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OCR for page 3372
Proc. Natl. Acad. SCZ. USA
Vol. 96, pp. 3372-3379, March 1999
Colloquium Paper
This paper was presented at the National Academy of Sciences colloquium "Geology, Mineralogy, and Human Welfare, "
held November 8-9, 1998 at the Arnold and Mabel Beckman Center in Irvine, CA.
Airborne minerals and related aerosol particles: Effects on
climate and the environment
PETER R. BUSECK* AND M1HALY POSFAI!
Departments of Geology and Chemistry/Biochemistry, Arizona State University, Tempe, AZ 85287
ABSTRACT Aerosol particles are ubiquitous in the tropo-
sphere and exert an important influence on global climate and
the environment. They affect climate through scattering, trans-
mission, and absorption of radiation as well as by acting as nuclei
for cloud formation. A significant fraction of the aerosol particle
burden consists of minerals, and most of the remainder
whether natural or anthropogenic consists of materials that
can be studied by the same methods as are used for fine-grained
minerals. Our emphasis is on the study and character of the
individual particles. Sulfate particles are the main cooling agents
among aerosols; we found that in the remote oceanic atmosphere
a significant fraction is aggregated with soot, a material that can
diminish the cooling effect of sulfate. Our results suggest oxi-
dization of SO2 may have occurred on soot surfaces, implying
that even in the remote marine troposphere soot provided nuclei
for heterogeneous sulfate formation. Sea salt is the dominant
aerosol species (by mass) above the oceans. In addition to being
important light scatterers and contributors to cloud condensa-
tion nuclei, sea-salt particles also provide large surface areas for
heterogeneous atmospheric reactions. Minerals comprise the
dominant mass fraction of the atmospheric aerosol burden. As
all geologists know, they are a highly heterogeneous mixture.
However, among atmospheric scientists they are commonly
treated as a fairly uniform group, and one whose interaction with
radiation is wideb assumed to be unpredictable. Given their
abundances, large total surface areas, and reactiv~ties, their role
in influencing climate will require increased attention as climate
models are refined.
There is widespread concern over the enhanced global warming
that might result from the buildup of "greenhouse gases" in the
atmosphere. The effects of aerosols (suspensions of solid or liquid
particles in air) on Earth's radiation balance is less widely realized,
and recognition of the role of airborne minerals has occurred only
relatively recently.
Climate is fundamentally influenced by Earth's energy budget,
which depends on radiation received from the sun and energy
radiated back to space. Incoming radiation is primarily in the
visible range, whereas exiting radiation is largely in the IR.
Greenhouse gases (H2O, CO2, CH4, N2O, etc.) absorb IR radi-
ation and radiate it back to Earth's surface. Anthropogenic
emissions of greenhouse gases cause increases in surface tem-
perature (the "greenhouse effect") and can have profound effects
on climate and thus on societal welfare (1, 2~.
Aerosol particles also have a major influence on global climate
and climate change; they can locally either intensify or moderate
the effects of the greenhouse gases through the scattering or
absorption of both incoming solar radiation and thermal radia-
tion emitted from Earth's surface. Aerosols also act as cloud
condensation nuclei (CCN) and thereby modify the radiative
properties of clouds. The profound effects of atmospheric aero
PNAS is available online at www.poas.org.
sots are surprising in view of their exceedingly low concentrations:
the volumetric ratio of aerosol particles to atmospheric gases is
between roughly 10-~° and 1O-~4 (3~. The focus of this paper is
on those particles, their compositions and structures and their
effects on climate and, to a lesser extent, on the environment.!
A growing awareness of the impact of particulate aerosols on
climate, and the incompletely recognized but serious effects of
anthropogenic aerosols, is summarized in several recent reviews
(4-6~. One reason for the relatively slow recognition of the role
of particulate aerosols is that their study has fallen to disparate
groups of scientists. Radiative transfer and other physical prop-
erties tend to be handled by one group (largely meteorologists
and physicists), whereas chemical effects such as acid rain are
emphasized by different scientists (mainly chemists). Perhaps the
least attention to date has been on the geochemistry and miner-
alogy of aerosol particles and the effects of speciation.
Efforts to control greenhouse gases have been formalized by
international treaty, e.g., the 1997 Kyoto Protocol on Climate
Change. A comparable international effort to understand and
control anthropogenic aerosol emissions has not (yet) occurred,
at least in part because the extent to which they affect climate is
not satisfactorily known. The incremental effects of anthropo-
genic increases in greenhouse gases are long lived (decades to
centuries), whereas those of aerosols are shorter (weeks) (7~.
However, the sizes, compositions, and atmospheric lifetimes of
particulate aerosols can vary spatially and temporally, and their
strongest effects tend to be near their sources. If aerosols indeed
offset climate responses to greenhouse gases, then the climate
effects of greenhouse gases are even more substantial than has
been recognized.
What role do mineralogists and geochemists have in addressing
these and related issues of fundamental importance for human
society and welfare? The main difference between most aerosol
particles and the materials that are routinely studied by miner-
alogists is that most terrestrial minerals are not as fine "rained.
There are major problems with studying fine-grained materials,
and therefore atmospheric chemists have traditionally empha-
sized bulk analyses to determine aerosol types. However, it is the
individual chemical species that affect the radiative balance and
Abbreviations: CCN, cloud condensation nuclei; TEM, transmission
electron microscope; SEMI scanning electron microscope; MBL'
marine boundary layer; FI,7 free troposphere; NSS7 non-sea salt; AFM'
atomic force microscopy; ACE, Aerosol Characterization Experi-
ments; FELINE' experiments in the equatorial Pacific; ASTEX/
IMAGE, experiments in the North Atlantic.
*To whom reprint requests should be addressed. e-mail: pbuseck@
asu.edu.
TPresent address: Department of Earth and Environmental Sciences'
University of Veszprem, Veszprem' POB 158' H8201 Hungary.
"Aerosol particles' to the extent they consist of nonanthropogenic
homogeneous inorganic solids of more or less uniform composition
and have ordered structures' fit generally accepted definitions of
minerals. However, except for the title, in this manuscript we follow
the usage common among atmospheric scientists and use ``minerals''
to refer to materials that once resided on Earthts land surface.
3372
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Colloquium Paper: Buseck and Posfai
climate as well as visibility and health. Paraphrasing a recent
statement (8) and allowing for slight exaggeration, interpreting
environmental and health effects of aerosols from bulk rather
than individual-particle analyses is like interpreting mortality
reports in a war zone from bulk airborne lead concentrations
rather than from bullets.
Our group has focused on the painstaking but necessary
analysis of individual particles. High-spatial-resolution methods,
using electron beams as the primary probes of both chemistry and
structure, have been developed to study increasingly fine-grained
minerals. We examine the inorganic and, in special cases, the
organic fraction of aerosol particles with electron microprobe
analyzers and scanning electron microscopes (SEMs) and trans-
mission electron microscopes (TEMs). In this paper we provide
a background to the above issues and indicate ways in which
mineralogical experience and experimental techniques can pro-
vide uniquely useful information. We first review the broad
problems and briefly describe the analytical techniques, then
discuss some of our recent transmission electron microscopy
results regarding sulfate, soot, sea salt, and mineral aerosols.
