required to grow the clusters into aerosol particles.
Under very polluted conditions, there is typically
sufficient availability of sulfuric acid to both form
stable clusters and to grow these clusters into aerosol particles tens to hundreds of nanometers in size.
During particle production events in background
air, the aerosol population typically increases from
concentrations of ~1,000 to ~100,000 cm
À3
, while
in polluted air concentrations increase from
~10,000 to ~100,000 cm
À3 [22, 23].
The most dramatic and intense nucleation events
occur in coastal air where background concentrations increase from ~500 to ~1,000,000 cm
À3 [26,
27]. These events occur when seaweed beds are
exposed during low tide conditions. The algae
emit I 2 and CH 2 I 2 which are rapidly photolyzed
and oxidized by O 3 resulting in the formation of
iodine oxides which readily nucleate into stable
clusters. In addition, there is sufficient iodine oxide
produced to account for the rapid growth of the
clusters into aerosol particles of 100 nm [28].
Multiphase Secondary Aerosol Production
The vast majority of secondary aerosol mass is not
produced through the nucleation process but
through condensation, aqueous phase reactions,
and multiphase processes. Condensation typically
involves both inorganic low-volatility gases, such
as sulfuric acid and nitric acid, and a vast range of
condensable organic species. Sulfuric acid results
from the oxidation of SO 2 by the OH radical,
while nitric acid results from the oxidation of
NO 2 in the atmosphere. NH 3 can also condense
on acidic particles resulting in fully or partially
neutralized particles. Nitric acid is regarded as a
semivolatile condensable gas that can be thermodynamically stable in either the gas phase or the
aerosol phase, depending on environmental conditions. At lower temperatures and higher relative
humidities, nitric acid is more likely to be found in
the condensed aerosol phase, while at warmer
temperatures and lower humidities, it is more
likely to evaporate. The chemistry of the preexisting aerosol also influences the gas phase
partitioning of nitric acid. For example, in
the presence of alkaline aerosol species such as
sea-salt and dust, nitric acid will preferentially
condense to these aerosol types. Since the bulk
of sea-salt and dust aerosol are found in supermicron size ranges, it is in these size ranges where
nitric acid can also be found. However, in polluted
regions, and in the absence of sea-salt and dust,
nitric acid is also found in significant amounts in
submicron-sized aerosol. Nitric acid and sulfate
aerosol are generally sourced from pollution
sources; however, in marine regions, the oxidation
of dimethylsulfide, a waste product from plankton
activity, can provide a significant source of natural
SO 2 and methane sulfonic acid, leading to natural
aerosol production.
Aqueous phase reactions can occur in liquid
aerosol particles but typically, the majority of
aqueous phase production occurs in cloud droplets. It is estimated that between 75% and 90% of
aerosol sulfate (SO 4
– – ) is produced through this
pathway. In cloud, the droplet surface area and
volume of the cloud droplets promote the dissolution of SO 2 into the aqueous phase where it can
be rapidly oxidized by ozone and/or hydrogen
peroxide [29]. The oxidation route by ozone is
highly pH dependent and dominant in alkaline
drops such as those formed on sea-salt aerosol,
while the hydrogen peroxide route dominates at
lower pH values [30]. When sea-salt is present in
cloud nuclei, the cloud behaves as a virtual buffer,
increasing alkalinity and accelerating ozonedriven aqueous phase oxidation of SO 2 to produce
sulfate [30]. This sulfate formation route led to the
significant production of acidic sulfate aerosol in
polluted air and was the primary cause of the acidrain problem in the 1970s–1980s.
Oxidation of volatile organic compounds
(VOCs) leads to the production of condensable
organic vapors, ultimately producing secondary
organic aerosol (SOA). The major oxidizing species are the OH radical, ozone, and the nitrate
radical (NO 3 ). Like in the case of nitric acid,
some of these condensable vapors can be semivolatile. There are both natural VOCs (biogenic:
BVOCs) and anthropogenic VOCs (AVOCs),
although it is estimated that the BVOC emissions
are of the order of ten times that of AVOC emissions. Some modeling studies suggest that in certain heavily populated regions, AVOCs can
contribute significantly to total organic carbon
levels [31]; other studies using carbon isotope
248
Aerosol in Global Atmosphere
Under very polluted conditions, there is typically
sufficient availability of sulfuric acid to both form
stable clusters and to grow these clusters into aerosol particles tens to hundreds of nanometers in size.
