CHAPTER 2 . The Chemical and Physical Properties of Marine Aerosols: An Introduction
45
Figures 2.3 and 2.4 clearly show that the major sources of aerosols, both pollution
and mineral dust, lie in the Northern Hemisphere. The contrast between the Northern and Southern Hemispheres is important both from the standpoint of climate issues and also the impact of aerosol transport on ocean biogeochemical processes.
2.1.4
Aerosol Composition
The large fraction of the aerosol mass over the ocean is comprised of a relatively small
number of species as shown in Table 2.1, which presents a selection of aerosol concentrations at various mostly remote ocean sites. The annual means are compiled from
long-term (multi-year) measurements made at ocean network stations operated by the
aerosol group at the University of Miami (Prospero et al. 1989; Savoie et al. 1989a;
Arimoto et al. 1995, 1996; Savoie et al. 1993, 1994). The sea salt aerosol concentration is
calculated from the measured concentration of Na + in aerosol based on the ratio of
the total concentration of salts in sea water to that ofNa + (i.e. sea salt aerosol = 3.256 x Na,
by weight). The nss-SO~- fraction is calculated by measuring the Na + concentration in
the aerosol; on the assumption that the SO~-INa + ratio in the sea-spray droplets is the
same as that in bulk sea water, the Na + value is converted to a sea-salt SO~- by multiplying by 0.2517. This quantity is subsequently subtracted from total SO~- yield of the
nss-SO~- concentration.
There are many other important marine-aerosol species that are not included in
Table 2.1. In particular, organic materials can constitute a substantial fraction of the
total aerosol mass (Jacobson et al. 2000). Most notable is soot ("black carbon" or "BC"),
produced by fuel combustion and biomass burning. BC is found almost everywhere,
even over the most remote oceans (Heintzenberg et al. 2000). In addition, as discussed
below, over many ocean regions a large fraction of the aerosol mass remains
uncharacterized (Quinn et al. 2000).
As one might expect, sea salt aerosol is a major constituent at most sites. (At some
sites the high concentrations are attributable in part to local surf conditions, which
are not representative of concentrations on larger scales.) Mineral dust shows an extremely wide range of concentrations. Note the very high concentrations in the tropical North Atlantic sites (Izana, Barbados) and the sites in the western North Pacific
(Cheju, Okinawa, Hong Kong). These reflect the impact of dust transport from North
Africa and China, respectively. Substantial dust concentrations are also noted in higher
latitude stations in the North Atlantic (Bermuda). Annual mean dust concentrations
over the central North Pacific (Shemya, Midway, Oahu) are low, but as will be shown
below, iliey are significant. Dust concentrations in ilie souiliern oceans tend to be extremely
low (for example, see American Samoa) due to the dearth of strong dust sources on ilie
souiliern continents and the very great transport distances to ilie central ocean regions.
The impact of pollutant sources is evident at many sites in the Northern Hemisphere.
Extremely high nss-SO~- and NO; concentrations are seen at the sites near the coast
of Asia in the western Pacific; these are attributable to the high levels of atmospheric
pollution found over much of Asia due to the limited use of emission controls. Moderately high pollutant levels are seen over the North Atlantic as well (Bermuda, Mace
Head) as a result of emissions from North America and Europe. In contrast, the concentrations of NO;- and nss-SO~- at American Samoa and the Antarctic stations,
45
Figures 2.3 and 2.4 clearly show that the major sources of aerosols, both pollution
and mineral dust, lie in the Northern Hemisphere. The contrast between the Northern and Southern Hemispheres is important both from the standpoint of climate issues and also the impact of aerosol transport on ocean biogeochemical processes.
2.1.4
Aerosol Composition
The large fraction of the aerosol mass over the ocean is comprised of a relatively small
number of species as shown in Table 2.1, which presents a selection of aerosol concentrations at various mostly remote ocean sites. The annual means are compiled from
long-term (multi-year) measurements made at ocean network stations operated by the
aerosol group at the University of Miami (Prospero et al. 1989; Savoie et al. 1989a;
Arimoto et al. 1995, 1996; Savoie et al. 1993, 1994). The sea salt aerosol concentration is
calculated from the measured concentration of Na + in aerosol based on the ratio of
the total concentration of salts in sea water to that ofNa + (i.e. sea salt aerosol = 3.256 x Na,
by weight). The nss-SO~- fraction is calculated by measuring the Na + concentration in
the aerosol; on the assumption that the SO~-INa + ratio in the sea-spray droplets is the
same as that in bulk sea water, the Na + value is converted to a sea-salt SO~- by multiplying by 0.2517. This quantity is subsequently subtracted from total SO~- yield of the
nss-SO~- concentration.
There are many other important marine-aerosol species that are not included in
Table 2.1. In particular, organic materials can constitute a substantial fraction of the
total aerosol mass (Jacobson et al. 2000). Most notable is soot ("black carbon" or "BC"),
produced by fuel combustion and biomass burning. BC is found almost everywhere,
even over the most remote oceans (Heintzenberg et al. 2000). In addition, as discussed
below, over many ocean regions a large fraction of the aerosol mass remains
uncharacterized (Quinn et al. 2000).
As one might expect, sea salt aerosol is a major constituent at most sites. (At some
sites the high concentrations are attributable in part to local surf conditions, which
are not representative of concentrations on larger scales.) Mineral dust shows an extremely wide range of concentrations. Note the very high concentrations in the tropical North Atlantic sites (Izana, Barbados) and the sites in the western North Pacific
(Cheju, Okinawa, Hong Kong). These reflect the impact of dust transport from North
Africa and China, respectively. Substantial dust concentrations are also noted in higher
latitude stations in the North Atlantic (Bermuda). Annual mean dust concentrations
over the central North Pacific (Shemya, Midway, Oahu) are low, but as will be shown
below, iliey are significant. Dust concentrations in ilie souiliern oceans tend to be extremely
low (for example, see American Samoa) due to the dearth of strong dust sources on ilie
souiliern continents and the very great transport distances to ilie central ocean regions.
The impact of pollutant sources is evident at many sites in the Northern Hemisphere.
Extremely high nss-SO~- and NO; concentrations are seen at the sites near the coast
of Asia in the western Pacific; these are attributable to the high levels of atmospheric
pollution found over much of Asia due to the limited use of emission controls. Moderately high pollutant levels are seen over the North Atlantic as well (Bermuda, Mace
Head) as a result of emissions from North America and Europe. In contrast, the concentrations of NO;- and nss-SO~- at American Samoa and the Antarctic stations,