CHAPTER 2 • The Chemical and Physical Properties of Marine Aerosols: An Introduction
41
space. One of the major objectives in the field of aerosol studies today is to understand
the factors affecting the variability of aerosols and to characterize that variability. There is
a dearth of information about many aspects of the atmospheric aerosol cycle; this is especially true for the marine atmosphere. The study of marine aerosols presents a particularly great challenge because of the vast areas involved, the wide range of sources that impact
on the marine atmosphere, and the many types of physical environments involved.
2.1.3
The Global Distribution of Aerosols Over the Oceans
Satellites provide us with graphic evidence of aerosol distributions over the oceans.
An example is AVHRR (Advanced Very High Resolution Radiometer), which measures
the radiation backscattered to space by aerosols over the oceans; these data are converted to equivalent aerosol optical thickness (EAOT), which is a measure of the column-integrated aerosol loading (Husar et al. 1997). Figure 2.3 shows the global average distribution of oceanic EAOT for the four seasons. Clear patterns are evident. First
of all, the highest values of EAOT (and hence, the greatest column concentrations of
aerosols) are found in regions close to the continents. This distribution affirms the
fact that in many ocean regions the aerosol character is defined to a large extent by
the transport of materials by winds from the continents. A second feature is that there
are large seasonal differences in aerosol concentrations as can be seen by comparing
the December-January distributions with June-August in Fig. 2.3. These differences
are due to a variety of factors, including the seasonal variability of source emissions
and meteorology. Third, some continents emit more aerosols than others, which suggests large differences in source and transport conditions. Especially notable is that
large "plume" of high EAOT values over the tropical Atlantic. The plume extends from
the coast of Africa to South America (in January) and to the Caribbean (in July); this
plume is attributed to the transport of African dust. The large region of high EAOT
over the Arabian Sea in June-August is due to dust carried from Africa and the Middle
East. A large plume is seen off the western coast of southern Africa in June-August
and in September-November; this is due to intense biomass burning in this region.
Substantial plumes are seen over the North Atlantic during all seasons but winter; these
are attributable to the transport of pollutants from North America and Europe. Similarly, large regions with high EAOT values are seen along the coast of Asia during most
seasons; these are largely due to pollution transport. The Asian plume is most prominent in the spring when large quantities of soil dust are carried out of Asia along with
pollution. The attribution of these plumes to these dominant aerosol types is supported
by evidence from field studies in the regions covered by the plumes (Husar et al.1997).
The Total Ozone Mapping Spectrometer (TOMS) also provides information on the
distribution of aerosols, especially absorbing species - predominantly mineral dust
and smoke (Herman et al. 1997). The TOMS product is especially useful, because (in
contrast to AVHRR) the system can detect aerosols over land as well as over water
surfaces. Figure 2.4 shows the global frequency of occurrence (days per month) of
moderate-to-high absorbing aerosol concentrations during January and July. Similarly
to AVHRR, TOMS shows prominent plumes over the tropical North Atlantic (January
and July) and the Arabian Sea (July) due to dust and the large plume in the South Atlantic in July due to smoke. In contrast to AVHRR, TOMS does not show large pollu-
41
space. One of the major objectives in the field of aerosol studies today is to understand
the factors affecting the variability of aerosols and to characterize that variability. There is
a dearth of information about many aspects of the atmospheric aerosol cycle; this is especially true for the marine atmosphere. The study of marine aerosols presents a particularly great challenge because of the vast areas involved, the wide range of sources that impact
on the marine atmosphere, and the many types of physical environments involved.
2.1.3
The Global Distribution of Aerosols Over the Oceans
Satellites provide us with graphic evidence of aerosol distributions over the oceans.
An example is AVHRR (Advanced Very High Resolution Radiometer), which measures
the radiation backscattered to space by aerosols over the oceans; these data are converted to equivalent aerosol optical thickness (EAOT), which is a measure of the column-integrated aerosol loading (Husar et al. 1997). Figure 2.3 shows the global average distribution of oceanic EAOT for the four seasons. Clear patterns are evident. First
of all, the highest values of EAOT (and hence, the greatest column concentrations of
aerosols) are found in regions close to the continents. This distribution affirms the
fact that in many ocean regions the aerosol character is defined to a large extent by
the transport of materials by winds from the continents. A second feature is that there
are large seasonal differences in aerosol concentrations as can be seen by comparing
the December-January distributions with June-August in Fig. 2.3. These differences
are due to a variety of factors, including the seasonal variability of source emissions
and meteorology. Third, some continents emit more aerosols than others, which suggests large differences in source and transport conditions. Especially notable is that
large "plume" of high EAOT values over the tropical Atlantic. The plume extends from
the coast of Africa to South America (in January) and to the Caribbean (in July); this
plume is attributed to the transport of African dust. The large region of high EAOT
over the Arabian Sea in June-August is due to dust carried from Africa and the Middle
East. A large plume is seen off the western coast of southern Africa in June-August
and in September-November; this is due to intense biomass burning in this region.
Substantial plumes are seen over the North Atlantic during all seasons but winter; these
are attributable to the transport of pollutants from North America and Europe. Similarly, large regions with high EAOT values are seen along the coast of Asia during most
seasons; these are largely due to pollution transport. The Asian plume is most prominent in the spring when large quantities of soil dust are carried out of Asia along with
pollution. The attribution of these plumes to these dominant aerosol types is supported
by evidence from field studies in the regions covered by the plumes (Husar et al.1997).
The Total Ozone Mapping Spectrometer (TOMS) also provides information on the
distribution of aerosols, especially absorbing species - predominantly mineral dust
and smoke (Herman et al. 1997). The TOMS product is especially useful, because (in
contrast to AVHRR) the system can detect aerosols over land as well as over water
surfaces. Figure 2.4 shows the global frequency of occurrence (days per month) of
moderate-to-high absorbing aerosol concentrations during January and July. Similarly
to AVHRR, TOMS shows prominent plumes over the tropical North Atlantic (January
and July) and the Arabian Sea (July) due to dust and the large plume in the South Atlantic in July due to smoke. In contrast to AVHRR, TOMS does not show large pollu-