Aerosols, Climate, and the Environment
Nature of Airborne Minerals and Other Inorganic Aerosols.
Andreae (9) estimated that the largest components of the global
atmospheric aerosol are, in decreasing mass abundances: mineral
aerosols primarily from soil deflation but also with a minor
component (<1%) from volcanoes (16.8 Tg), sea salt (3.6 Tg),
natural and anthropogenic sulfates (3.3 Tg), products of biomass
burning excluding soot (1.8 Tg) and of industrial sources including
soot (1.4 Tg), natural and anthropogenic nonmethane hydrocar-
bons (1.3 Tg), natural and anthropogenic nitrates largely from
NOX (0.6 Tg), and biological debris (0.5 Tg).
In general, a distinction is made between primary particles,
which are injected into the troposphere, and secondary particles,
which form within the troposphere. Sea salt from spray, desert
dust, volcanic mineral emissions, and re-entrained road dust are
examples of primary aerosols. In contrast, particles produced by
condensation of gases result in secondary aerosols. Primary
aerosol particles tend to be larger, dominating the "coarse"
fraction, which is >1 Am in diameter and mostly mechanical in
origin. The fine fraction is enriched in secondary particles, largely
between 0.1 and 1 ,um in diameter and mainly chemical in origin.
The smaller size range is also called the "accumulation mode," in
distinction to the "coarse mode." In addition, there is a "nucle-
ation mode," with particles smaller than 0.1 ~m. Recently "nano-
particles" in the 3- to 10-nm range have also been distinguished
(10~.
Human activities affect the amounts, types, and distributions of
aerosols that enter the atmosphere. Anthropogenic particles are
especially abundant in the submicrometer portion of the aerosol,
and they provide a major uncertainty in estimating climatic effects
(11, 12~.
Different particle types (mineral dust, sulfates, carbonaceous
materials, sea salt, organic compounds) can occur in the same air
mass. An important question that we commonly address in our
individual-particle work is, to use the terminology of atmospheric
chemistry, whether the particles are internally or externally mixed.
Phases that are externally mixed occur within the same aerosol
but in discrete separate particles. If they are internally mixed, then
they occur within the same particles; inhomogeneous internal
mixtures are much like minerals in a rock, whereas homogeneous
internal mixtures are solutions. Changes from predominantly
external to internal mixtures can occur when particles grow by
coagulation with different species, a process that is especially
common during entrainment into clouds.
The differences between internal and external mixtures can
significantly affect the optical properties and radiative efficiency
of the aerosol and its ability to act as CCN. The nature and
magnitude of these effects have received considerable attention
(13, 14), but there are differences of opinion. For example, some
Proc. Natl. Acad. Sci. USA 96 (1999) 3373
authors (15, 16) indicate that scattering calculations for mixed
aerosols are not significantly affected by assumptions regarding
internal or external mixing, whereas others (17, 18) reach the
opposite conclusion for mixtures of sulfate and soot. Clearly, the
problems have not been resolved, and most current models do not
yet consider the subtleties of internal mixtures.
Atmospheric aerosol particles can also profoundly influence
the environment. Dust, smoke, and haze locally impair visibility
and health in both urban and rural regions. The harmful respi-
ratory health effects of certain mineral and anthropogenic par-
ticles are well documented and have led to a plethora of federal
rules and regulations (19, 20~. Knowledge of the compositions
and microphysical properties of aerosols is critical first for un-
derstanding and then for ameliorating some of these pernicious
environmental and health effects.
Effects of Airborne hIinerals and Other Particles on Climate
Radiative Forcing. Forcing is the term used to describe
changes imposed on the planetary energy balance. It is measured
in watts per square meter (Wm-2~. Aerosol radiative forcing,
which refers to the effects of aerosols, is termed direct if it results
from backscattering and absorption of radiation by the aerosol
particles themselves, and indirect if it results from the influence of
the particles on the optical properties, amounts, and lifetimes of
clouds. Positive forcing results in a net warming at Earth's surface,
and negative forcing results in a net cooling.
The magnitude of the radiative effects of aerosol particles
depends on their compositions, sizes and size distributions, abun-
dances, hygroscopicities, surface properties, densities, and refrac-
tive indices (15, 21-23~. Some of these parameters are interde-
pendent, and they can vary with locality, sources, and environ-
mental variables such as intensity of sunlight and relative
humidity. Also, inventories of concentrations of particle types,
especially those with a broad range of spatial and temporal
distributions such as from industrial, arid urban, and particular
geological sources (deserts, volcanoes) need to be well known for
specific particulate assemblages. Their vertical distributions and
underlying surface albedos are also important (244. The com-
posite effect is complex and will require extensive measurement
before it is well understood.
Sulfates are thought to be the most important scatterers of solar
radiation on a global scale, producing a net cooling at Earth's
surface, whereas soot tends to be a major absorber of the solar
radiation and so has a net warming effect (44. The role of mineral
dust is more ambiguous (9, 25~. Particles tend to be most efficient
in scattering radiation having wavelengths comparable to their
physical sizes; submicron dust particles are efficient scatterers of
the incoming sunlight and thus can have a cooling effect, espe-
cially near Earth's surface. On the other hand, mineral particles
also absorb light and thus have a heating effect at the altitude at
which they occur (cf. Mineral Dust). Because the outgoing radi-
ation is in the IR, and silicate minerals have bands in which they
absorb in the IR, they can act as "greenhouse particles." The
larger mineral aerosol particles tend to have shorter atmospheric
lifetimes and to be most concentrated in the lower troposphere,
near their source areas, and so produce localized effects.
Cloud droplets form on aerosol particles as nuclei. The num-
ber, sizes, and compositions of such CCN have major influences
on cloud formation. Hygroscopic materials such as sulfates and
sea salts are especially efficient as nuclei; mineral dust and
combustion products can also be effective, especially if they are
wettable or acquire hygroscopic coatings (26~. Increased numbers
of CCN lead to more cloud droplets and concurrent decreases in
droplet sizes (for given cloud water contents) (27-29~. Because of
multiple scattering within the cloud, cloud albedo tends to
increase with numbers and small sizes of hygroscopic aerosol
particles, which results in increased cooling. In addition, clouds
with more and smaller droplets are less prone to rain and drizzle
formation and therefore persist longer, having more time to exert
their cooling effect.
OCR for page 3374
3374 Colloquium Paper: Buseck and Postal
Mineralogical Techniques Applied to Single Aerosol
Particles
Electron-Beam Analyses. The traditional method of studying
the chemistry of aerosol particles is through bulk methods. By
using such techniques, large numbers of particles are analyzed en
masse. However, the results provide no information about indi-
vidual aerosol species and the mixing state of the aerosol;
knowing whether the aerosol particles are attached to one
another or to other types of particles is critical for understanding
processes that involve or modify the particles during atmospheric
transport.
