Environmental Engineering Reference
and by lightning discharge. Minor quantities are emitted from fossil fuel combustion and in some
chemical manufacturing processes (e.g., nitric acid, chemical fertilizer, and nylon production). Its
current concentration in the atmosphere is about 0.3 ppmv and is growing by about 0.25% per year.
While N 2 O is a strong far-infrared absorbing gas in the 7- to 8-
m range, currently measured small
concentrations of this gas and its slower growth rate compared to CO 2 mean that in about 2100 it
may contribute about 10% to global warming.
Chlorofluorocarbons (CFC) are entirely man-made products, because they are produced in chemical
factories for use as refrigerants, propellants in spray cans, foam-blowing agents, solvents, and so
on. By international conventions, they are gradually being phased out of production. However,
because of their slow venting from existing appliances and foam insulation materials, coupled with
their long lifetime in the atmosphere (hundreds of years), they will contribute to global warming
for a long time after they cease to be produced. In 2100, chlorofluorocarbons may contribute about
5-10% to global warming.
In addition, chlorofluorocarbons reduce the concentration of ozone in the stratosphere (see
In Chapter 9 we noted that there are two layers of O 3 in the atmosphere: one in the stratosphere
(the “good” ozone), the other in the troposphere (the “bad” ozone). Stratospheric O 3 is produced
naturally from molecular oxygen under the influence of solar UV radiation. Some tropospheric O 3
has diffused down from the stratosphere, and the rest is produced from anthropogenic precursors,
nitric oxides (NO x ), and volatile organic compounds (VOC), under the influence of solar radiation.
About 10% of the total column density of ozone resides in the troposphere. Because the upper
troposphere and lower stratosphere is colder than the earth's surface, ozone molecules residing in
those layers absorb part of the outgoing far-infrared terrestrial radiation and then re-radiate back
to the surface, thus adding to global warming.
Ozone column densities vary greatly as a function of latitude, altitude, and seasons, and
they also vary from urbanized-industrialized parts of the continents to remote areas. Thus, it is
difficult to assign a global, annual average column density and altitude profile for O 3 . Currently,
the stratospheric ozone is being depleted by chlorofluorocarbons, which results in a small negative
feedback to global warming because less of the outgoing terrestrial infrared radiation is absorbed
by stratospheric ozone. With the phasing out of CFC production worldwide, it is estimated that
stratospheric ozone concentrations will return slowly (tens to hundreds of years) to pre-CFC times.
On the other hand, tropospheric ozone is on the increase because of increased emission of the
ozone precursors, NO x and VOC (see Section 9.2.5). In the upper troposphere, where ambient
temperatures are lower than at the earth's surface, the anthropogenic ozone absorbs some of the
outgoing terrestrial radiation, causing a positive feedback to global warming. On balance, one may
assume that in the foreseeable future the stratospheric ozone deficit and the tropospheric ozone
surplus will cancel each other, and no appreciable contribution to global warming is to be expected