Question

Describe the role of ozone in the atmosphere & analyze the kinetics of ozone in the troposphere.

Answer 1

Role of ozone in atmosphere :

The ozone molecules in the upper atmosphere (stratosphere) and the lower atmosphere (troposphere) are chemically identical, because they all consist of three oxygen atoms and have the chemical formula O3. However, they have very different roles in the atmosphere and very different effects on humans and other living beings. Stratospheric ozone (sometimes referred to as "good ozone") plays a beneficial role by absorbing most of the biologically damaging ultraviolet sunlight (called UV-B), allowing only a small amount to reach the Earth's surface. The absorption of ultraviolet radiation by ozone creates a source of heat, which actually forms the stratosphere itself (a region in which the temperature rises as one goes to higher altitudes). Ozone thus plays a key role in the temperature structure of the Earth's atmosphere. Without the filtering action of the ozone layer, more of the Sun's UV-B radiation would penetrate the atmosphere and would reach the Earth's surface. Many experimental studies of plants and animals and clinical studies of humans have shown the harmful effects of excessive exposure to UV-B radiation.

At the Earth's surface, ozone comes into direct contact with life-forms and displays its destructive side (hence, it is often called "bad ozone"). Because ozone reacts strongly with other molecules, high levels of ozone are toxic to living systems.

Kinetics :

The steppwise reactions are :

In this mechanism, O2 undergoes photodissociation to form two oxygen atoms in equation (1) using light of energy E=hn. An oxygen atom then combines with an oxygen molecule in equation (2) to produce ozone. In equation (2), M is any third body in the atmosphere (likely N2 or O2) that carries away the excess energy and allows the ozone to remain intact. Thus in simplistic terms, O3 is produced by reaction (1) followed by (2). Ozone appears to be lost via reactions (3) and (4). However, reaction (3) does not represent a true loss for ozone because O atoms formed in (3) can reform O3 via reaction (2).

Order of the reaction :

Looking at each elementary reaction, we would expect that reaction (1) follows first order kinetics, reaction (2) follows third order kinetics, reaction (3) follows first order kinetics and reaction (4) follows second order kinetics. Note that reaction (2) requires three species to come together at the same time, i.e. it is a termolecular reaction. If we look at the relative concentration of species in the atmosphere, O2 and M (= O2 + N2) have much higher concentrations than O and O3, and so they can be considered to be essentially constant over time. Thus, we might expect reaction (1) to obey zero order kinetics (a “pseudo” zero order reaction) and reaction (2) to obey first order kinetics (“pseudo” first order reaction).