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Module 6: Air Pollutants and Control Techniques - Ozone - Formation Mechanisms
Tropospheric Ozone
Stratospheric Ozone
Practice
Problems
Objective
- Describe the basic formation mechanisms for ozone.
Ozone is a pollutant that forms photochemically (i.e. a reaction caused by sunlight) in the troposphere (lowest portion of the atmosphere) due to the presence of nitrogen oxides, volatile organic compounds (VOCs), and carbon monoxide (CO). The emission rates of these precursor compounds are regulated in order to minimize the formation rate of ozone and the other photochemical reaction products.
The general nature of the photochemical reactions is illustrated by the pollutant profiles (Figure 1) found in smog chamber studies that simulate urban air masses. The reactions begin quickly following the injection of nitrogen oxides, organic compounds, and carbon monoxide. Nitric oxide (NO) emitted from fossil-fuel-fired boilers, automobiles, trucks, and other fuel burning emission sources is rapidly converted to nitrogen dioxide (NO2) due to photochemically initiated reactions. The formation of nitrogen dioxide further stimulates the photochemical smog-forming reactions because nitrogen dioxide is very efficient at absorbing sunlight in the ultraviolet portion of its spectrum.
As the reactions proceed further, nitrogen dioxide reacts to form particulate and vapor phase nitrates. As the nitric oxide concentration drops, the levels of ozone rise rapidly. Along with the increase in ozone, the levels of various partial oxidation products also increase. Many of these compounds are in the form of particulate matter, which scatter light. These particles create the smog associated with photochemical reactions.
The formation of ambient levels of ozone is highest during the summer months from May through September (Northern hemisphere). This is often termed the ozone season. The intensity of sunlight necessary for the photochemically initiated reactions is highest during this time period.
Ozone can also form in apparently clean rural environments. In these areas, the "pollutants" are apparently low levels of organic compounds emitted from trees and vegetation and low levels of nitrogen oxides emitted from natural biological activity. The photochemical reactions are similar to those in polluted urban areas; however, the concentrations of rural ozone are limited by the very low concentrations of nitrogen oxides that are usually available. Rural ozone is considered natural.
In the stratosphere, ozone forms naturally from molecular oxygen. The presence of the ozone in the stratosphere is beneficial because ozone absorbs ultraviolet radiation from the sun. Maintaining adequate ozone concentrations in the stratosphere is critical to ensuring that large areas of the Earth are not subjected to harmful ultraviolet radiation. The stratospheric ozone concentrations are decreasing over North America due to the presence of chlorinated and fluorinated organic compounds, which are not reactive at the Earth's surface. Once these compounds are transferred convectively to the stratosphere, they can initiate free radical chain reactions that reduce the equilibrium concentrations of ozone.
The control of precursor gases such as nitrogen oxides emitted into the troposphere to minimize ground level ozone concentrations will not have an adverse effect on the beneficial ozone levels in the stratosphere. The formation mechanisms for ozone in the stratosphere are entirely different than those in the troposphere.
Practice Problems
Ozone - Formation Mechanisms
- Instructions:
- Complete the Practice Problems before proceeding to the next section. Click on the button below.