Journal of Geophysical Research, Vol.102, NO. D14, 16889-16909, 1997
Characterization of troposphereic
aerosols over the oceans with the AVHRR
advanced high resolution radiometer
optical thickness product
Rudolf B. Husar
Center for Air Pollution Impact and Trend Analysis
Washington University
St. Louis, MO 63130-4899
Joseph M. Prospero
University of Miami, RSMAS
Miami, FL 33149
Larry L. Stowe
NOAA/NESDIS
World Weather Building
Washington, DC 20233
Abstract
1.0 Introduction
2.0 The NOAA AVHRR Aerosol Product
3.0 The AVHRR Aerosol Record: July 1989 - June 1991
3.1 General Features of Aerosol Distribution Patterns
3.2 Atlantic Ocean
3.3 Indian Ocean, Arabian Sea and the Bay of Bengal
3.4 Pacific Ocean
4.0 Conclusions
5.0 References
Abstract. The National Oceanic and Atmospheric Administration (NOAA) Advanced
Very High-Resolution Radiometer (AVHRR) is a polar orbiting satellite that provides
information on aerosol distributions based on backscatter radiation measurements that
yield a measure of the "radiatively equivalent" aerosol optical thickness (EAOT) over the
oceans. Seasonally-composited EAOT data for the period July 1989 to June 1991 reveal
many spatially-coherent plume-like patterns that can usually be interpreted in terms of
known (or reasonably-hypothesized) sources in association with climatological wind
fields. The largest and most persistent areas of high EAOT values are associated with
wind-blown dust and biomass burning sources; especially prominent are sources in
Africa, the middle East and the Asian sub-continent. Prominent plumes over the midlatitude North Atlantic are attributed to pollution emissions from North America and
Europe. Large plumes attributed to pollution aerosols and dust from sources in Asia are
clearly visible over the western and central North Pacific. On a global scale, the annually
averaged northern hemisphere EAOT values are about 1.7 times greater than those in the
southern hemisphere. Considering each hemisphere separately, EAOT values in the
summer are about twice those in the winter; within the mid latitude band 30o-60o (i.e.,
where anthropogenic emissions are greatest), the summer/winter ratio is about three. The
temporal variability of monthly mean EAOT in specific ocean regions often shows
characteristic seasonal patterns that are usually consistent with aerosol measurements
made in the marine boundary layer. Nonetheless, there are many features in the EAOT
distributions that can not be readily interpreted at this time. The AVHRR EAOT
distributions demonstrate that satellite products can serve as a useful tool for the planning
and implementation of focused aerosol research programs and that they will be especially
important in studies of climate-related processes.
1.0 Introduction
Tropospheric aerosols play an important biogeochemical role in many earth processes;
they can also affect human health, visibility and climate. Aerosols can impact climate by
radiative processes, directly through the scatter and absorption of solar radiation and
indirectly through cloud effects. Recent modeling of direct effects indicates that
anthropogenic sulfate could scatter a substantial amount or radiation back to space
[Charlson et al., 1992] thereby having a cooling effect on climate which appears to be
substantiated by observed temperature trends [Karl et al., 1995]. Until recently, research
has primarily focused on anthropogenic sulfate aerosols; however, other types of aerosols
could be important, among them soil dust and the products of biomass burning [Penner et
al., 1994; Penner, 1995; Andreae, 1995; Duce, 1995; Prospero, 1996a,b]. At present the
estimates of direct aerosol forcing are highly uncertain because of many unknown factors
about aerosol properties [Kiehl and Briegleb, 1993; Penner et al., 1994; Andreae, 1995;
Karl et al., 1995; Lacis et al., 1995; Taylor and Penner, 1994] and their temporal and
spatial variability over the earth, especially over the oceans where so little data is
available. The concentration, composition and physical properties of particles in the
marine atmosphere can vary greatly, depending on the distribution of sources, the
controlling meteorological processes in the source regions, the large-scale wind systems
involved in transport and, finally, the removal processes that act on the particles and
deposit them in the ocean or carry them to other land masses. Consequently it is difficult
to characterize aerosols solely on the basis of sporadic in situ measurements.
