Many urban centers in the world undergo episodes of air pollution during which relatively high levels of ground-level ozone—an undesirable constitu- ent of air if present in appreciable concentrations at low altitudes in the air that we breathe—are produced as a result of the light-induced chemical reac- tion of pollutants. This phenomenon is called photochemical smog, and is sometimes characterized as “an ozone layer in the wrong place,” to contrast it with the beneficial stratospheric ozone discussed in Chapter 1. The word smog is a combination of smoke and fog. The process of smog formation involves hundreds of different reactions, involving dozens of chemicals, occurring simultaneously. Indeed, urban atmospheres have been referred to as giant chemical reactors. The most important reactions that occur in such air masses will be discussed in greater detail in Chapter 17. In the following material, we investigate the nature and origin of the pollutants—especially nitrogen oxides—and see how they combine to produce photochemical smog.
The chief original reactants in an episode of photochemical smog are molecules of nitric oxide, NO, and of unburned hydrocarbons and par- tially oxidized hydrocarbons that are emitted into the air as pollutants from internal combustion engines; nitric oxide is also released from electric power
Some NO is oxidized instead by organic peroxy free radicals, ROO, as discussed in Chapter 17.
Review Questions 1–4 are based on the material above.
NO2* itself may react with water molecules to produce OH radicals directly, rather than exclusively by prior ozone production.
Urban Ozone: The Photochemical Smog Process 77
plants. The concentrations of these chemicals are orders of magnitude greater than are found in clean air.
Collectively, the substances, including hydrocarbons and their deriva- tives, that readily vaporize into the air are called volatile organic compounds, or VOCs, many of which react in photochemical smog.
The emissions, by sector, of VOCs in the United States and Canada are illustrated by the pie charts in Figure 3-2. Emissions from on-road transporta- tion especially have fallen drastically over the last two decades.
Gaseous hydrocarbons and partially oxidized hydrocarbons are VOCs that are present in urban air as a result of the evaporation of solvents, liquid fuels, and other organic compounds. For example, vapor is released into the air when a gasoline tank is filled unless the hose’s nozzle is specially designed to minimize this loss. Evaporated, unburned gasoline is also emitted from the tail- pipe of a vehicle before its catalytic converter has been warmed sufficiently to operate. Two-cycle engines such as those in outboard motor boats are particu- larly notorious for emitting significant proportions of their gasoline unburned into the air. Personal watercraft manufactured in the 1990s, before pollution controls came into effect, emitted more smog-producing emissions in a day’s operation than an automobile of the same era driven for several years! In many regions, new lawn mowers are required to be outfitted with a catalytic converter, though this issue is controversial since some mower manufacturers claim that a hot converter could pose a fire hazard to the engine. Some outboard motors and domestic firepit fireplaces are also now equipped with catalytic converters.
Formally, VOCs are defined as organic compounds having boiling points that lie between 50°C and 260°C.
Nonindustrial 7%
Other 16%
Solvents 16%
Off-road vehicles 14%
On-road vehicles 12%
Industrial 35%
Electric generating units
<1%
Nonindustrial 9%
Industrial Other 8%
18%
Solvents 26%
Off-road vehicles 16%
Electric generation
<1%
On-road vehicles 23%
United States Total: 16.7 million tons/year (15.2 million tonnes/year)
Canada Total: 2.9 million tons/year (2.7 million tonnes/year)
FIGURE 3-2 VOC emission sources in North America in 2006. [Source: International Joint Commission, Canada–United States Air Quality Agreement: 2008 Progress Report, Washington, D.C. and Ottawa, Ontario, 2008.]
Another vital ingredient in producing photochemical smog is sunshine, which serves indirectly to increase the concentration of free radicals that participate in the chemical processes of smog formation. Substances that are emitted directly into air are called primary pollutants; in the smog reaction, these are NO and VOCs, most of which are relatively innocuous with regard to human health. Substances into which the primary ones are trans- formed are called secondary pollutants; in smog they include ozone, nitric acid, HNO3, and partially oxidized (and in some cases nitrated) organic com- pounds, which are much more toxic than the reactants. An approximate overall chemical equation for the smog reaction is
Overall reaction:
VOCs NO O2 sunlight 9:9: mixture of O3, HNO3, organics
Primary pollutants Secondary pollutants
Other than those that absorb sunlight and subsequently decompose, most atmospheric molecules that are transformed in air begin by reacting with the hydroxyl free radical. The most reactive VOCs in urban air are alkene hydrocarbons, since they contain a carbon–carbon double bond, C R C; and aldehydes, which contain a C R O bond. These compounds are particularly reactive since their reactions with OH, in which the hydroxyl radical adds to a carbon atom participating in the double bond, analogous with the case of carbon monoxide discussed previously, are very fast.
Hydroxyl initiates the reaction in the atmosphere of a hydrogen- containing molecule that does not contain a multiple bond by abstracting a hydrogen atom from it, thereby forming a water molecule and leaving a free radical fragment of the hydride. For example, the first reaction of meth- ane molecules in air is their loss of hydrogen to hydroxyl:
CH4 OH 9: CH3 H2O
The fragment CH3 contains an odd number of electrons, and is the methyl free radical. Such free radicals do react with diatomic oxygen in a sequence of reactions that involves their oxidation to carbon monoxide (and ulti- mately to carbon dioxide):
CH49: CH39:9: other intermediates 9: CO
As a result of reactions in the sequence in which peroxy free radicals oxidize NO to NO2, the hydroxyl free radical is eventually regenerated in the same manner as it is in the CO oxidation. Thus OH acts as a catalyst in the atmospheric oxidation of most species and is effective in air even in tiny concentration.
In smog episodes, aldehydes are among the intermediates, and their pho- tochemical decomposition by UV-A produces additional free radicals. Since on average more than 1.0 molecule of HOO is produced in each cycle as a result, more than 1.0 OH molecule is created by their reaction with nitric oxide. Over time, the total concentration of OH and HOO free radicals
O2 O2
OH
Recall that a catalyst is defined as a substance that speeds up a chemical reaction, but is regenerated during the process.
Urban Ozone: The Photochemical Smog Process 79
builds up during a smog reaction, thereby accelerating it. Notice that the operation of the OH/HOO cycle and the smog it produces depends upon the simultaneous presence of NO and of reactive VOCs; without one or the other of these key reactants the cycle could not proceed nearly as quickly. The NO and VOCs here are said to act in synergism; their overall effect is much greater than would be the sum of either acting in isolation.
When the reaction sequence of oxidizing CH4 to CO is combined with that of CO oxidation to CO2, the overall reaction is seen to be the complete oxidation of methane to carbon dioxide and water, the same reaction as occurs when natural gas is burned in air:
CH4 2 O29: CO2 2 H2O
Details of the complete sequences by which such free-radical reactions oxidize the gases emitted into clean and polluted air are explained in Chapter 17.