Our approach has been to emphasize the individual particles
because the radiative, environmental, and health effects of par-
ticles depend on their speciation rather than their averaged bulk
compositions. Our primary instruments are the analytical SEM,
electron microprobe analyzer (EMPA), and TEMs. The SEM
and EMPA can operate in automated mode, running unattended
around the clock and producing large numbers of analyses
(30-35~.
Particle analysis using TEMs, which are the main instruments
used for the results reported here, cannot be automated because
of the complexity of the method (sample tilting in diffraction
mode to obtain critical crystal orientation, combined with
changes to imaging and analysis modes to obtain size, shape, and
chemical information). Thus, the TEM does not readily provide
the statistical depth produced by the SEM or EMPA, but it can
be used to analyze far smaller particles (down to <1 nm vs. '0.1
~m) and, perhaps more importantly, it can provide crystallo-
graphic, morphologic, size, and chemical information for indi-
vidual particles. Many of the aerosol particles of greatest interest
for influencing climate fall within a size range that is accessible
only by using the TEM (36-44~. A general background for
mineralogical transmision electron microscopy is given by ref. 45.
Aerosol Time-of-night Mass Spectrometry. A recent instru-
mental development of considerable interest is the aerosol time-
of-flight mass spectrometer (ATOFMS), which can be used for
rapid measurements of both sizes and compositions of individual
particles (46-48~. Particles pass through a laser beam, which
blasts each particle into positively and negatively charged ions.
These are then analyzed in the ATOFMS to determine the atomic
or molecular weight of each ion, from which fragment compo-
sitions can be determined. Particle sizes are calculated from their
velocities as they move through the ATOFMS. A result is that
chemical and size information is obtained from individual par-
ticles in real time, although back calculations are required to infer
parent species from their ion fragments.
Samples. Because the oceans cover ~70% of the globe, much
attention has been given to the profound effects of the oceans on
climate. A recent area of interest is the role of aerosol particles
in the marine troposphere and the effects they have on climate.
Marine aerosols are important because there is generally enough
humidity to form clouds, but there can be a shortage of CCN;
thus, the indirect climate effect of aerosols is far more pro-
nounced over sea than land. Aerosols in such areas are also the
most likely to represent unpolluted, "global background" condi-
tions, and the contributions of natural and anthropogenic sources
are easier to identify than in continental or polluted oceanic
atmospheres.
Two recent international research programs are shedding
important new insights onto the role of aerosols in the tropo-
sphere. The Aerosol Characterization Experiments (ACE) took
place in late 1995 in the Southern Ocean near Tasmania (ACE-1)
and in 1997 in the North Atlantic (ACE-24. They produced
integrated measurements from ships, airplanes, and ground sta-
tions; ACE-1 involved researchers from 45 institutions in North
America, Europe, Australia, and Asia (49~.
In the ACE-1 campaign we used two platforms. At Cape Grim,
Tasmania, we collected aerosol particles onto filters in a two-
stage impactor (for SEM analysis) and directly onto TEM grids.
Proc. Natl. Acad. Sc'. USA 96 (1999)
We also used a one-stage impactor that was mounted on a C-130
aircraft. In this paper we use results from a Lagrangian experi-
ment in which a tagged air mass was followed and the aerosol
evolution within it studied over time. The airplane flew "stacked
circles," with each circle at a different altitude.
From the ACE-2 experiment we use data obtained from
particles collected onto TEM grids that were placed on filters.
The sampling station was on a mountain top (at 2,600 m altitude)
on Izana, Canary Islands. We also include results from earlier
experiments in the equatorial Pacific (FELINE; ref. 41 ) and the
North Atlantic (ASTEX/MAGE; refs. 42, 50~.
Open Problems, Issues, and Results
Sulfates§ and Associated Soot and Organic Species. Sulfates
are probably the main climate-cooling aerosols (51-53~. They
scatter solar radiation and are effective as CCN; the result is
negative forcing and thus cooling at Earth's surface. The radiative
forcing of sulfate aerosol particles, especially in the Northern
Hemisphere, is roughly equivalent in magnitude but opposite in
sign to the combined forcing by the greenhouse gases (12, 51,
53-55~.
If sulfates are internally mixed with other aerosol species, their
hydroscopic behavior and optical properties can change dramat-
ically (14, 17, 18) and result in diminished cooling. It is therefore
important to determine the state of mixing of these particles. We
studied sulfate particles in sample sets obtained from (i) the
polluted marine boundary layer (MBL) near the Azores Islands
(ASTEX/MAGE), (ii) the remote unpolluted Southern Ocean
MBL and free troposphere (FT) (ACE-1), and (iii) the essentially
clean North Atlantic FT at Izana, Canary Islands (ACE-2~. In
distinction to the relatively small amount of primary sulfate that
is in sea water and thus occurs in every sea-salt aerosol particle,
the secondary sulfate aerosol, which is what interests us here, is
called non-sea salt (NSS) sulfate.
TEM Observations. The compositions of hydrated sulfate
particles may change during sample processing or in the vacuum
of a TEM. For such samples we must rely on indirect evidence,
such as morphological features, to identify the compositions of
§Here we use "sulfate" for particles that consist purely or predomi-
nantly of sulfates and are formed in the atmosphere by either
homogeneous or heterogeneous reactions involving SO2 gas. They
have compositions ranging from H2SO4 to (NH4)2SO4. Sea-salt
sulfate and the NSS sulfate formed by conversion of the original
sea-salt particles are not considered in this section.
~ ~ ~ O , j O
a. ~ do ~
O #
4~3' ~ ~ 911, ~ ~
pica · ' ~ ~ I.
;':e ~ {e ~ ~
tic 48~ ~ ~ ~ ~
~0 4' ~ ~ i,
i' ~ ~ ~ ~ ~ ~O ~
~ pm ~..
. . · ~ ~
{; ~ ~ ~ '~v
to ~ ~ ' hi,, ~ ~9~
~ .e ~ ~ ~(
. ~ 0 ~_
Oe ~_ '.
.~. ~ ~-
I:
~_ .~ ~' 's .~-
~ : ~ ::::: it: ~ :: ~500:nm : ~.~.3 · .~+
. . ,, ,, O
. ~.. .
b
FIG. 1. TEM images of (NH4)2SO4. (a) The selected-area electron-
diffraction pattern (upper left) confirms the identification. The arrow
points to a soot aggregate. (Azores, North Atlantic, ASTEX/MAGE);
(b) Rings of small (NH4)2SO4 crystals that formed as the sulfate
particles dehydrated. The dimensions of the halos can be used to
distinguish among particles that likely had different water contents
while still airborne. (Southern Ocean, ACE-1.)
