southeast USA. Significant levels of the order of
1 mg m
À2 are also seen over most oceanic regions.
Non-sea-salt sulfate exhibits a global average burden of ~3.5 mg m
À2 , with peak values of the order
of 50 mg m
À2 . Again, the main developed and
developing countries with significant population
densities appear as burden “hot spots” as for the
other anthropogenic aerosol types (e.g., black carbon and particulate organic matter). In contrast to
other anthropogenic sources, there are notable
burdens over remote oceanic regions, reflecting
also its natural oceanic source.
Finally, the sea-salt burden is 13.5 mg m
À2 ,
with peak burdens of ~50 mg m
À2 . The peak
burdens occur at midlatitudes, or the “roaring
40s,” north and south of the equator, and are
associated with regions of intensive cyclonic
activity.
It is interesting to note that in Table 1, sea-salt
was highlighted as being the single most important
aerosol emission type, but it had a mass burden
slightly lower than dust which had significantly
lower emissions than sea-salt. In the AEROCOM
(http://dataipsl.ipsl.jussieu.fr/AEROCOM; [57])
study, dust also has a higher burden for generally
lower emissions. The explanation is that significant
dust plumes are vented into the Free Troposphere
where removal rates are slower and residence times
are longer than in the marine boundary layer where
removal rates are likely higher due to higher precipitation rates and, for the larger-sized particles,
shorter gravitational settling timescales (i.e., seasalt is not injected at as high altitudes as dust, so
they settle out more rapidly).
At any one location, the chemical composition
of atmospheric can vary significantly depending
on the air mass back trajectory arriving at that
location. An example of the chemical variability
encountered is demonstrated for the Northeast
Atlantic region where clean marine and polluted
continental air masses are frequently encountered
at the Mace Head Atmospheric Research Station,
which is representative of the region. Figure 8
illustrates the absolute chemical composition and
the relative chemical composition observed in two
contrasting air masses (continental polar, maritime polar) of different origin over the Northeast
Atlantic. Typically, the main ionic compounds
such as sodium, chloride, nitrate, sulfate, and
ammonium are easily detected; however, it is
also possible to quantify the water-soluble and
water-insoluble organic component. It is readily
seen that the absolute and relative contributions of
the above species varies greatly with air mass, or
source origin [58]. Peak submicron mass loadings
occur for the continental air, while peak supermicron masses occur for marine air. In marine
polar air, the supmicron sizes are almost exclusively composed of sea-salt, while submicron
sizes are marginally dominated by nss-sulfate,
with the second largest contribution derived
from sea-salt. Small amounts of ammonia,
water-soluble organic matter and water-insoluble
organic matter are also evident. By contrast, in the
continental air, sea-salt and nitrate are the two
dominant species in supermicron sizes, with
smaller amounts of organic matter visible and in
the submicron sizes, nss-sulfate and water-soluble
organic matter dominate. It should be noted that
marine aerosol can be significantly enriched in
organic matter depending on the degree to which
air masses pass over biologically rich waters.
Mass is only one important parameter of an
aerosol size distribution. Perhaps the other most
important parameter is the number concentration,
and in particular, the concentration in the so-called
accumulation mode (100–500 nm diameter). This
size range is not only important for scattering and
radiative effects of atmospheric aerosols, it is crucially important in terms of influencing the number concentration of cloud condensation nuclei.
Figure 9 illustrates the horizontal and vertical
(zonal) average concentration in the accumulation
mode size range [59]. It can be seen that the
highest number concentration of aerosols
(N ¼ 3,000–5,000 cm
À3 ) occurs over regions in
Europe such as the Netherlands and Northern
Italy, China, India, and Brazil.
Vertical Distributions
The vertical distribution of aerosol particles, measured as aerosol backscatter using a LiDAR (Light
Detection And Ranging), typically reveals maximum particle mass and number concentrations in
the atmospheric boundary layer (lower
~1,000–2,000 m) as shown in Fig. 10. In this figure,
Aerosol in Global Atmosphere
255
1 mg m
À2 are also seen over most oceanic regions.