Satellites can be used to map aerosol distributions because aerosol particles, when
illuminated by the sun, scatter a fraction of the solar radiation back to space. At the
simplest level, aerosol transport events can be detected by visible inspection of imagery
[e.g., Ferrare et al., 1990]; aerosol patches appear as diffuse gray areas which are clearly
different from cloud which has a brighter and more textured appearance. The quantitative
interpretation of satellite-sensed aerosol backscatter in terms of aerosol optical thickness
(AOT) requires an accurate knowledge of a number of factors. Especially important are
the aerosol angular scattering (phase) function, which is strongly dependent on the
aerosol size distribution and chemical composition, and the albedo of the underlying
surface. Retrievals are especially difficult over land surfaces which have complex and
variable radiative properties. The process is simplified over the oceans because the ocean
surface has a relatively low and constant albedo [Griggs, 1983; Durkee et. al., 1986,
1991]. Satellite-sensed AOT measurements over the ocean are generally in reasonable
agreement with concurrent surface-based measurements of AOT as measured with
upward-looking radiation instruments such as sun-photometers [Durkee et al., 1991;
Villevalde et al.,1994; Ignatov, et al., 1995]. In this paper we present AOT values over
the ocean.
2.0 The NOAA AVHRR Aerosol Product
The NOAA National Environmental Satellite Data and Information Service (NESDIS)
has an operational program that routinely derives and archives estimates of AOT for the
global ocean using polar-orbiting meteorological satellites. A detailed description of the
history of the AVHRR program, the evolution of the aerosol retrieval algorithm and the
analysis procedure is found in Stowe et al. (this volume). In this paper we present only a
brief overview of the procedures used for the data presented herein.
The AOT is estimated from backscatter radiation measurements made at an effective
wavelength of 0.63 m. The NOAA/11 AVHRR aerosol retrieval algorithm is based on a
simple Junge aerosol size distribution (size parameter, =3.5) and a real index of refraction
of 1.5. The algorithm corrects for Rayleigh scattering by atmospheric gases, for ozone
absorption using global mean climatological ozone distributions, and for surface
reflectance by assuming a constant global Lambertian ocean reflectance of 1.5%. The
AOT distributions are presented as 1ox1o composites of the retreived data in 2x2 arrays of
4-km global area coverage pixels over the oceans. AVHRR data are available through the
National Climatic Data Center (Asheville, NC).
To avoid solar specular reflection from the ocean surface, measurements are made only
on the side of the orbit away from the sun, excluding data within a 40o half angle interval
on the specular ray. To minimize the possibility of errors over coastal waters where water
color and reflectance may be variable (because of sediment load from rivers, marine
organisms, pigments, etc.) data from coastal waters are rejected. In heavy seas, the bright
foam from whitecaps could cause spurious backscatter that could be interpreted as
aerosol. During the data processing, this possible source of interference was not
considered. However the AVHRR product (Fig. 1) does not show substantially increased
EAOT values in the winter hemisphere where winds and whitecap coverage [Erickson et
al., 1986] are expected to be highest; this suggests that whitecaps do not present a major
problem.
Cloud screening procedures are based on the fact that the cloud reflectance is high and
relatively constant across much of the visible and infra-red spectrum; in contrast, aerosol
backscattering is much stronger in the visible than in the near and far infrared [McClain,
1989]. Pixels containing clouds are identified and removed by comparing the observed
signal in the near-IR with that expected in the absence of clouds. The resulting estimated
aerosol pattern may be spatially and temporally biased according to the fraction of the
data that is retrieved for a specific region, a factor that is largely determined by cloudcover statistics, by the equator crossing time of the satellite, and by not scanning the solar
side of each orbit.
Validation tests [Ignatov et al., 1995] show good agreement between the AVHRR EAOT
measurements and ground based AOT measurements. The good agreement is somewhat
surprising, given the relatively simple algorithm that is used for the retrievals and the
complex mixture of pollutant and natural aerosols that is found over many ocean regions.
Pollutant aerosols and biomass burning products lie mostly in the submicron diameter
size range. Mie scattering calculations (R. Husar, unpublished data) show that, for
aerosols having a narrow size distribution at the low end of the submicron size range, the
phase function is nearly symmetrical with strong forward and backward lobes. In
contrast, for mineral dust and sea-salt, aerosols which have mass median diameters of
several microns or greater [Duce, 1995; Savoie and Prospero, 1982], the phase function
has a dominant forward scattering peak. Nonetheless, Mie calculation show that for
"reasonable" size distributions (that is, a relatively broad size distribution comparable to
that found in ambient aerosols) the backscatter fraction in the scattering angle range
viewed by AVHRR is relatively insensitive to aerosol size over the range from 0.2-3.0 m
diameter [R. Husar, unpublished data].