OCR for page 3375
Colloquium Paper: Buseck and Posfai
the original atmospheric particles. Fig. la shows a typical particle
that was collected above the North Atlantic Ocean when a
polluted air mass of European origin was sampled. An energy-
dispersive x-ray spectrum shows O. N. and S. and the selected-
area electron-diffraction pattern was used to confirm its identity
as crystalline (NH4~2SO4.
Acidic particles are more hydroscopic than pure (NH4~2SO4
and so will contain more water and spread farther on the TEM
grid than neutral species. The halos of discrete smaller particles
that form (Fig. lb) presumably result from the acidity of the
original sulfate (37) and are likely related to the amount of
adsorbed water. Different samples typically have characteristic
distributions of sulfates with or without halos. For example, the
images in Figs. 1 a and b and 2a were obtained from different
samples; we assume that the halo-free particle (Fig. la) was
(NH4~2SO4 even while in the air. The sulfate with a single ring of
smaller particles (Fig. 2a) was slightly acidic, whereas the particles
with multiple rings (Fig. lb) were more acidic. Other differences
in appearance reflect the degree of decomposition produced by
the electron beam of the TEM and contrast of the image.
Internal Mixing of Sulfate and Soot. There is increasing
evidence that carbonaceous aerosols can have major impacts on
aerosol radiative forcing (56, 57~. Soot both scatters and absorbs
radiation at solar wavelengths and thus produces cooling at
Earth's surface while warming the atmosphere around the par-
ticle. The net effect is a slight positive forcing. The resulting
warming of the atmosphere can have particularly significant
effects over highly reflective areas such as those covered with
snow. When soot derived from fossil fuels is entered into models,
it produces a positive forcing that can offset 40% or more of the
negative global radiative forcing of sulfate (17~. Organic particles
can be the dominant CON aerosols above tropical rain forests (58)
and densely populated areas such as the East Coast (56, 57~.
Internal mixtures of sea salt, sulfate, and material that is presum-
ably organic carbon occur widely (59~.
We found that many sulfate particles are internally mixed with
soot in the marine troposphere (Figs. la and 2a). Soot/sulfate
aggregates had been observed in polluted urban environments
(40, 60-62), but it is surprising that even in the remote oceanic
atmosphere a significant fraction of sulfates contain soot inclu-
sions (44~. Up to ~90% of the submicron ammonium sulfate
particles contain soot aggregates in samples collected above the
North Atlantic Ocean during a pollution episode (ASTEX/
MAGE). In the clean atmosphere above the Southern Ocean
during the ACE-1 experiment, 11 to 46~o of the sulfate particles
contain soot. Soot/sulfate aggregates comprise about 20~o of all
sulfate particles in two samples that we studied from Izana
(ACE-24. Because we observed similar relative numbers of
soot/sulfate internal mixtures in the aerosol from two essentially
clean but geographically distant locations (Southern Ocean and
Izana, North Atlantic), we believe it likely that soot/sulfate
internal mixing is a globally important phenomenon and must be
considered when the climate effects of sulfate aerosols are
modeled (44~.
We observed variations in the compositions and microstruc-
tures of soot particles and in their associations with other species
(i) The polluted North Atlantic ASTEX/MAGE samples have
~-~ ~: ~ ·~ ~
of ~500-nm
Proc. Natl. Acad. Sci. USA 96 (1999J 3375
soot associated with silica fly ash spherules that contain various
metals; the association suggests that a coal-burning power plant
was the likely source of the soot. (ii) Depending on the sample,
between 20% and 50% of soot particles from both the Southern
Ocean and Izana contain significant K which is typical of biomass
burning (63-66~. (iii) Only C and O are detectable in most soot
from the Southern Ocean; such particles typically have "com-
pact" microstructures, as described below. Because they contain
no associated fly ash, K V, or other metals, any of which would
suggest alternate sources, we conclude they were likely emitted by
aircraft.
Most soot particles from the Southern Ocean atmosphere
consist of only a few 10- to 50-nm globules. The (NH~2SO4
particle in Fig. 2a contains such a typical small soot inclusion.
Individual graphitic layers are wavy and subparallel, forming the
poorly crystalline turbostratic structure characteristic of soot (Fig.
2b). Large branching soot aggregates (Fig. 2~) are less common
than the small particles.
Our observations of soot/sulfate particles are significant for
several reasons. Internally mixed soot and sulfate raise the
question of whether they became attached through (i) coagula-
tion, (~i) processing in cloud droplets, or (zii) condensation and
oxidization of SO2 on the soot surfaces. If mechanism (iii) is
responsible for the observed aggregates, then soot particles
provided nuclei for heterogeneous sulfate formation even in the
remote marine troposphere, a pathway for sulfate formation that
has not been considered in climate models. Further studies are
needed to unravel the processes that bring atmospheric sulfate
and soot particles together.
A soot inclusion within a sulfate particle changes the optical
properties of the sulfate. That particle, if soot-free, would have a
cooling effect on the atmosphere; however, if it contained enough
soot, it could emit IR radiation and thereby heat its immediate
environment. The magnitude of the effect depends on the size of
the inclusion. On the other hand, nucleation of sulfate particles
on soot would result In additional CON particles that exert
increased indirect cooling. The presence of absorbing soot ~nclu-
sions in clouds can have a major offset of the expected Increase
of cloud albedo. Clearly, the aggregation of soot with sulfate can
cause changes in both the direct and indirect radiative forcing of
sulfates and thus needs further study.
Internal Mixing of Sulfate and Organic Species. For deter-
mining climate effects, it is important to know whether inorganic
particles contain organic coatings. Such coatings can influence
the hydroscopic behavior of particles by retarding water evapo-
ration (67, 68) and increasing or decreasing water adsorption by
inorganic aerosols (69~. Many particles of ammonium sulfate,
when sublimated with the electron beam in the TEM, leave visible
residues on the grid. The residues in Fig. 3 include a soot
aggregate and dark films that indicate only S in their energy-
dispersive x-ray spectra. The most reasonable identification of the
residue films is that they consist of organic compounds that
coated the original sulfate particles.
Indirect evidence for the probable presence of organic coatings
on sulfates is provided by combining atomic force microscopy
(AFM) and TEM images of the same sulfate particles (434. The
AFM image (Fig. 4) was obtained under ambient conditions at a
FIG. 2. TEM images of an internal mix-
ture of (NH4~2SO4 and soot. (a) The halo is
similar to those in Fig. 1. The arrow points
to a soot aggregate. (Southern Ocean,
ACE-1~; (b) High-resolution image of the
arrowed tip of the soot aggregate in a. A
degree of ordering is evident in the onion-
like graphitic layers, seen edge on. (c) A
large branching soot aggregate; such aggre-
gates are typical of combustion processes
(95~. (Southern Ocean, ACE-1.)
OCR for page 3376
3376 Colloquium Paper: Buseck and Posfai
~ .;0''
is.
