light energy to chemical energy. Raising this value to only 1% would make a vast
difference in the economics of producing biomass as a substitute for fossil carbon and
would greatly increase the quantities of available biomass fuel.
Green chemistry can also be applied in the prevention of release of greenhouse
gases other than carbon dioxide. This has been done, for example, in the replacement
of chlorofluorocarbons (Freons) with analogous compounds having at least one C-H
bond, that are rather readily destroyed in the troposphere. Both kinds of compounds
act as greenhouse gases, but the latter last for much shorter times during which they
are available to absorb infrared radiation. Another approach is to limit the emissions of
methane, CH 4. Large quantities of methane are released by anaerobic bacteria growing in
flooded rice paddies. By developing strains of rice and means of cultivation that enable
the crop to be grown on unflooded soil, this source of methane can be greatly reduced.
Methane collection systems placed in municipal waste landfills can prevent the release
of methane from this source and provide a source of methane fuel.
Green chemistry, biochemistry, and biology can be used to deal with global warming
when it occurs. Crops, fertilizers, and pesticides can be developed that enable plants to
grow under the drought conditions that would follow global warming. Another approach
is the development of salt-tolerant crops that can be grown on soil irrigated with saline
water, where fresh water supplies are limited.
8.10. Photochemical Smog
Photochemical smog is one of the most common urban air pollution problems. It
occurs in dry, stagnant air masses, usually stabilized by a temperature inversion (see
Figure 8.1), that are subjected to intense sunlight. A smoggy atmosphere contains ozone
(O 3 ), organic oxidants, nitrogen oxides, aldehydes, and other noxious species. In latter
stages of smog formation visibility in the atmosphere is lowered by the presence of a
haze of fine particles formed by the oxidation of organic compounds in smog.
The chemical ingredients of smog are nitrogen oxides and organic compounds, both
released from the automobile, as well as from other sources. The driving energy force
behind smog formation is electromagnetic radiation with a wavelength at around 400 nm
or less, in the ultraviolet region, just shorter than the lower limit for visible light. Energy
absorbed by a molecule from this radiation can result in the formation of active species,
thus initiating photochemical reactions.
Although methane, CH 4 , is one of the least active hydrocarbons in terms of forming
smog, it will be used here to show the smog formation process because it is the simplest
hydrocarbon molecule. Smog is produced in a series of chain reactions. The first of these
occurs when a photon of electromagnetic radiation with a wavelength less than 398 nm
is absorbed by a molecule of nitrogen dioxide,
NO 2 + hν → NO + O (8.10.1)to produce an oxygen atom, O. The oxygen atom is a very reactive species that can
abstract a hydrogen atom from methane,
Chap. 8. Air and the Atmosphere 217