Non-sea-salt sulfate exhibits a global average burden of ~3.5 mg m
À2 , with peak values of the order
of 50 mg m
À2 . Again, the main developed and
developing countries with significant population
densities appear as burden “hot spots” as for the
other anthropogenic aerosol types (e.g., black carbon and particulate organic matter). In contrast to
other anthropogenic sources, there are notable
burdens over remote oceanic regions, reflecting
also its natural oceanic source.
Finally, the sea-salt burden is 13.5 mg m
À2 ,
with peak burdens of ~50 mg m
À2 . The peak
burdens occur at midlatitudes, or the “roaring
40s,” north and south of the equator, and are
associated with regions of intensive cyclonic
activity.
It is interesting to note that in Table 1, sea-salt
was highlighted as being the single most important
aerosol emission type, but it had a mass burden
slightly lower than dust which had significantly
lower emissions than sea-salt. In the AEROCOM
(http://dataipsl.ipsl.jussieu.fr/AEROCOM; [57])
study, dust also has a higher burden for generally
lower emissions. The explanation is that significant
dust plumes are vented into the Free Troposphere
where removal rates are slower and residence times
are longer than in the marine boundary layer where
removal rates are likely higher due to higher precipitation rates and, for the larger-sized particles,
shorter gravitational settling timescales (i.e., seasalt is not injected at as high altitudes as dust, so
they settle out more rapidly).
At any one location, the chemical composition
of atmospheric can vary significantly depending
on the air mass back trajectory arriving at that
location. An example of the chemical variability
encountered is demonstrated for the Northeast
Atlantic region where clean marine and polluted
continental air masses are frequently encountered
at the Mace Head Atmospheric Research Station,
which is representative of the region. Figure 8
illustrates the absolute chemical composition and
the relative chemical composition observed in two
contrasting air masses (continental polar, maritime polar) of different origin over the Northeast
Atlantic. Typically, the main ionic compounds
such as sodium, chloride, nitrate, sulfate, and
ammonium are easily detected; however, it is
also possible to quantify the water-soluble and
water-insoluble organic component. It is readily
seen that the absolute and relative contributions of
the above species varies greatly with air mass, or
source origin [58]. Peak submicron mass loadings
occur for the continental air, while peak supermicron masses occur for marine air. In marine
polar air, the supmicron sizes are almost exclusively composed of sea-salt, while submicron
sizes are marginally dominated by nss-sulfate,
with the second largest contribution derived
from sea-salt. Small amounts of ammonia,
water-soluble organic matter and water-insoluble
organic matter are also evident. By contrast, in the
continental air, sea-salt and nitrate are the two
dominant species in supermicron sizes, with
smaller amounts of organic matter visible and in
the submicron sizes, nss-sulfate and water-soluble
organic matter dominate. It should be noted that
marine aerosol can be significantly enriched in
organic matter depending on the degree to which
air masses pass over biologically rich waters.
Mass is only one important parameter of an
aerosol size distribution. Perhaps the other most
important parameter is the number concentration,
and in particular, the concentration in the so-called
accumulation mode (100–500 nm diameter). This
size range is not only important for scattering and
radiative effects of atmospheric aerosols, it is crucially important in terms of influencing the number concentration of cloud condensation nuclei.
Figure 9 illustrates the horizontal and vertical
(zonal) average concentration in the accumulation
mode size range [59]. It can be seen that the
highest number concentration of aerosols
(N ¼ 3,000–5,000 cm
À3 ) occurs over regions in
Europe such as the Netherlands and Northern
Italy, China, India, and Brazil.
Vertical Distributions
The vertical distribution of aerosol particles, measured as aerosol backscatter using a LiDAR (Light
Detection And Ranging), typically reveals maximum particle mass and number concentrations in
the atmospheric boundary layer (lower
~1,000–2,000 m) as shown in Fig. 10. In this figure,
Aerosol in Global Atmosphere
255