500 me
FIG. 3. TEM image of ammonium sulfate (a) before and (b) after it
was sublimated by the electron beam. We believe the dark films in b are
residues of organic material that coated the aerosol particle before
sampling. The arrow marks a small soot particle. (Southern Ocean,
ACE-1.)
relative humidity of 31%. Ike TEM image shows that the same
particles are significantly smaller in the vacuum of the TEM (at
Otto relative humidity) than during the AFM study. The amount
of water lost when they were inserted into the TEM was calcu-
lated and was in excess of what could be expected for pure
(NH4~2SO4. We believe that organic coatings on the particles are
responsible for the observed anomaly in the hydroscopic behavior
of these sulfates.
Sea Salt. Sea-salt aerosol particles are generated when rising
bubbles burst at the surface of the ocean and water drops are
ejected into the atmosphere (Fig. 5 a and b). The droplets are
typically 0.1 to 100 ,um in diameter (70~. Sea-salt particles can
dominate light scattering by aerosols and comprise a significant
fraction of CCN above some regions of the unpolluted oceans
(48~. The large mass of sea-salt aerosol means that it dominates
the particulate surface area in the marine troposphere and,
through heterogeneous surface reactions, plays an important role
in the atmospheric cycles of Cl, S. and N (Fig. Sc).
There is controversy about the significance of sea salt for
aerosol radiative effects in the MBL. It had been thought that
sea-salt particles are mostly too large to be efficient scatterers of
solar radiation and that the smaller NSS sulfate is the main
contributor to radiative effects in the MBL. However, recent data
indicate that in some regions sea salt can be a major source of
CCN and can even dominate NSS sulfate (21, 70~. Individual-
particle analysis can be used to determine sea salt vs. sulfate
number concentrations and sizes to obtain a better understanding
s:
.. - If<..
<~ ~ ~ ~ ~ ~x '''
L A: ~ ~
~ ~2''.~
~ ~ ~ .~
~ . d~ jet ~
it' ! ' ~ ~ : ~
~ 1,um
. '
j
~ .
· ~ ~ ~,L,~ ~
~ I.
. ~ .
. . at. .
~ , ~
~ +~v
· i .
~.~d 'a C
~ em.
8-~.,r
6~= ~ . it
.:~< b
FIG. 4. (a) AFM and (b) TEM images of identical sulfate particles.
Note the decrease in size caused by dehydration within the TEM. The
amount of lost water is larger than expected and suggests increased
hygroscopicity through organic coatings. (AFM image by Huifang Xu)
(Azores, North Atlantic, ASTEX/MAGE.)
Proc. Natl. Acad. Sci. USA 96 (1999J
NaCI
sulfate
200 rim
500 rim
FIG. 5. TEM images of sea salt. (a and b) Subhedral halite (NaCl)
and euhedral sulfate crystals. The particle in b belongs to the smallest
sea-salt particles that occur in the ACE-1 samples. (Southern Ocean,
Cape Grim, ACE-1~; (c) Halite particles in various stages of conver-
sion to sulfate and nitrate. Grain A is partly converted, whereas C has
been completely converted to nitrate and grains B to sulfates. (Azores,
North Atlantic, ASTEX/MAGE.)
OCR for page 3377
Proc. Natl. Acad. Sci. USA 96 (1999) 3377
S
Colloquium Paper: Buseck and Posfai
Na
>~x x x~
\ /
=
Na
R24 /
/ x
lax x
I\a
/ x \
-
xXxx\
x REV
\
S 1200 C]
S
xXxxx: A
Xx art
\~P
A
Na
x§~
\
Na
\
S 150 C! \
S 30 Cl
Na
R26 /x \
,/ x xNxxxxt
Day 1 \ / Day 2
Cl S
of the radiative importance of sea salt. During ACE-1 we found
that in the windy Southern Ocean, sea salt is a constituent in
almost all MBLparticles; onlyless than 1% were pure sulfate (48),
although in other areas sulfate is a major particle type (31, 71~.
Changes occur with altitude in the compositions of sea-salt
particles collected above the Southern Ocean (Fig. 6), and the
reaction pathway from A (fresh sea salt) to B (sulfate) can be
traced. Unreacted particles dominate the samples collected
within the MBL (at ~30, 150, and 600 m); their S contents are
consistent with the presence of sea-salt sulfate (i.e., sulfate that
was in the seawater droplet when it was ejected into the atmo-
sphere). There is more variation in particle compositions at 150
and 600 m than near the sea surface (at 30 m), but the differences
are small. In addition to unreacted sea salt, S-nch particles occur
at 1,200 and 2,100 m, above the thermal inversion. Several
particles in the 2,100-m sample were completely converted to
sulfate. More reacted particles occur in the FT, i.e., at high
altitudes, presumably because they spent more time in the atmo-
sphere and had more time to react with SO2 than the particles in
the MBL. Some particles in the FT may have been transported
from remote locations where SO2 concentrations could have been
higher than at the ACE-1 study area.
Temporal changes in sea-salt compositions can also be ob-
served from the ACE-1 samples. During Lagrangian experiment
"B." an air mass was tagged by balloons and sampled from flights
on 3 consecutive days. Sea-salt compositions from samples col-
lected during the first (R24) and last (R26) flights are summa-
rized in the lower part of Fig. 6. The temporal changes resemble
those with increasing altitude; reacted sulfate-rich particles
formed during the 26 hr between the flights. Particle aging is
noticeable in the FT samples, with more sulfate particles in the
second sample set than in the first. The longer the sea-salt
particles reside in the air, the longer they are exposed to SO2 and
the more likely they are to convert to sulfates.
Cl
FIG. 6. Na-Cl-S plots showing
changes in composition of aerosol par-
ticles with altitude (Upper) and time
(`Lower). Each of the upper triangles
represents a sample from a single alti-
tude (meters above sea level are indi-
cated) and contains data from between
30 to 70 particles. The upper samples
were collected during a series of flights
on one day. The lower triangles contain
composites of samples from several al-
titudes; the one on the left contains all
particles indicated above, whereas the
one on the right contains those collected
26 hr later. Each contains data from
about 250 particles. Mg, K, and Ca are
not included in the diagrams because
their ratios to Na do not change (or are
within our analytical error). (Southern
Ocean, ACE-1.)
Mineral Dust. A significant fraction of the atmospheric par-
ticulate burden consists of mineral dust; its injection rate into the
atmosphere can vary temporally and spatially, e.g., as a result of
dust storms, volcanic eruptions, and anthropogenic activities. The
mineral-dust burden tends to be especially high near source
regions. Important examples include semiarid and arid lands,
areas where land use is changing, and, in general, in the tropical
and subtropical belts (72, 73). Moreover, changes in climate as
well as land use can profoundly affect the amount of mineral dust
that enters the troposphere. Drought and increased desertifica-
tion by human activities can dramatically increase the dust
available for deflation, and the tropospheric dust burden will
increase appreciably. The relatively short atmospheric lifetime of
much such dust means its radiative forcing adjusts relatively
rapidly to changes in emissions.