The AVHRR retreival algorithm has subsequently undergone a number of changes that
will substantially improve the accuracy and precision or the resulting AOT values and
may permit the retreival of additional aerosol parameters (see Stowe et al., this volume).
3.0 The AVHRR Aerosol Record: July 1989 - June 1991
This work presents a summary of the global oceanic aerosol distribution patterns as
detected by the NOAA/11 polar-orbiting satellite between July 1989 and June 1991. The
aerosol data after June, 1991 were greatly influenced by the Mt. Pinatubo volcanic
eruption. During the pre-Pinatubo period the stratosphere was unusually clean [Dutton et
al., 1994] and the backscattering signal was dominated by the tropospheric aerosol
[Stowe et al., 1992; Long and Stowe, 1994; Stowe et al., this volume]. The results are
presented in four seasonal (quarterly) maps (Fig. 1); in subsequent discussions we refer to
the individual maps by the initials of the seasonal months (i.e., June-July-August as JJA).
Figure 1 (a-d) Radiatively equivalent aerosol optical thickness (EAOT x 1000) over
the oceans derived from NOAA AVHRR satellites for the four seasons. The figure
incorporates data for the period July 1989 to June 1991.
The seasonal EAOT data displayed in Figure 1 are available in electronic format by
downloading ppsfeaot.zip in latitude, longitude grids in hdf files.
3.1 General Features of Aerosol Distribution Patterns
The EAOT distributions in Fig. 1 clearly show that the most prominent areas of increased
EAOT are associated with continental sources. In many regions the continents are fringed
by areas of high EAOT. In some regions the continents appear, in effect, to emit long
"plumes" of increased EAOT. The continental aerosol plumes are generally characterized
by high values of EAOT near the coasts, the values declining with distance from the
coast. This aerosol pattern is consistent with the transport of aerosols from continental
sources by large scale wind systems, followed by atmospheric dispersion and removal in
the downwind direction. In well-defined flow fields such as the trade winds (e.g., the
tropical NAO), the continental plumes are markedly elongated and can extend over
several thousand kilometers.
A second type of EAOT distribution consists of isolated patches that do not appear to be
linked to continental sources. These distributions show much weaker spatial gradients
which would be consistent with more diffuse large-scale sources (e.g., oceanic
emissions). An example is the belt of slightly increased EAOT values that extends over
much of the southern oceans at 30o-60oS.
On the basis of the temporal and spatial distribution of EAOT in Fig. 1 we can identify
ocean regions where the seasonal variability of EAOT displays a characteristic pattern;
these regions are delineated in Fig. 2., using a rectangular geometry for computational
convenience. The monthly averages and annual average EAOT for each region are
presented in Table 1. The monthly average EAOT in each of these regions is shown in a
series of graphs in Fig. 3a-p.
Figure 2 also shows the location of various types of potential aerosol sources on the
continents; these provide a qualitative picture of the geographic distributions of emissions
that might be important sources for the EAOT plumes depicted in Fig. 1. Most
anthropogenic emissions are located in the northern hemisphere (NH) and they are
concentrated in the mid-latitudes [Hameed and Dignon, 1992; Benkovitz et al., 1996;
Husar and Husar, 1990], especially eastern North America, Europe, and eastern Asia.
Figure 2 shows the distribution of sulfur sources which, in this context, is also broadly
representative of other types of anthropogenic aerosol emissions and their precursors
(e.g., organic species, nitrates, black carbon, etc.). Arid regions and deserts are potential
sources of wind blown dust [Pye, 1987, 1989]. In Fig.2 we show the location of sand
desert regions [Olson, 1986]. However, it should be noted that sand deserts are not
necessarily good or exclusive sources of wind-blown dust, a point that will be
emphasized in this paper.
Biomass burning is an important source of aerosols, especially black carbon [see Levine,
1991]. Emissions from the burning of savannas comprise about 50% of all tropical
biomass burning sources [Hao and Liu, 1994]. To reflect the distribution of this type of
source we show the distribution of tropical savanna [Olson, 1986]. The map also shows
the locations where biomass burning was reported by NASA astronauts during space
flights [Andreae, 1993].
