ATMOSPHERIC INPUT OF POLLUTANTS
R. A. Duce, Texas A&M University, College Station,
TX, USA
Copyright & 2001 Elsevier Ltd.
Introduction
For about a century oceanographers have tried to
understand the budgets and processes associated
with both natural and human-derived substances
entering the ocean. Much of the early work focused
on the most obvious inputs – those carried by rivers
and streams. Later studies investigated sewage outfalls, dumping, and other direct input pathways for
pollutants. Over the past decade or two, however, it
has become apparent that the atmosphere is also not
only a significant, but in some cases dominant,
pathway by which both natural materials and contaminants are transported from the continents to
both the coastal and open oceans. These substances
include mineral dust and plant residues, metals, nitrogen compounds from combustion processes and
fertilizers, and pesticides and a wide range of other
synthetic organic compounds from industrial and
domestic sources. Some of these substances carried
into the ocean by the atmosphere, such as lead and
some chlorinated hydrocarbons, are potentially
harmful to marine biological systems. Other substances, such as nitrogen compounds, phosphorus,
and iron, are nutrients and may enhance marine
productivity. For some substances, such as aluminum
and some rare earth elements, the atmospheric input
has an important impact on their natural chemical
cycle in the sea.
In subsequent sections there will be discussions of
the input of specific chemicals via the atmosphere to
estuarine and coastal waters. This will be followed
by considerations of the atmospheric input to open
ocean regions and its potential importance. The atmospheric estimates will be compared with the input
via other pathways when possible. Note that there
are still very large uncertainties in all of the fluxes
presented, both those from the atmosphere and those
from other sources. Unless otherwise indicated, it
should be assumed that the atmospheric input rates
have uncertainties ranging from a factor of 2 to 4,
sometimes even larger.
Estimating Atmospheric Contaminant
Deposition
Contaminants present as gases in the atmosphere can
exchange directly across the air/sea boundary or they
may be scavenged by rain and snow. Pollutants present on particles (aerosols) may deposit on the ocean
either by direct (dry) deposition or they may also be
scavenged by precipitation. The removal of gases
and/or particles by rain and snow is termed wet
deposition.
Direct Deposition of Gases
Actual measurement of the fluxes of gases to a water
surface is possible for only a very few chemicals at
the present time, although extensive research is
underway in this area, and analytical capabilities for
fast response measurements of some trace gases are
becoming available. Modeling the flux of gaseous
compounds to the sea surface or to rain droplets
requires a knowledge of the Henry’s law constants
and air/sea exchange coefficients as well as atmospheric and oceanic concentrations of the chemicals
of interest. For many chemicals this information is
not available. Discussions of the details of these
processes of air/sea gas exchange can be found in
other articles in this volume.
Particle Dry Deposition
Reliable methods do not currently exist to measure
directly the dry deposition of the full size range of
aerosol particles to a water surface. Thus, dry deposition of aerosols is often estimated using the dry
deposition velocity, v d . For dry deposition, the flux is
then given by:
F d ¼ v d Á C a
½1
where F d is the dry deposition flux (e.g., in g m
À2 s
À1
),
v d is the dry deposition velocity (e.g., in m s
À1
), and C a
is the concentration of the substance on the aerosol
particles in the atmosphere (e.g., in g m
À3
). In this
formulation v d incorporates all the processes of dry
deposition, including diffusion, impaction, and
gravitational settling of the particles to a water surface. It is very difficult to parameterize accurately the
dry deposition velocity since each of these processes is
acting on a particle population, and they are each
dependent upon a number of factors, including wind
speed, particle size, relative humidity, etc. The following are dry deposition velocities that have been
281
R. A. Duce, Texas A&M University, College Station,
TX, USA
Copyright & 2001 Elsevier Ltd.
Introduction
For about a century oceanographers have tried to
understand the budgets and processes associated
with both natural and human-derived substances
entering the ocean. Much of the early work focused
on the most obvious inputs – those carried by rivers
and streams. Later studies investigated sewage outfalls, dumping, and other direct input pathways for
pollutants. Over the past decade or two, however, it
has become apparent that the atmosphere is also not
only a significant, but in some cases dominant,
pathway by which both natural materials and contaminants are transported from the continents to
both the coastal and open oceans. These substances
include mineral dust and plant residues, metals, nitrogen compounds from combustion processes and
fertilizers, and pesticides and a wide range of other
synthetic organic compounds from industrial and
domestic sources. Some of these substances carried
into the ocean by the atmosphere, such as lead and
some chlorinated hydrocarbons, are potentially
harmful to marine biological systems. Other substances, such as nitrogen compounds, phosphorus,
and iron, are nutrients and may enhance marine
productivity. For some substances, such as aluminum
and some rare earth elements, the atmospheric input
has an important impact on their natural chemical
cycle in the sea.
In subsequent sections there will be discussions of
the input of specific chemicals via the atmosphere to
estuarine and coastal waters. This will be followed
by considerations of the atmospheric input to open
ocean regions and its potential importance. The atmospheric estimates will be compared with the input
via other pathways when possible. Note that there
are still very large uncertainties in all of the fluxes
presented, both those from the atmosphere and those
from other sources. Unless otherwise indicated, it
should be assumed that the atmospheric input rates
have uncertainties ranging from a factor of 2 to 4,
sometimes even larger.
Estimating Atmospheric Contaminant
Deposition
Contaminants present as gases in the atmosphere can
exchange directly across the air/sea boundary or they
may be scavenged by rain and snow. Pollutants present on particles (aerosols) may deposit on the ocean
either by direct (dry) deposition or they may also be
scavenged by precipitation. The removal of gases
and/or particles by rain and snow is termed wet
deposition.
Direct Deposition of Gases
Actual measurement of the fluxes of gases to a water
surface is possible for only a very few chemicals at
the present time, although extensive research is
underway in this area, and analytical capabilities for
fast response measurements of some trace gases are
becoming available. Modeling the flux of gaseous
compounds to the sea surface or to rain droplets
requires a knowledge of the Henry’s law constants
and air/sea exchange coefficients as well as atmospheric and oceanic concentrations of the chemicals
of interest. For many chemicals this information is
not available. Discussions of the details of these
processes of air/sea gas exchange can be found in
other articles in this volume.
Particle Dry Deposition
Reliable methods do not currently exist to measure
directly the dry deposition of the full size range of
aerosol particles to a water surface. Thus, dry deposition of aerosols is often estimated using the dry
deposition velocity, v d . For dry deposition, the flux is
then given by:
F d ¼ v d Á C a
½1
where F d is the dry deposition flux (e.g., in g m
À2 s
À1
),
v d is the dry deposition velocity (e.g., in m s
À1
), and C a
is the concentration of the substance on the aerosol
particles in the atmosphere (e.g., in g m
À3
). In this
formulation v d incorporates all the processes of dry
deposition, including diffusion, impaction, and
gravitational settling of the particles to a water surface. It is very difficult to parameterize accurately the
dry deposition velocity since each of these processes is
acting on a particle population, and they are each
dependent upon a number of factors, including wind
speed, particle size, relative humidity, etc. The following are dry deposition velocities that have been
281