Mineral aerosols have a dual forcing role, producing both
warming and cooling effects. The details are not well known
because the scattering characteristics of mineral aerosols are hard
to determine (74~. Duce (75) estimates a direct forcing of -0.75
Wm-2, which is roughly equivalent to that from sulfate from
biogenic gases (-0.68 Wm-2). It has been suggested that the
direct radiative forcing of mineral aerosols approaches that of
anthropogenic sulfates (73) and that in the tropical and subtrop-
ical North Atlantic region mineral dust is the dominant light-
scattering aerosol (72). Tegen et al. (76, 77) estimate that dis-
turbed soils contribute roughly half of the total atmospheric dust,
and that the negative shortwave forcing and the positive IR
forcing approximately cancel at the top of the atmosphere, but
that internally the energy is redistributed, leading to climate
change. Because of the lack of sufficient data, all models make
many assumptions about the optical and radiative properties of
mineral dust and are subject to revision as improved data are
acquired.
OCR for page 3378
3378 Colloquium Paper: Buseck and Posfa,
b
~ :~ ~ :~ ~ . :~ ~ ~ ~
200nm ~ ~
hi. :_
:~::
:: ~ ~_
f.i ~. ::
~Q
500 rim
NaCI
_.
_'
A
-
1_`
. W~
~ silicate
_
~:~:: : 1 Am ~
-,~_:~:
~: _,
~ - ~ ~.~;.~.~.~ I, ~,' -
FIG. 7. TEM unages of mineral dust collected from the marine
troposphere. (a) Internal mixture of presumably terrestrial silicate and
anhydnte with sea salt (Azores, North Atlantic, ASTEX/MAGE); (b)
smectite (clay) and quartz (Q). The small grain size of the clay is visible
at the thin edge (the arrows mark hexagonal platelets). Selected-area
electron-diffraction patterns of clay and quartz are at the upper left and
lower right, respectively. (Canary Islands, North Atlantic, ACE-2; (c)
TEM unage of goethite, FeO(OH), collected 2,600 m above sea level.
Fe-beanug minerals like this could be important nutrient sources In
remote oceans. (Canary Islands, North Atlantic, ACE-2.)
Anthropogenically generated dust such as arises from trans-
portation and industry can result in massive injections of minerals
into the atmosphere. Between 30 and 50% of the soil dust burden
may result from human activities (78~. The resulting climatic
effect can equal or even exceed that from aerosols generated by
burning of fossil fuels and locally can be comparable to that from
clouds (79~.
Mineral-dust particles provide important reactive surfaces for
atmospheric reactions. Mineral aerosols significantly affect the
cycles of N. S. and atmospheric oxidants (80~. An annual average
of 40% (locally reaching 70%) of total sulfate is associated with
mineral dust over Asia, the western United States, Australia, and
North Africa, and large parts of the oceans have over 10~o of the
sulfate associated with mineral aerosol (81~.
The association of sulfates with the larger mineral particles
means that the local cooling effects of the sulfate aerosol are
diminished because of a decrease in the incremental mass scat-
tering efficiency of the sulfate. The mineral particles, being partly
coated with hydroscopic sulfates- sometimes as the result of
cloud processing can also become CON. Such processing of
mineral-dust particles over the eastern Mediterranean has con-
verted them into giant CON that influence precipitation and the
concentration of ice crystals in convective clouds (82~.
Our TEM studies of individual mineral-dust particles aim at
identifying major mineral species in the aerosol and determining
whether they are aggregated with other aerosol species. Such
information is useful for obtaining improved refractive indices
and size and shape data for model calculations of the radiative
Proc. Natl. Acad. Sci. USA 96 (1999J
forcing of dust. The observation of internal or external mixtures
is important for determining which chemical reactions take place
on mineral surfaces and how the original particles change during
atmospheric transport.
The compositions and aggregations of the mineral aerosols
collected when a polluted air mass of European origin arrived
at our sampling site near the Azores Islands reflect their
transport over the ocean. An unidentified silicate is associated
with euhedral anhydrite, CaSO4, and sea salt (Fig. 7a). Smec-
tite (Fig. 7b) is the most common mineral, and anhydrite is also
widespread (42~. Because the aerosol was transported in an air
mass that contains high concentrations of SO2 from European
pollution sources (83), it is likely that original carbonate
particles reacted with SO2 to form anhydrite. Such conversions
were observed in Asian (84) and Saharan (85) dust plumes.
Under high-dust conditions, mineral particles provide an
important pathway for SO2 removal from the atmosphere (81~.
Internal mixtures of sea salt, sulfate, and mineral particles were
also observed by Andreae et al. (86), who concluded that such
aggregates likely formed in clouds.
An interesting consequence of the transport of mineral aerosol
particles to the oceans is that they can serve as sources of
biological productivity. Large areas of the ocean, ranging from
the tropical equatorial Pacific to the polar Antarctic and the
Southern Ocean, contain fewer phytoplankton/zooplankton than
expected from the abundance of nutrients in the sea water. A
correlation exists between a relative lack of Fe and this under-
productivity (87, 884. Iron in the oceans far from land is mainly
provided by continental dust (89-91), which can be transported
across large oceanic expanses (92-944. We have identified goe-
thite (Fig. 7c) and other Fe minerals from the aerosol In the FT.
It appears as if atmospheric transport of mineral dust is wide-
spread and can have major effects on life at the bottom of the food
chain In the large areas of the oceans that are far from the
nutrients provided by river flow from the continents. Nutrients
derived from mineral dust may be [uniting factors on primary
productivity.
We thank Dr. J. R. Anderson for help with sample collection, general
assistance, and many useful discussions. Helpful reviews and comments
were provided by Drs. J. R. Anderson, M. O. Andreae, J. M. Prospero,
and S. E. Schwartz. This research was funded through grants from the
National Science Foundation. Electron microscopy was performed by
using instruments of the Arizona State University Facility for High
Resolution Electron Microscopy.
1. Farquhar, G. D. (1997) Science 278, 1411.
2. Mahlman, J. D. (1997) Science 278, 1416-1417.
3. Molina, M. J., Molina, L. T. & Kolb, C. E. (1996) Annul Rev. Phys.
Chem. 47, 327-367.
4. Charlson, R. J. & Heintzenberg, J., eds. (1995) Aerosol Forcing of
Climate (Wiley, New York).
5. Houghton, J. T., Meiro Filho, L. G., Callander, B. A., Harris, N.,
Kattenberg, A. & Maskell, K., eds. (1996) Climate Change 1995: The
Science of Climate Change, Contribution of Working Group I to the Second
Assessment Report of the Intergovernmental Panel on Climate Change
(IPCC) (Cambridge Univ. Press, Cambridge, U. K).
6. National Research Council, Panel on Aerosol Radiative Forcing and
Climate Change (1996) A Plan for a Research Program on Aerosol
Radiative Forcing and Climate Change (Natl. Acad. Press, Washington,
DC).
