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Tropospheric Ozone
Tropospheric ozone is a chemical that may be a trace constituent of natural
biogeochemical cycles, but which presents an environmental problem primarily due
to large anthropogenic sources in major cities. In future learning units
we will discuss the problem of diminishing amounts of ozone in the stratosphere,
but for now we are focusing on an excess of ozone in the lower levels in the
atmosphere. Ozone is a very reactive chemical that interacts with plant and animal
living tissue in a detrimental way. It can reduce lung functioning in humans and
suppress plant growth enough to reduce yields of agricultural and horticultural crops.
Ozone is formed in the troposphere by various chemical reactions involving a combination of the oxides of nitrogen, reactive volatile organic compounds (VOC) produced by a combination of exhaust from automobiles in the form of unburned hydrocarbons, and diatomic oxygen in the presence of sunlight (Figure 12). High atmospheric temperatures and/or intense radiation at high altitudes are required to form ozone.
Typical reactions to create ozone include nitrogen dioxide being illuminated by sunlight in the wavelength region of 295 and 430 microns to give NO plus an oxygen. The free oxygen combines with diatomic oxygen in the presence of another molecule (to give a momentum balance) to produce ozone. A rapid ozone destruction reaction consists of NO interacting with ozone to produce and . With ozone continually being produced and destroyed by these two reactions, an equilibrium condition is established in which the intermediate constituent, namely ozone, exists at a concentration determined by the ratio of the abundance of and NO. Oxidation of photochemically reactive VOCs forms highly reactive chemical species known as free radicals, which are capable of converting NO and without the destruction of . As a result, these volatile organic compounds, mainly produced by automobiles, can interfere with the natural ozone destruction process and allow ozone concentrations to become elevated.
If any one of the four ingredients (, VOC, , and sunlight) is absent, ozone concentrations diminish. Morning and evening rush-hour traffic in major cities on clear warm days provide the sufficient amounts of all ingredients, but elimination of any one reduces ozone levels significantly.
Anything that is susceptible to oxidation is a potential target for the adverse effects of elevated ozone concentrations (Figure 13). The human lung, for instance, particularly for people who have respiratory problems, can experience diminished functioning or reduced capacity. Figures 14, 15, 16, and 17 show examples of estimated yield losses for a variety of agricultural and horticultural crops due to elevated ozone levels.
Soybeans, for instance, under an ozone concentration of 0.081 parts per million will experience about a 30% loss of yield. The EPA threshold standard for ozone is 0.12 ppm, so even at levels below the EPA standard, soybeans could suffer loss of yield.
Other grain crops, such as wheat for instance, seem to be less vulnerable than soybeans. A quick scan of the vegetables listed also shows that ozone concentrations below the EPA standard will lead to significant yield reductions in all varieties.
