68 AIR POLLUTION METEOROLOGY
in some of the factors affecting climate. Until such models
are improved, then, we cannot really estimate quantitatively
climatic changes produced by pollutants.
The concentration of CO 2 is about 340 parts per million
(ppm). According to observations at Mauna Loa in Hawaii,
over the last forty years or so, it has increased at the rate of
0.7% per year. This is less than half the amount put into the
atmosphere by industry. The other half goes into the ocean
or into vegetation; but it is not known how much goes into
each. Further, we do not know whether the same fraction
can disappear out of the atmosphere in the future—e.g., the
amount going into the ocean is sensitive to temperature, and
the amount going into vegetation may be limited by other fac-
tors. However, a reasonable guess is that the fraction of CO 2 in
the atmosphere will double in the middle of the 21st century.
The basic effect of CO 2 on climate is due to the fact that
it transmits short-wave radiation from the sun, but stops a part
of the infrared radiation emitted by the earth. Hence, the more
CO 2 , the greater the surface temperature. This is known as the
greenhouse effect. Also, since CO 2 increases the radiation into
space, the high atmosphere is cooled by increasing CO 2.
The heating rate at the ground expected with a doubling
of CO 2 has been calculated by many radiation specialists.
The answers differ, depending on how many other vari-
ables (such as cloud cover) are allowed to change as the
CO 2 changes. The best current estimates are that doubling
CO 2 would increase the surface temperature about 2C, and
decrease the temperature aloft a little more. But these esti-
mates do not treat changes of cloud cover and oceanic effects
realistically, and these estimates may yet be corrected. Still,
if we expect only a 20% change in CO 2 by the end of the cen-
tury, the climatic change due to this factor should be small.
However, a serious problem could arise in the next century,
particularly because it is difficult to see how a trend in CO 2
concentration can be reversed. It is therefore of great impor-
tance to continue monitoring CO 2 concentration accurately.
As of 1987, it appears likely that increases of concentra-
tion of other trace gases (e.g. fluorocarbons) may, in combi-
nation, have as strong a warming effect at the surface as CO 2.
So far, no significant warming has been detected.OzoneOzone (O 3 ) is an important part of photochemical smog;
originating mostly from the effect of sunlight on automobile
exhaust. The concentration is critically dependent on chemi-
cal reactions as well as on diffusion. Chemistry is beyond
the scope of this paper as O 3 and ozone pollution near the
ground will not be discussed further.
More important, 90% of the ozone exists in the strato-
sphere (above about 11 km). Its concentration even there is
small (usually less than 10 ppm). If all ozone were to be
brought to the surface of the ground, its thickness would
average about 0.3 cm.
Most of the ozone occurs at high latitudes, and there is
a spring maximum. The great importance of stratospheric
ozone is due to its ability to absorb ultraviolet (UV) light,
particularly in the UVB region (290–320 μ m) where humanskin is extremely sensitive. Thus, decreased ozone would
increase skin cancer.
We now realize that small fractions (10 −9 ) of certain
gases can destroy ozone by catalytic reactions. The most
important are oxides of nitrogen and chlorine. Nitrogen
oxides could originate for example, from supersonic trans-
ports. However calculations show that, unless the number of
SSTs is increased significantly, this problem is not serious.
More important is the problem of chlorofluorometh-
anes (CFM) the use of which has been rapidly increasing.
They are used in sprays, foams and refrigeration, CFMs
are so stable that most of them are still in the atmosphere.
Eventually, however, CFMs will seep into the stratosphere
(about 1%/year). In the high stratosphere, UV will dissociate
CFMs producing chlorine, which destroys ozone.
A slow decrease of ozone in the stratosphere has indeed
been indicated by recent satellite observations. For total
ozone, the results are much more controversial. Chemical–
meterological models show only a very small decrease so far,
too small to isolate from the “noisy” observations. However,
the accuracy of the models can be questioned, particularly
since new relevant reactions have been discovered every few
years, so that model results have varied.
Of special interest has been the recognition of an “ozone
hole,” centered at the South Pole, and lasting a month or so
in the Southern Spring. Total column ozone falls to about
half its normal value. The phasing out of chlorofluorocar-
bons, or CFCs began in 1989 with the implementation of the
Montreal Protocol.
Editors Notes: Scientists at NASA and various U.S.
universities have been studying satellite data taken over the
past 2 decades. They found the rate of ozone depletion in the
upper stratosphere is slowing partially as a result of a reduc-
tion in the use of CFCs (see Newchurch, et al., 2005).
In the troposphere, aerosol formation from the combus-
tion of fossil fuels and biomass is a precursor to the forma-
tion of brown clouds, which are intricately linked to climate
changes (Ramanathan and Ramana, 2003). Ozone, a com-
ponent of smog, also forms in the troposphere, when NOx
combines with volatile organic compounds in the presence of
sunlight. There is growing scientific evidence that the inter-
continental transport (ICT) of aerosols and ozone influences
surface air quality over downwind continents (Fiore, et al.,
2003). For example during the dust storm events in Asia in
April of 2001, the ground level aerosol concentrations in the
western U.S. and Canada increased by as much as 40 μ g/m^3
resulting from the ICT of aerosols. Fiore, et al. found there
are global dimensions to the aerosol and ozone problems.
It has also been suggested that ozone changes can pro-
duce climate changes, but these appear rather unimportant at
present, except that they may worsen slightly the CO 2 green-
house effect.SummaryIn summary, increasing air pollution can modify the climate in
many ways. There is no evidence that any significant change
has occurred so far; but eventually, large effects are likely.C001_005_r03.indd 68C001_005_r03.indd 68 11/18/2005 10:13:39 AM11/18/2005 10:13:39 AM