7. Schwartz, S. E. & Andreae, M. O. (1996) Science 272, 1121-1122.
8. Michaels, R. A. (1997) Science 278, 1696.
9. Andreae, M. O. (1995) in Future Climates of the World: A Modeling
Perspective, World Survey of Climatology, ed. Henderson-Sellers, A.
(Elsevier, Amsterdam), Vol. 16, pp. 341-392.
10. Weber, R. J., Marti, J. J., McMurry, P. H., Eisele, F. L., Tanner, D. J. &
Jefferson, A. (1997) J. Geophys. Res. 102, 4375-4385.
11. Hegg, D., Larson, T. & Yuen, P. F. (1993) J. Geophys. Res. 98,
18435-18439.
12. Schwartz, S. E. (1996) J. Aerosol Sci. 3, 359-382.
13. Ackerman, T. P. & Toon, O. B. (1981) Appl. Opt. 20, 3661-3668.
14. Chylek, P., Videen, G., Ngo, D., Pinnick, R. G. & Klett, J. D. (1995)
J. Geophys. Res. 100, 16325-16332.
15. Pilinis, C., Pandis, S. N. & Seinfeld, J. H. (1995) J. Geophys. Res. 100,
18739-18754.
OCR for page 3379
Colloquium Paper: Buseck and Posfai
16. Tang, I. (1997) J. Geophys. Res. 102, 1883-1893.
17. Haywood, J. M. & Shine, K. P. (1995) Geophys. Res. Lett. 22, 603-606.
18. Haywood, J. M., Roberts, D. L., Slingo, A., Edwards, J. M. & Shine,
K. P. (1997) J. Clim. 10, 1562-1577.
19. Abelson, P. H. (1998) Science 281, 1609.
20. National Research Council, Committee on Research Priorities for
Airborne Particulate Matter (1998) Research Priorities for Airborne
Particulate Matter: I. Immediate Priorities and a Long-Range Research
Portfolio (Natl. Acad. Press, Washington, DC).
21. Pszenny A., Keene, W., O'Dowd, C., Smith, M. & Quinn, P. (1998)
Intl. Global Atmospheric Chem. (IGAC) IGACtivities NewsLetter 11,
6-12.
22. Ogren, J. A. (1995) inAerosol Forcing of Climate, eds. Charlson, R. J.
& Heintzenberg, J. (Wiley, New York), pp. 215-226.
23. Lacis, A. A. & Mishchenko, M. I. (1995) inAerosol Forcing of Climate,
eds. Charlson, R. J. & Heintzenberg, J. (Wiley, New York), pp. 11-42.
24. Haywood, J. M. & Ramaswamy, V. (1998) J. Geophys. Res. 103,
6043-6058.
25. Andreae, M. O. (1996) Nature (London) 380, 389-390.
26. Kaufman, Y. J. & Fraser, R. S. (1997) Science 277, 1636-1639.
27. Twomey, S. (1977a) Atmospheric Aerosols (Elsevier, New York).
28. Twomey, S. (1977b) J. Atmos. Sci. 34, 1149-1152.
29. Baker, M. B. (1997) Science 276, 1072-1078.
30. Germani, M. S. & Buseck, P. R. (1991) Anal. Chem. 63, 2232-2237.
31. Anderson, J. R., Buseck, P. R. & Saucy, D. A. (1992)Atmos. Environ.
26A, 1747-1762.
32. De Bock, L. A., Van Malderen, H. & Van Grieken, R. E. (1994)
Environ. Sci. Technol. 28,1513-1520.
33. Katrinak, K. A., Anderson, J. R. & Buseck, P. R. (1995) Environ. Sci.
Technol. 29, 321-329.
34. Buseck, P. R. & Anderson, J. R. (1998) in advanced Mineralogy 3, ed.
Marfunin, A. S. (Springer, Berlin), pp. 292-300.
35. Anderson, J. R. & Buseck, P. R. (1998) inAdvanced Mineralogy 3, ed.
Marfunin, A. S. (Springer, Berlin), pp. 301-312.
36. Sheridan, P. (1989) Atmos. Environ. 23, 2371-2386.
37. Sheridan, P. J., Schnell, R. C., Kahl, J. D., Boatman, J. F. & Garvey,
D. M. (1993) Atmos. Environ. 27A, 1169-1183.
38. Parungo, F., Kopcewicz, B., Nagamoto, C., Schnell, R., Sheridan, P.,
Zhu, C. & Harris7 J. (1992) J. Geophys. Res. 97, 15867-15882.
39. McInnes, L. M., Covert, D. S., Quinn, P. K. & Germani, M. S. (1994)
J. Geophys. Res. 99, 8257-8268.
40. Katrinak, K. A., Rez, P. & Buseck, P. R. (1992) Environ. Sci. Technol.
26, 1967-1976.
41. Posfai, M., Anderson, J. R., Buseck, P. R., Shattuck, T. W. & Tindale,
N. W. (1994) Atmos. Environ. 28, 1747-1756.
42. Posfai, M., Anderson, J. R., Buseck, P. R. & Sievering, H. (1995) J.
Geophys. Res. 100, 23063-23074.
43. Posfai, M., Xu, H., Anderson, J. R. & Buseck, P. R. (1998a) Geophys.
Res. Lett. 25,1907-1910.
44. Posfai, M., Anderson, J. R., Buseck, P. R. & Sievering, H. (1998b) J.
Geophys. Res. 103, in review.
45. Buseck, P. R. (1992) in Minerals and Reactions at the Atomic Scale:
Transmission Electron Microscopy, ed. Buseck, P. R. (Min. Soc. Am.,
Washington, DC), pp. 1-36.
46. Prather, K. A., Nordmeyer, T. & Salt, K. (1994) Anal. Chem. 66,
1403-1407.
47. Gard, E. E., Kleeman, M. J., Gross, D. S., Hughes, L. S., Allen, J. O.,
Morrical, B. D., Fergenson, D. P., Dienes, T., Galli, M. E., Johnson,
R. J., et al. (1998) Science 279, 1184-1187.
48. Murphy, D. M., Anderson, J. R., Quinn, P. K., McInnes, L. M., Brechtel,
F. J., Kreidenweis, S. M., Middlebrook, A. M., Posfai, M., Thomson, D. S.
& Buseck, P. R. (1998) Nature (London) 392, 62-65.
49. Bates, T. S., Huebert, B. J., Gras, J. L., Griffiths, F. B. & Durkee, P. A.
(1998) J. Geophys. Res. 103, 16297-16318.
50. Huebert, B. J., Pszenny, A. & Blomquist, B. (1996) J. Geophys. Res.
101, 4319-4329.
51. Charlson, R. J., Langner, J., Rodhe, H., Leovy, C. B. & Warren, S. G.
(1991) Tellus 43AB, 152-163.
