76
J. M. Prospero
shows the measured distribution of nss-SO~-; also shown is the computed 'distribution of biogenic ally-produced (from DMS) nss-SO~-. This figure shows that the Northern Hemisphere is hugely impacted by continental anthropogenic sources and that the
biogenic source is trivially small. The latitudinal distribution of Be (middle panel) is
broadly similar to that of nss-SO~-; one subtle difference is the presence of a small
Southern Hemisphere maximum in the mid-latitudes (30-45° S), which is attributed
to biomass burning. Finally, the bottom panel shows the relative latitudinal distribution of "metals"; this category includes V, Cr, Ni, Cu, Zn, Cd, and Pb - all species that
are closely identified with pollution sources. The distribution of these metals also shows
a large maximum in the Northern Hemisphere mid-latitudes.
Thus, there is clear evidence of the widespread impact of anthropogenic aerosols
over large areas of the global ocean. This impact is evident for a wide range of materials, both inorganic and organic. The chemistry of these complex aerosol mixtures is
expected to be very different from that of aerosols in remote ocean regions where
pollution impacts are minimal.
2.7
Conclusions
Research shows that aerosols can play an important role in a number of important
marine biogeochemical processes and in climate. However, at this time it is difficult
to quantify these impacts. There are huge areas of the ocean for which we have little
or no detailed knowledge of aerosol physical and chemical properties. We also lack
detailed knowledge of the processes by which aerosols can affect climate. As for the
impact of aerosol material on ocean biogeochemical processes, there are two problems that preclude a clear assessment. First of all, we have only a very rough idea of
how much aerosol material is actually deposited; estimates are largely based on models that thus far have yielded widely varying results. Secondly, we have a very poor
understanding of the ways in which deposited aerosol materials participate in sea water
chemistry and biology. For example, we have very little information on the rates at
which dust-borne metals are released to the water column, the speciation and chemical properties of the released materials, and their bioavailability. These are all ripe areas
for research.
This review could only cover a relatively short list of species. While these species
make up a substantial fraction of the aerosol mass, in many ocean regions there is a
large fraction of the marine aerosol mass that has not been characterized. For example,
the work of Quinn et al. (2000) suggests that 9-45% of the submicrometre aerosol mass
over the remote Pacific Ocean consists of species other than nss-SO~-, sea salt, NH;,
and MSA. We know that in some regions mineral dust and organic materials could
make up much of the balance of the mass. Nonetheless, it is clear that there is much
work yet to be done in this regard. In particular, there is considerable evidence that
organic materials are ubiquitous over the oceans and that much of this material is
transported from the continents. Unfortunately, much of the data on organic aerosols
is semiquantitative. Thus, there is a great need to characterize this aerosol component.
In the absence of such data it is not possible to assess the impact of organic aerosols
on marine biogeochemistry.
J. M. Prospero
shows the measured distribution of nss-SO~-; also shown is the computed 'distribution of biogenic ally-produced (from DMS) nss-SO~-. This figure shows that the Northern Hemisphere is hugely impacted by continental anthropogenic sources and that the
biogenic source is trivially small. The latitudinal distribution of Be (middle panel) is
broadly similar to that of nss-SO~-; one subtle difference is the presence of a small
Southern Hemisphere maximum in the mid-latitudes (30-45° S), which is attributed
to biomass burning. Finally, the bottom panel shows the relative latitudinal distribution of "metals"; this category includes V, Cr, Ni, Cu, Zn, Cd, and Pb - all species that
are closely identified with pollution sources. The distribution of these metals also shows
a large maximum in the Northern Hemisphere mid-latitudes.
Thus, there is clear evidence of the widespread impact of anthropogenic aerosols
over large areas of the global ocean. This impact is evident for a wide range of materials, both inorganic and organic. The chemistry of these complex aerosol mixtures is
expected to be very different from that of aerosols in remote ocean regions where
pollution impacts are minimal.
2.7
Conclusions
Research shows that aerosols can play an important role in a number of important
marine biogeochemical processes and in climate. However, at this time it is difficult
to quantify these impacts. There are huge areas of the ocean for which we have little
or no detailed knowledge of aerosol physical and chemical properties. We also lack
detailed knowledge of the processes by which aerosols can affect climate. As for the
impact of aerosol material on ocean biogeochemical processes, there are two problems that preclude a clear assessment. First of all, we have only a very rough idea of
how much aerosol material is actually deposited; estimates are largely based on models that thus far have yielded widely varying results. Secondly, we have a very poor
understanding of the ways in which deposited aerosol materials participate in sea water
chemistry and biology. For example, we have very little information on the rates at
which dust-borne metals are released to the water column, the speciation and chemical properties of the released materials, and their bioavailability. These are all ripe areas
for research.
This review could only cover a relatively short list of species. While these species
make up a substantial fraction of the aerosol mass, in many ocean regions there is a
large fraction of the marine aerosol mass that has not been characterized. For example,
the work of Quinn et al. (2000) suggests that 9-45% of the submicrometre aerosol mass
over the remote Pacific Ocean consists of species other than nss-SO~-, sea salt, NH;,
and MSA. We know that in some regions mineral dust and organic materials could
make up much of the balance of the mass. Nonetheless, it is clear that there is much
work yet to be done in this regard. In particular, there is considerable evidence that
organic materials are ubiquitous over the oceans and that much of this material is
transported from the continents. Unfortunately, much of the data on organic aerosols
is semiquantitative. Thus, there is a great need to characterize this aerosol component.
In the absence of such data it is not possible to assess the impact of organic aerosols
on marine biogeochemistry.