During particle production events in background
air, the aerosol population typically increases from
concentrations of ~1,000 to ~100,000 cm
À3
, while
in polluted air concentrations increase from
~10,000 to ~100,000 cm
À3 [22, 23].
The most dramatic and intense nucleation events
occur in coastal air where background concentrations increase from ~500 to ~1,000,000 cm
À3 [26,
27]. These events occur when seaweed beds are
exposed during low tide conditions. The algae
emit I 2 and CH 2 I 2 which are rapidly photolyzed
and oxidized by O 3 resulting in the formation of
iodine oxides which readily nucleate into stable
clusters. In addition, there is sufficient iodine oxide
produced to account for the rapid growth of the
clusters into aerosol particles of 100 nm [28].
Multiphase Secondary Aerosol Production
The vast majority of secondary aerosol mass is not
produced through the nucleation process but
through condensation, aqueous phase reactions,
and multiphase processes. Condensation typically
involves both inorganic low-volatility gases, such
as sulfuric acid and nitric acid, and a vast range of
condensable organic species. Sulfuric acid results
from the oxidation of SO 2 by the OH radical,
while nitric acid results from the oxidation of
NO 2 in the atmosphere. NH 3 can also condense
on acidic particles resulting in fully or partially
neutralized particles. Nitric acid is regarded as a
semivolatile condensable gas that can be thermodynamically stable in either the gas phase or the
aerosol phase, depending on environmental conditions. At lower temperatures and higher relative
humidities, nitric acid is more likely to be found in
the condensed aerosol phase, while at warmer
temperatures and lower humidities, it is more
likely to evaporate. The chemistry of the preexisting aerosol also influences the gas phase
partitioning of nitric acid. For example, in
the presence of alkaline aerosol species such as
sea-salt and dust, nitric acid will preferentially
condense to these aerosol types. Since the bulk
of sea-salt and dust aerosol are found in supermicron size ranges, it is in these size ranges where
nitric acid can also be found. However, in polluted
regions, and in the absence of sea-salt and dust,
nitric acid is also found in significant amounts in
submicron-sized aerosol. Nitric acid and sulfate
aerosol are generally sourced from pollution
sources; however, in marine regions, the oxidation
of dimethylsulfide, a waste product from plankton
activity, can provide a significant source of natural
SO 2 and methane sulfonic acid, leading to natural
aerosol production.
Aqueous phase reactions can occur in liquid
aerosol particles but typically, the majority of
aqueous phase production occurs in cloud droplets. It is estimated that between 75% and 90% of
aerosol sulfate (SO 4
– – ) is produced through this
pathway. In cloud, the droplet surface area and
volume of the cloud droplets promote the dissolution of SO 2 into the aqueous phase where it can
be rapidly oxidized by ozone and/or hydrogen
peroxide [29]. The oxidation route by ozone is
highly pH dependent and dominant in alkaline
drops such as those formed on sea-salt aerosol,
while the hydrogen peroxide route dominates at
lower pH values [30]. When sea-salt is present in
cloud nuclei, the cloud behaves as a virtual buffer,
increasing alkalinity and accelerating ozonedriven aqueous phase oxidation of SO 2 to produce
sulfate [30]. This sulfate formation route led to the
significant production of acidic sulfate aerosol in
polluted air and was the primary cause of the acidrain problem in the 1970s–1980s.
Oxidation of volatile organic compounds
(VOCs) leads to the production of condensable
organic vapors, ultimately producing secondary
organic aerosol (SOA). The major oxidizing species are the OH radical, ozone, and the nitrate
radical (NO 3 ). Like in the case of nitric acid,
some of these condensable vapors can be semivolatile. There are both natural VOCs (biogenic:
BVOCs) and anthropogenic VOCs (AVOCs),
although it is estimated that the BVOC emissions
are of the order of ten times that of AVOC emissions. Some modeling studies suggest that in certain heavily populated regions, AVOCs can
contribute significantly to total organic carbon
levels [31]; other studies using carbon isotope
248
Aerosol in Global Atmosphere