Question

The good ozone in the stratosphere and the bad ozone in the troposphere are the same chemical compound.

Describe

1) the ozone formation and decomposition in the stratosphere.

2) the photochemical cycle of ozone formation and removal in the troposphere.

3) Why are the two ozone formation processes different.

4) how do human activities interrupt the two processes?

Answer 1

1) Stratospheric ozone is formed naturally by chemical reactions involving solar ultraviolet radiation (sunlight) and oxygen molecules, which make up 21% of the atmosphere. In the first step, solar ultraviolet radiation breaks apart one oxygen molecule (O_{​​​​​​2}) to produce two oxygen atoms (2 O) (see Figure below). In the second step, each of these highly reactive atoms combines with an oxygen molecule to produce an ozone molecule (O_{​​​​​​3}). These reactions occur continually whenever solar ultraviolet radiation is present in the stratosphere. As a result, the largest ozone production occurs in the tropical stratosphere.

The depletion of the protective O_{​​​​​​3} layer is because of the presence of particular chemicals in the stratosphere of earth’s atmosphere. The constant release of compounds like carbon tetrachloride, carbon tetrafluoride, CFCs (chlorofluorocarbon) or freons and other chlorine or bromine containing halogens in the atmosphere is the main reason for the depletion.

CFC compounds are non-inflammable, non-toxic, nonreactive organic molecules. Hence, it is used in air conditioners, refrigerators, plastic foam production, cleaning computer parts, etc.

However, these chemicals mix with normal atmospheric gases and finally reach the stratosphere. Thus, these compounds break down into free chlorine radicals in the presence of powerful UV radiation in the stratosphere.

CF₂Cl_​​2 (g) → Cl(g) + CF₂Cl(g)

(in presence of powerful UV Radiation)

The chlorine radicals combine with the stratospheric O₃ thereby forming molecular oxygen and chlorine monoxide radicals.

Cl(g) + O_{​​​​​​3}(g) → ClO(g) + O_{​​​​​​2}(g)

Chlorine monoxide radicals will further react with atomic oxygen to form more chlorine radicals.

ClO(g) + O(g) → Cl(g) + O_{​​​​​​2}(g)

This process will continue and constantly regenerate chlorine radicals. This, in turn, will lead to the breakdown of ozone. Hence, CFCs are transporting agents that are responsible for damaging the ozone layer.

2) In the troposphere near the Earth’s surface, ozone forms through the splitting of molecules by sunlight as it does in the stratosphere. However in the troposphere, nitrogen dioxide, not molecular oxygen, provides the primary source of the oxygen atoms required for ozone formation. Sunlight splits nitrogen dioxide into nitric oxide and an oxygen atom.

A single oxygen atom then combines with an oxygen molecule to produce ozone.

Ozone then reacts readily with nitric oxide to yield nitrogen dioxide and oxygen.

The process described above results in no net gain in ozone. Concentrations occur in higher amounts in the troposphere than these reactions alone account for. In the 1950s, chemists discovered that two additional chemical constitutents of the troposphere contribute to ozone formation. These constituents are nitrogen oxides and volatile organic compounds, and they have both natural and industrial sources

Photochemical pollution is formed from emissions of nitrogen oxides (NOx, where NOx = NO + NO_{​​​​​2}) and of volatile organic compounds (VOCs) and carbon monoxide (CO) in the presence of sunlight. Ozone (O_{​​​​​​3}), the major photochemical pollutant, is transported across national boundaries. Emissions of NOx are responsible for much of the ozone formation occurring in rural areas. In more densely populated regions, in particular close to cities, ozone formation is enhanced by VOC emissions. VOCs are mainly released from road traffic and the use of products containing organic solvents. NOx and CO are mostly emitted from transport and combustion processes. After release, these precursors are dispersed by wind and atmospheric turbulence. The freshly emitted pollutants mix with other pollutants, including ozone, present in background air, and a complicated process of chemical reactions and continuous dilution takes place.

Ozone is removed from the troposphere by surface deposition and by photochemical loss processes.

As in the stratosphere, ozone in the troposphere is destroyed by naturally occurring chemical reactions and by reactions involving human-produced chemicals. Tropospheric ozone can also be destroyed when ozone reacts with a variety of surfaces, such as those of soils and plants.

3) Ozone occurs in two layers of the atmosphere. The layer closest to the Earth's surface is the troposphere. Here, ground-level or "bad" ozone is an air pollutant that is harmful to breathe and it damages crops, trees and other vegetation. It is a main ingredient of urban smog. Bad” ozone is found at ground level. In cities, it’s made when emissions from vehicles, power plants, chemical plants, and other sources react with heat and sunlight. The hotter the day and the stronger the sun, the more ozone is formed. The troposphere generally extends to a level about 6 miles up, where it meets the second layer, the stratosphere. The stratosphere or "good" ozone layer extends upward from about 6 to 30 miles and protects life on Earth from the sun's harmful ultraviolet (UV) rays. Stratospheric ozone is formed naturally by chemical reactions involving solar ultraviolet radiation (sunlight) and oxygen molecules, which make up 21% of the atmosphere.

4) The initial step in the depletion of stratospheric ozone by human activities is the emission, at Earth’s surface, of gases containing chlorine and bromine. Most of these gases accumulate in the lower atmosphere because they are unreactive and do not dissolve readily in rain or snow. Natural air motions transport these accumulated gases to the stratosphere, where they are converted to more reactive gases. Some of these gases then participate in reactions that destroy ozone. Finally, when air returns to the lower atmosphere, these reactive chlorine and bromine gases are removed from Earth’s atmosphere by rain and snow.

Emission, accumulation, and transport. The principal steps in stratospheric ozone depletion caused by human activities are shown in Figure. The process begins with the emission, at Earth’s surface, of source gases containing the halogens chlorine and bromine. The halogen source gases, often referred to as ozone-depleting substances (ODSs), include manufactured chemicals released to the atmosphere in a variety of applications, such as refrigeration, air conditioning, and foam blowing. Chlorofluorocarbons (CFCs) are an important example of chlorine-containing gases. Emitted source gases accumulate in the lower atmosphere (troposphere) and are transported to the stratosphere by natural air motions. The accumulation occurs because most source gases are highly unreactive in the lower atmosphere. Small amounts of these gases dissolve in ocean waters. The low reactivity of these manufactured halogenated gases is one property that makes them well suited for specialized applications such as refrigeration. Some halogen gases are emitted in substantial quantities from natural sources. These emissions also accumulate in the troposphere, are transported to the stratosphere, and participate in ozone destruction reactions.

Ozone depletion by halogen source gases occurs globally. Large seasonal ozone losses occur in polar regions as a result of reactions involving polar stratospheric clouds (PSCs). Ozone depletion ends when reactive halogen gases are removed by rain and snow in the troposphere and deposited on Earth’s surface.