4.3 General reactions in the troposphere and stratosphere

Some reactions in the atmosphere involve major gases and occur on global scale, however, others involve trace chemical species on smaller scales (e.g., continental, regional, local scales). Composition of the troposphere shows high variability in mixing ratios of reactive trace species. These compounds are present in low concentration and many of them have short residence time and they are very reactive. Nitrogen oxide (NO) is generated during the combustion processes (e.g., fossil fuel burning, forest fires, lightning discharges)

.

(R4.11)

Nitrogen oxide can oxidized further producing nitrogen dioxide (NO2)

.

(R4.12)

It should be noted that, however, the concentration of oxygen is the highest in the atmosphere reaction above (R4.12) would not produce a significant amount of nitrogen dioxide, because this reaction kinetically is very slow. Actually, O2 is not the most important oxidant for this oxidation reaction. There are other very reactive species, which have the major effect in the oxidation of nitrogen oxide and are present in photochemical smog: ozone (O3), peroxy radicals (ROO•) and oxyradicals (RO•)

.

(R4.13)

One important tropospheric photochemical reaction is the decomposition of nitrogen dioxide

.

(R4.14)

The atomic oxygen produced in this reaction reacts with molecular oxygen to produce ozone, and this is only the significant pathway to produce ozone in the troposphere.

Radicals also play a very significant role in many atmospheric chemical reactions because of their high affinity to react. The most important radical that plays a very crucial role in many atmospheric chemical reactions is the hydroxyl radical (OH•). This reactive species forms from the following consecutive steps involving photochemical reactions

,

(R4.15)

,

(R4.16)

,

(R4.17)

.

(R4.18)

Atomic oxygen produced in the reaction (R4.15) is ground state oxygen, while oxygen atom produced in reaction (R4.17) is in excited state. For reaction (R4.15) radiation in the visible range supplies sufficient energy, however, in reaction (R4.18) higher energy (UV) radiation is required. Excited state oxygen atoms are essential to form radical with water.

Hydroxyl radical can react with various stable species in the atmosphere and provides other reactive compounds

,

(R4.19)

.

(R4.20)

The hydrogen () and methyl () radicals formed react further

,

(R4.21)

.

(R4.22)

The peroxy radical () is an important oxidants in the oxidation of nitric acid (Seinfeld and Pandis, 2006).

We should remember that many important atmospheric processes are mostly photochemical processes. The energy of solar radiation increases with increasing altitude, therefore photochemistry becomes even more important in the upper atmosphere. Radicals play a dominant role in many atmospheric reactions and the hydroxyl radical is the most reactive compound and it contributes to the oxidative nature of photochemical smog.

A detailed description of reactions of chemical species of carbon, nitrogen, sulphur and oxygen is provided in Chapters 5–8.

References

Seinfeld J.H. and Pandis S.N.. 2006. Atmospheric Chemistry and Physics: From Air Pollution to Climate Change. Wiley.

Sharma A.. 2007. Environmental Chemistry. GOEL Publishing House, Meerut.