52. Rivera-Carpio, C. A., Corrigan, C. E., Novakov, T., Penner, J. E.,
Rogers, C. F. & Chow, J. C. (1996) J. Geophys. Res. 101, 19483-19493.
53. Charlson, R. J., Schwartz, S. E., Hales, J. M., Cess, R. D., Coakley,
J. A., Jr., Hansen, J. E. & Hofmann, D. J. (1992) Science 255, 423-430.
54. Kiehl, J. T. & Briegleb, B. P. (1993) Science 260, 311-314.
55. Taylor, K. E. & Penner, J. E. (1994) Nature (London) 369, 734-737.
Proc. Natl. Acad. Sci. USA 96 (1999J 3379
58.
59.
75.
78.
79.
80.
81.
90.
56. Hegg, D. A., Livingston, J., Hobbs, P. V., Novakov, T. & Russell, P.
(1997) J. Geophys. Res. 102, 25293-25303.
57. Hobbs, P. V. (1998) Intl. Global Atmospheric Chem. (IGAC) IG~C
tivities NewsLetter 11, 3-5.
Novakov, T. & Penner, J. E. (1993) Nature (London) 365, 823-826.
McInnes, L. M., Bergin, M. H., Ogren, J. A. & Schwartz, S. E. (1998)
Geophys. Res. Lett. 25, 513-516.
60. Okada, K. (1985) Atmos. Environ. 19, 743-757.
61. Meszaros, A. & Meszaros, E. (1989) Aerosol Sci. Tech. 10, 337-342.
62. Parungo, F., Nagamoto, C., Zhou, M.-Y., Hansen, A. D. A. & Harris,
J. (1994) Atmos. Environ. 28, 3251-3260.
63. Andreae, M. O. (1983) Science 220, 1148-1151.
64. Cachier, H., Ducret, J., Bremond, M.-P., Yoboue, V., Lacaux, J.-P.,
Gaudichet, A. & Baudet, J. (1991) in Global Biomass Buming, ed. Levine,
J. S. (MIT Press, Cambridge, MA), pp. 174-180.
65. Artaxo, A., Gerab, F., Yamasoe, M. A. & Martins, J. V. (1994) J.
Geophys. Res. 99, 22857-22868.
66. Turn, S. Q., Jenkins, B. M., Chow, J. C., Pritchett, L. C., Campbell,
D., Cahill, T. & Whalen, S. A. (1997) J. Geophys. Res. 102, 3683-3699.
67. Gill, P. S., Graedel, T. E. & Weschler, C. J. (1983) Rev. Geophys. Space
Phys. 21, 903-920.
68. Andrews, E. & Larson, S. M. (1993) Environ. Sci. Technol. 27,
857-865.
69. Saxena, P., Hildemann, L. M., McMurry, P. H. & Seinfeld, J. H. (1995)
J. Geophys. Res. 100,18755-18770.
70. O'Dowd, C. D., Smith, M. H., Consterdine, I. E. & Lowe, J. A. (1997)
Atmos. Environ. 31, 73-80.
71. Anderson, J. R., Buseck, P. R., Patterson, T. L. & Arimoto, R. (1996)
Atmos. Environ. 30, 319-338.
72. Li, X., Maring, H., Savoie, D., Voss, K. & Prospero, J. M. (1996)
Nature (London) 380, 416-419.
73. Bergametti, G. & Dulac, F. (1998) Intl. Global Atmospheric Chem.
(IGAC) IGACtivities NewsLetter 11, 13-17.
74. Penner, J. E., Charlson, R. J., Hales, J. M., Laulainen, N. S., Leifer,
R., Novakov, T., Ogren, J., Radke, L. F., Schwartz, S. E. & Travis, L.
(1994) Bull. Am. Meteorol. Soc. 75, 375-400.
Duce, R. A. (1995) in aerosol Forcing of Climate, eds. Charlson, R. J.
& Heintzenberg, J. (Wiley, New York), pp. 43-72.
76. Tegen, I., Lacis, A. A. & Fung, I. (1996) Nature (London) 380,
419-422.
77. Tegen, I., Hollrig, P., Chin, M., Fung, I., Jacob, D. & Penner, J. (1997)
J. Geophys. Res. 102, 23895-23915.
Tegen, I. & Fung, I. (1995) J. Geophys. Res. 100, 18707-18726.
Sokolik, I. N. & Toon, O. B. (1996) Nature (London) 381, 681-683.
Andreae, M. O. & Crutzen, P. J. (1997) Science 276, 1052-1058.
Dentener, F. J., Carmichael, G. R., Zang, Y., I elieveld, J. & Crutzen,
P. J. (1996) J. Geophys. Res. 101, 22869-22889.
82. Levin, Z., Ganor, E. & Gladstein, V. (1996) J. Appl. Meteorol. 35,
1511-1523.
83. Sievering, H., Gorman, E., Ley, T., Pszenny, A., Springer-Young, M.,
Boatman, J., Kim, Y., Nagamoto, C. & Wellman, D. (l995)J. Geophys.
Res. 100, 23075-23081.
84. Winchester, J. W. & Wang, M.-X. (1989) Tellus 41B, 323-337.
85. Glaccum, R. A. & Prospero, J. M. (1980) Marine Geol. 37, 295-321.
86. Andreae, M. O., Charlson, R. J., Bruynseels, F., Storms, H., van
Grieken, R. E. & Maenhaut, W. (1986) Science 232, 1620-1623.
87. Duce, R. A. (1986) in The Role of Air-Sea Exchange in Geochemical
Cycling, ed. Buat-Menard, P. (D. Reidel Pub. Co., Dordrecht), pp.
497-529.
88. Martin, J. H. & Gordon, R. M. (1988) Deep-Sea Res. 35, 177-196.
89. Duce, R. A. & Tindale, N. W. (1991) Limnol. Oceanogr. 36, 1715
1726.
Martin, J. H., Gordon, R. M. & Fitzwater, S. E. (1991) Limnol.
Oceanogr. 36, 1793-1802.
91. Martin, J. H., Coale, K. H., Johnson, K. S., Fitzwater, S. E., Gordon,
R. M., Tanner, S. J., Hunter, C. N., Elrod, V. A., Nowicki, J. L., Coley,
T. L., et al. (1994) Nature (London) 371, 123-129.
92. Arimoto, R., Duce, R. A., Savoie, D. L. & Prospero, J. M. (1992) J.
Atmos. Chem. 14, 439-457.
93. Prospero, J. M., Uematsu, M. D. & Savoie, L. (1989) in Chemical
Oceanography, eds. Riley, J. P. & Chester, R. (Academic, New York),
Vol. 10, pp. 187-218.
94. Prospero, J. M. (1999) Proc. Natl. Acad. Sci. USA 96, 3396-3403.
95. Katrinak, K. A., Rez, P., Perkes, P. R. & Buseck, P. R. (1993) Environ.
Sci. Technol. 27, 539-547.
Representative terms from entire chapter:
mineral dust