Oceanic sources of atmospheric aerosols are also of interest. In the absence of pollutant
aerosols, the dominant sub-micron aerosol component over the oceans is believed to be
nss-SO4= that results from the oxidation of biologically-produced dimethylsulfide (DMS)
emitted from the oceans [Shaw, 1987; Charlson et al., 1987; Covert et al., 1992].
Unfortunately, there are no satisfactory proxy indicators for oceanic sulfur emission
patterns. For example, oceanic emissions of DMS do not appear to be related to
chlorophyll distributions in the ocean [Bates et al., 1993; Covert et al., 1992] which can
be measured by satellite.
Figure 1 shows that on a global scale the highest EAOT values occur during the summer
season, and the lowest during the winter. The seasonal variability is more pronounced at
mid latitudes than at low latitudes. The hemispheric seasonality is depicted in Figure 3a
(see also Table 1) which shows the average optical depth for the NH, the SH, and the
globe. (Note, however, that the hemispheric values do not include the polar regions which
are not covered by the satellite data.) The global average shows only a small annual
variability (0.10 < <0.14); individually, the NH and NH show a seasonal amplitude of
about a factor of two. The NH average peaks at =0.20 in April-June, and the lowest
values (=0.10) are in November and December. The SH has the highest monthly average
in January (=0.13) and lowest in May (=0.07). The seasonal patterns in the two
hemispheres are not quite symmetric: the NH peaks in late spring and early summer,
while the SH is highest in mid summer.
The impact of industrial anthropogenic aerosol sources is most evident in the mid-latitude
aerosol distributions between 30-60 in the NH and SH (Fig. 3p); to facilitate comparison,
the plot of the SH monthly averages is shifted by six months to correspond seasonally to
the NH. In the NH mid-latitudes the annual average oceanic EAOT (=0.12) is 1.5 times
that of the SH (=0.08). It is interesting to note, however, that much of the increased
EAOT in the NH occurs during the spring months (April-June) and it is attributable to
aerosols over the North Pacific Ocean (NPO), as evidenced by the very large plume in
Fig. 1/MAM.
In the following sections, we discuss the EAOT distributions in terms of our current
knowledge of aerosols over the oceans. We compare the temporal and spatial variability
of EAOT with aerosol chemical measurements obtained from various ocean regions.
Because we are interested on seasonal time-scale variability, we refer principally to data
sets that were acquired over relatively long time periods; this means that we must rely
mainly on measurements made at the surface in the marine boundary layer. Satellite
estimates of AOT integrate over the thickness of the atmospheric column whereas the
aerosol measurements, at best, approximate the concentrations in the boundary layer.
Nonetheless, over the time scale of the seasons, we might expect to see some
correspondence between the ground-based and satellite data sets; indeed, one objective of
this exercise is to see how well they agree.
We focus our discussions of aerosol data on those constituents that are most likely to
have the greatest impact on the radiative properties of the atmosphere as viewed by
AVHRR: non-sea-salt (nss) sulfate, mineral dust and biomass burning products. We do
not include nitrate because previous studies have shown that over the ocean, most of the
nitrate mass is associated with sea-salt aerosol in the supra-micron size range [Savoie and
Prospero, 1982] where it constitutes only a small fraction of the aerosol mass. Thus the
radiative effects of nitrate will be relatively small compared to other constituents [Li et
al., 1996]. Similarly, ammonium, an important aerosol constituent, is concentrated in the
submicron size range in association with nss-sulfate; as such it only constitutes a small
fraction of the aerosol mass in the sub-micron fraction. We do not discuss sea-salt
aerosols because they do not appear to have a sufficiently strong impact on atmospheric
radiative processes to be readily detectable by AVHRR; note, for example the absence of
any regions with substantially increased EAOT values duirng winter in either hemisphere
in Fig 1.
In discussing the distribution and properties of aerosols in the context of the AVHRR
data, we do not attempt to make a comprehensive review of all relevant aerosol literature;
instead we only cite as examples those containing measurements made during the time of
the AVHRR data shown in Fig. 1 or literature that contains relatively comprehensive or
unique coverage.
In our assessment of the AVHRR product, we make extensive reference to Nimbus7/TOMS data (Herman et al., this volume). On the basis of the relative radiance
measured at 340nm and 380nm, Herman et al. obtain information on the global
distribution of absorbing aerosols, largely mineral dust and black carbon derived from
biomass burning (and, to a lesser extent, coal combustion). Because TOMS is not
sensitive to sulfate aerosols and other pollution products, it does not "see", for example,
the large pollution plumes in the mid-latitude North Atlantic Ocean (NAO). Thus, by
comparing the AVHRR product with that from TOMS, we can distinguish among the
various types of sources.
