ACTIVITY Vehicles
Power plants NO NO2
O2
O3
+OH
HNO3 Sunlight
Sunlight O3 or HOO
Emissions Internal cycling Smog products
FIGURE 3-4 Nitrogen- based components of photochemical smog.
their origins as atmospheric pollutants. Nitrogen oxide gases are produced by two different reactions whenever a fuel is burned in air with a hot flame.
• Some nitric oxide is produced from the oxidation of nitrogen atoms contained in molecules of the fuel itself; it is called fuel NO. About 30–60% of a fuel’s nitrogen is converted to NO during combustion.
However, most fuels do not contain much nitrogen, so this process accounts for only a small fraction of NO emissions.
• Nitric oxide is produced by the oxidation at high combustion temperatures of atmospheric nitrogen, and it is called thermal NO.
At high flame temperatures, some of the nitrogen and oxygen gases in the air passing through the flame combine to form NO:
N2 O2 9: 2 NO
The higher the flame temperature, the more NO is produced. Since this reac- tion is very endothermic, its equilibrium constant is very small at normal temperatures but increases rapidly as the temperature rises.
One might expect that the relatively high concentrations of NO that are produced under combustion conditions would revert back to molecular nitro- gen and oxygen as the exhaust gases cool, since the equilibrium constant for the above reaction is much smaller at lower temperatures. However, the activation energy for the reverse reaction is also quite high, so the process cannot occur to an appreciable extent except at high temperatures. The flow of gases through the combustion zone is so rapid that the NO does not have sufficient time to react at the high temperatures its reaction requires. Thus the relatively high concentrations of nitric oxide produced during combus- tion are maintained in the cooled exhaust gases. Equilibrium cannot be quickly re-established and the nitrogen is “frozen” as NO.
Because the reaction between N2 and O2 has a high activation energy, it is negligibly slow except at the very high temperatures such as occur in the modern combustion engines of vehicles—particularly when they are travel- ing at high speeds—and in power plants. Very little NO is produced by the burning of wood and other natural materials since the flame temperatures involved in such combustion processes are relatively low.
Two distinct mechanisms are involved in the initiation of the reaction of molecular nitrogen and oxygen to produce thermal nitric oxide; in one it is atomic oxygen that attacks intact N2 molecules, whereas in the other it is free radicals, such as CH, that are derived from the decomposition of the fuel.
The initial reaction steps of the first mechanism are O2!1 2 O O N29: NO N
The rate of the second, slower step is proportional to [O] [N2]. However, since, from the equilibrium in the first step, [O] is proportional to the square
hot flame
LeChatelier’s principle; see also Additional Problem 4.
Recall from introductory 0!
chemistry that the rate of a reaction step is proportional to the product of the concentrations of its reactants.
Urban Ozone: The Photochemical Smog Process 83
root of [O2], it follows that the rate of NO formation will be proportional to [N2] [O2]1/2. Consequently, this process is relatively slow under oxygen-poor conditions.
The nitric oxide released into air is gradually oxidized to nitrogen diox- ide over a period of minutes to hours, the rate depending upon the concen- tration of the pollutant gases present. Collectively, NO and NO2 in air is referred to as NOX, pronounced “nox.” The yellow-brown color in the atmosphere of a smog-ridden city is due in part to the nitrogen dioxide pres- ent, since this gas absorbs visible light, especially near 400 nm (see its spec- trum in Figure 3-1), removing sunlight’s purple component while allowing most yellow light to be transmitted. The small levels of NOX in clean air result in part from the operation of the above reactions in the very energetic environment of lightning flashes and in part from the release of NOX and of ammonia, NH3, from biological sources. NOX is also emitted from coniferous trees when sunlight shines on them and when the ambient concentrations of these gases are low.
The sources of anthropogenic NOX emissions in North America are shown by sector in Figure 3-5. The quantities from on-road transportation, and from electric power generation in the United States, fell substantially over the last dozen years.
Although our analysis above has identified ozone as the main product of smog, the situation is actually more complicated, as a detailed study in Chapter 16 indicates. For example, nitrogen dioxide reacts with organic free radicals to produce organic nitrates. Many of the products form particles or are incorpo- rated into them, as discussed in Section 3.24.
Nonindustrial 3%
Solvents
<1%
Off-road vehicles 32%
On-road vehicles 22%
Industrial 33%
Electric generating units 10%
Nonindustrial 4%
Industrial 16%
Other 2%
Solvents
<1%
Off-road vehicles 24%
Electric generation On-road 20%
vehicles 34%
United States Total: 17.4 million tons/year (15.8 million tonnes/year)
Canada Total: 2.6 million tons/year (2.4 million tonnes/year)
Other
<1%
FIGURE 3-5 Anthropogenic NOX sources in North America, by sector in 2006.
[Source: International Joint Commission, Canada–United States Air Quality Agreement: 2008 Progress Report, Washington, D.C. and Ottawa, Ontario, 2008.]