36
J. M. Prospero
can assess the relative impacts of anthropogenic and natural species and thereby to
be able to partition changes that might occur in both through time; so that we can
assess any changes in natural emissions that might occur as a result of human -induced
climate change. Knowledge of these natural climate-related species also helps us to
understand the factors that played a role in pre-human climate change. Motivated by
these concerns, a very substantial part of the community effort is now devoted to a
wide range of aerosol species.
The study of the role of aerosols is complicated by the fact that the physical and
chemical properties of aerosols are highly variable in both time and space. In contrast, most common "greenhouse" gases (e.g. CO 2 , CH 4 , halocarbons) have relatively
long lifetimes (months to years) in the atmosphere, and as a consequence, their distribution is relatively uniform over the Earth, and changes occur relatively slowly.
Aerosols, on the other hand, have a residence time in the troposphere that ranges from
a few days to a few weeks. Consequently, the concentration, composition and physical
properties of particles in the atmosphere can vary greatly, depending on the distribution of sources, the meteorological processes in the source regions, the large scale circulation systems that subsequently control long-range transport, and finally, the various removal processes that act on the particles. In order to assess the transport of
continental materials to the oceans and the general impact on climate, we must have \l
good understanding of all these processes.
This paper is intended to be a brief review of the chemical and physical properties
of aerosol species over the oceans. The emphasis is on those aerosol species that play
an important role in marine biogeochemical and climate processes:
• Sea salt aerosols: Sea salt aerosols are produced by the bursting of bubbles at the
surface of the ocean. In remote ocean regions, sea salt aerosol is often the primary
light scattering aerosol component, and it is an important source of cloud-forming
nuclei. Consequently, sea salt has a role in climate. Furthermore, over large areas of
the ocean, sea salt aerosol offers a large surface area for chemical and physical interactions with other aerosol species and with gases; because of its large size and high
settling velocity, sea salt aerosol can serve as a major sink for other atmospheric
components.
• Non-sea-Salt (nss) sulphate aerosols: Nss-SO~- is that fraction of marine SO~- aerosol that is not derived from sea water aerosol droplets. Nss-SO~- plays a major role
in radiation and cloud processes. As such, it is important to understand the factors
affecting its distribution over the oceans. Nss-SO~- is the major acidic aerosol species in the atmosphere; over large areas of the world, the pH of aerosols and precipitation is largely controlled by the concentration of nss-SO~- and ammonium. NssSO~- aerosol over the oceans has two major sources: the oxidation of dimethyl-sulphide (DMS) emitted by marine organisms and pollution transported from the continents.
• Nitrate and ammonium: The global budgets of both these species have been hugely
impacted by human activities. Ammonium is the major neutralizing species in the
atmosphere. Both species can play an important role as nutrient inputs to coastal
waters and oceans.
J. M. Prospero
can assess the relative impacts of anthropogenic and natural species and thereby to
be able to partition changes that might occur in both through time; so that we can
assess any changes in natural emissions that might occur as a result of human -induced
climate change. Knowledge of these natural climate-related species also helps us to
understand the factors that played a role in pre-human climate change. Motivated by
these concerns, a very substantial part of the community effort is now devoted to a
wide range of aerosol species.
The study of the role of aerosols is complicated by the fact that the physical and
chemical properties of aerosols are highly variable in both time and space. In contrast, most common "greenhouse" gases (e.g. CO 2 , CH 4 , halocarbons) have relatively
long lifetimes (months to years) in the atmosphere, and as a consequence, their distribution is relatively uniform over the Earth, and changes occur relatively slowly.
Aerosols, on the other hand, have a residence time in the troposphere that ranges from
a few days to a few weeks. Consequently, the concentration, composition and physical
properties of particles in the atmosphere can vary greatly, depending on the distribution of sources, the meteorological processes in the source regions, the large scale circulation systems that subsequently control long-range transport, and finally, the various removal processes that act on the particles. In order to assess the transport of
continental materials to the oceans and the general impact on climate, we must have \l
good understanding of all these processes.
This paper is intended to be a brief review of the chemical and physical properties
of aerosol species over the oceans. The emphasis is on those aerosol species that play
an important role in marine biogeochemical and climate processes:
• Sea salt aerosols: Sea salt aerosols are produced by the bursting of bubbles at the
surface of the ocean. In remote ocean regions, sea salt aerosol is often the primary
light scattering aerosol component, and it is an important source of cloud-forming
nuclei. Consequently, sea salt has a role in climate. Furthermore, over large areas of
the ocean, sea salt aerosol offers a large surface area for chemical and physical interactions with other aerosol species and with gases; because of its large size and high
settling velocity, sea salt aerosol can serve as a major sink for other atmospheric
components.
• Non-sea-Salt (nss) sulphate aerosols: Nss-SO~- is that fraction of marine SO~- aerosol that is not derived from sea water aerosol droplets. Nss-SO~- plays a major role
in radiation and cloud processes. As such, it is important to understand the factors
affecting its distribution over the oceans. Nss-SO~- is the major acidic aerosol species in the atmosphere; over large areas of the world, the pH of aerosols and precipitation is largely controlled by the concentration of nss-SO~- and ammonium. NssSO~- aerosol over the oceans has two major sources: the oxidation of dimethyl-sulphide (DMS) emitted by marine organisms and pollution transported from the continents.
• Nitrate and ammonium: The global budgets of both these species have been hugely
impacted by human activities. Ammonium is the major neutralizing species in the
atmosphere. Both species can play an important role as nutrient inputs to coastal
waters and oceans.