). The presence of atomic sodium (Na) was first detected from its noctural emission by Slipher (1929) and is discussed by Chapman (1939). In recent decades several other metallic atoms (e.g., K, Li, Fe, Ca, etc.) and ions (e.g., Mg+, Fe+, A1+, Na+, Ca+) have been identified. These species are present in permanent layers from 85 to 110 km, which have been studied in detail using resonance fluorescence lidar techniques (see e.g., Granier et al., 1989 Bills and Gardner, 1990 Qian and Gardner, 1995). 

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Several of the systems reported in the previous section occur in solution under conditions of base or acid catalysis. Thus, it is not surprising that the positive ion chemistry of such systems will resemble the acid-catalysed process. 

As in the case of the positive ion chemistry, the problem is to describe the chemical steps which convert the primary negative ions to the observed clusters. The primary negative ions can only be O- and 02 formed in the electron attachment reactions ... 

Note the generation again of NO+ and 02 as well as the ion N02. A detailed study has recently been reported of the reactions of the primary and secondary stratospheric ions with several molecules152) and of the reactions of O4 and Os with several stratospheric neutrals1 S3). It seems clear from these studies that although fast binary ion-molecule reactions are important first steps in the positive ion chemistry of the lower atmosphere, the subsequent chemistry is controlled by ternary association reactions (Sect. 3.2.3). 

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. 

A. Thermochemistry, Structure and Reactivity Related to the Gas-phase Positive Ion Chemistry of Ge, Sn and Pb Compounds... 

The observed positive ions are protonated clusters containing water and high proton affinity species such as acetonitrile in the lower stratosphere (26) or ammonia in the lower troposphere (20). Other high proton affinity species such as pyridine and picolines may enter into the positive ion chemistry of the lower troposphere (27,28). Further discussion of these studies and the experimental techniques can be found elsewhere (28,29). 

It has been very difficult to limit the scope of this review. After some consideration, chose to divide it into sections in close correspondence with the tradition of text books in physical organic chemistry. I also decided to concentrate on literature from the last decade, although it has been necessary to cite classical papers of the field for the sake of completeness and to give an overview. I have to admit that I have chosen to focus on the literature of positive ion chemistry rather than that of negative ion chemistry, since excellent reviews have been published on that subject recently [1-3]. 

In summary, it seems that our understanding of the stratospheric positive ion chemistry is far from satisfactory because independent information on both underlying processes and reactant trace gases is largely lacking. 

Thus, our understanding of stratospheric ion chemistry, like that of positive ion chemistry, is far from satisfactory due to the lack of independent information on reactant trace gases and laboratory data. 

Other potential reactant trace gases besides H2O and EhS04 are NIL,. CILCN, and acids such as HNO3 and HCl. Ammonia, for example, is highly soluble in water and therefore may become depleted from the gas phase. According to in situ measurements, tropospheric ammonia vapor abundances greatly exceed the critical reactant trace gas level. Consequently, NHj may markedly influence the positive ion chemistry. The same may be true for CILCN which seems to originate from the troposphere. 

Figure 7.21. Schematic diagram of D-region positive ion chemistry. From Ferguson (1979).

ICR studies of phosphorus esters have been reported (25-27). However, phosphorylation of neutral nucleophiles is not well documented. Accordingly, we examined the positive-ion chemistry of alcohols with phosphorus esters. 

The effects of added water on G(NH3) and G(propionic) from the sodium salt of acetylalanine are summarized in Figure 3. The ammonia yield which as we have noted is derived from a number of reaction modes shows but a small decrease, AG(NH3), — 1, as the acetylalanine concentration is decreased to 1M. And, even more striking is the fact that the yield of the major organic product, propionic acid, is essentially independent of acetylalanine concentration over the entire range of Figure 3. Our tentative conclusion is then that cleavage of the N—C bond to yield propionic acid does not arise in the main from the positive-ion chemistry of Reactions 16, 16a, 18, and 18a. 

The ion chemistry of the lower TA (below 100 km) is dominated by termolecular (three-body) reactions of both positive ions and negative ions. At altitudes between about 50 to 90 km, in the ionospheric D-re-gion, most ionizing solar radiations have been filtered out except for and Lp radiation, which can selectively ionize NO and 02( Ag) molecules. So the initial ions in the positive ion chemistry are NO+ and... 

Figure 5 The ion chemistry of the lower atmosphere. The positive ion chemistry (left column) and the negative ion chemistry (right column) are dominated by termolecular reactions, finally producing the cluster ions in the lower boxes.

Bohme, D. K. (2000) Experimental studies of positive ion chemistry with flow-tube mass spectrometry birth evolution, and achievements in the 20th century. Int. J. Mass Spectrom., 200,97. 

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