ATMOSPHERIC CHEMISTRY 131
catalyzed SO 2 oxidation paths as a function of pH. In the
case of the H 2 O 2 - catalyzed oxidation of S(IV), the rate of
oxidation will be limited by the H 2 O 2 present in the cloud
or available to the cloud. This leads to the rate of S(IV) con-
version to S(VI) being limited by the rate at which gaseous
H 2 O 2 is incorporated into the aqueous phase of the clouds by
updrafts and entrainment.Natural Sources of Acids and Organic AcidsThere are a variety of potential natural sources of acids in
the atmosphere. Dimethyl sulfide (DMS) is one of the most
important natural sulfur compounds emitted from the oceans
(Cocks and Kallend, 1988). Hydroxyl radicals may react
with DMS by either of two possible routes:OH CH 3 SCH 3 → CH 3 S(OH)CH 3 (38)
OH CH 3 SCH 3 → CH 3 SCH 2 H 2 O (39)addition^ to the sulfur or abstraction of a hydrogen atom from
one of the methyl groups. For the first case, the product is
proposed to react with oxygen:CH 3 S(OH)CH 3 2O 2 → CH 3 SO 3 H CH 3 O 2 (40)eventually forming methane sulfonic acid (CH 3 SO 3 H, or
MSA). Many organic S(IV) compounds are easily hydro-
lyzed to inorganic S(IV), which can be oxidized to S(VI).
For the second path, the alkyl-type radical is expected to
react with molecular oxygen to form a peroxy-type radical,
followed by the oxidation of NO to NO 2 :CH 3 SCH 2 O 2 → CH 3 SCH 2 O 2 (41)
CH 3 SCH 2 O 2 NO 2O 2 →
NO 2 HCHO SO 2 CH 3 O 2 (42)The details of this mechanism are not well established,
but the suggestion is that DMS, which is produced by bio-
genic processes, can be partially oxidized to SO 2 , hence
contributing to the SO 2 observed in the atmosphere. This
SO 2 would be oxidized by the same routes as the anthropo-
genic SO 2. Several of the papers included in the volume by
Saltzman and Cooper (1989) have presented a much more
complete discussion of the role of biogenic sulfur in the
atmosphere.
In recent years, it has become increasingly obvious that
there are substantial contributions of organic acids (carboxylic
acids) in the atmosphere (Chebbi and Carlier, 1996). It has been
found that formic acid (HCOOH) and acetic acid (CH 3 COOH)
are the most important gas-phase carboxylic acids identified
in the atmosphere. Concentrations in excess of 10 ppb of
these compounds have been observed in polluted urban areas.
Concentrations of these acids have been observed in excess of
1 ppb, in the Amazon forest, particularly during the dry season.
A very wide range of mono- and dicarboxylic acids have been
observed in the aqueous phase, rain, snow, cloud water, and fog
water. Dicarboxylic acids are much more important in aerosol
particles, since they have much lower vapor pressures than do
monocarboxylic acids. Carboxylic acids have been observedas direct emissions from biomass burning, in motor-vehicle
exhaust, and in direct biogenic emissions. Carboxylic acids
are also produced in the atmosphere. The most important gas-
phase reactions for the production of carboxylic acids are as
a product of the ozone oxidation of alkenes. Aqueous-phase
oxidation of formaldehyde is believed to be a major source of
formic acid, maybe more important than the gas-phase pro-
duction. Carboxylic acids are, in general, relatively unreactive;
their primary loss processes from the atmosphere are believed
to be wet and dry deposition.SummaryMuch of the atmospheric acidity results from the oxidation
of nitrogen oxides and sulfur oxides. In the case of nitro-
gen oxides, this oxidation is primarily due to the gas-phase
reaction of OH with NO 2. In the case of sulfur oxides, the
comparable reaction of OH with SO 2 is much slower, but is
likely to be the dominant oxidation process in the absence
of clouds. When clouds are present, the aqueous-phase oxi-
dation of SO 2 is expected to be more important. At higher
pH, the more important aqueous oxidation of SO 2 is likely
to be catalyzed by ozone, while at lower pH, the H 2 O 2 -
catalyzed oxidation is likely to be more important. Organic
acids also contribute significantly to the acidity observed in
the atmosphere.POLAR STRATOSPHERIC OZONEIn 1974, Molina and Rowland proposed that chlorofluoro-
carbons (CFCs) were sufficiently long-lived in the tropo-
sphere to be able to diffuse to the stratosphere, where effects
on ozone would be possible. They shared in the 1995 Nobel
Prize in chemistry for this work.
More recently an ozone “hole” has been observed in the
stratosphere over Antarctica, which becomes particularly
intense during the Southern Hemispheric spring, in October.
This led attention to be shifted to the polar regions, where
effects of CFCs on stratospheric ozone content have been
observed. Before dealing with this more recent discovery, it
is necessary to provide some of the background information
about the stratosphere and its chemistry.
The stratosphere is the region of the atmosphere lying
above the troposphere. In the troposphere, the temperature of
the atmosphere decreases with increasing altitude from about
290 K near the surface to about 200 K at the tropopause. The
tropopause is the boundary between the troposphere and
the stratosphere, where the temperature reaches a minimum.
The altitude of the tropopause varies with season and latitude
between altitudes of 10 and 17 km. Above the tropopause, in
the stratosphere, the temperature increases with altitude up to
about 270 K near an altitude of 50 km. In the troposphere, the
warmer air is below the cooler air. Since warmer air is less
dense, it tends to rise; hence there is relatively good vertical
mixing in the troposphere. On the other hand, in the strato-
sphere the warmer air is on top, which leads to poor vertical
mixing and a relatively stable atmosphere.C001_008_r03.indd 131C001_008_r03.indd 131 11/18/2005 10:15:23 AM11/18/2005 10:15:23 AM