Before discussing the aerosol data, it is important to note that cloud contamination (that
is, the misinterpretation of cloud as aerosol) does not appear to be a major problem. If the
cloud screening algorithm introduced errors, we would expect to see, for example, high
rates of anomalous EAOT from the regions bordering the intertropical convergence zone
(ITCZ) and symmetrical with it. As can be seen in Fig. 1, aerosol plumes are often seen
to the north of the climatological position of the ITCZ, but not to the south. This suggests
that, in general, the EAOT "plumes" are not cloud-contamination artifacts. Clouds can
bias the EAOT results presented here to the extent that the aerosol concentrations and
properties during cloud free conditions are different from those during cloudy conditions
and to the extent that the clouds reduce the data coverage from any specific region.
Nonetheless, from the standpoint of direct radiative forcing, it is the aerosol distributions
shown here, obtained under clear-air (i.e., cloud free) conditions, that are most relevant.
3.2 Atlantic Ocean
The quarterly composites (Fig. 1) clearly show that EAOT values for the NAO and the
low-latitude SAO are relatively high during much of the year. In all cases the EAOT
spatial distributions have a well-defined plume-like character that suggests that the
aerosols are derived from continental sources. As shown in Fig. 2, there is a high density
of pollutant sources in North America and Europe which together account for about half
of the global total of anthropogenic sulfur and nitrogen emissions [Levy and Moxim
1989; Hameed and Dignon 1992]. Also, large quantities of soil dust are transported out of
North Africa all year long, affecting much of the tropical NAO and Caribbean. Biomass
burning is carried out extensively in Africa and South America (see Fig. 2) and
combustion products are carried over large areas of the NAO and SAO in the lowlatitudes. In the following sections we discuss the impacts of these sources on various
regions in the Atlantic.
Tropical Atlantic Ocean and Caribbean. On a global scale, the largest and most persistent
areas of high EAOT values are found over the tropical NAO. The plume is at its
maximum extent in the summer when it reaches into the Caribbean, the Gulf of Mexico
and the southeast coast of the United States. The relatively steep gradient on the southern
boundary of the tropical NAO EAOT plume roughly corresponds to the seasonal
climatological position of the northern boundary of the ITCZ. The plume shifts
seasonally in a manner that is consistent with seasonal changes in the large scale
circulation and the migration of the ITCZ towards the summer hemisphere.
The EAOT plume is clearly associated with the transport of dust from sources in North
Africa. Large quantities of mineral dust are transported across the tropical Atlantic during
much of the year [Carlson and Prospero, 1972; Prospero and Carlson, 1972; Karyampudi
and Carlson, 1988]. Satellite imagery shows that it takes about one week for dust
outbreaks to cross the tropical NAO from the coast of Africa to the Caribbean [Ott et al.,
1991]. The main transport occurs at higher altitudes in a layer (the Saharan air layer) that
typically reaches to several km and often to 5-6 km [Prospero and Carlson, 1972; Talbot
et al., 1986; Karyampudi and Carlson, 1988; Westphal et al., 1987; 1988].
Concentrations aloft are usually several times greater than in the marine boundary layer.
Because of the unusually high temperature and low relative humidity of the dust layer, it
can be identified in routine meteorological soundings as far west as the Caribbean
[Carlson and Prospero, 1972; Ott et al., 1991] and over the Amazon basin in eastern
Brazil [Swap et al., 1992; Artaxo et al., 1994]. The placement and extent of the EAOT
plume in JJA matches that obtained with sun photometers aboard a network of ships in
the tropical NAO during the summer of 1974 [Prospero et. al., 1979].
The seasonal movement of the EAOT plume conforms precisely with the mineral dust
measurements that have been made almost continuously at Barbados (13.17oN, 59.43oW)
since 1965 [Prospero and Nees, 1986]. In Fig. 4a we show two years of Barbados daily
dust concentration data that was acquired during the time corresponding to that of the
satellite record. Dust concentrations typically rise and fall in a coherent manner over the
period of several days with the passage of easterly waves [Carlson and Prospero, 1972;
Prospero and Carlson, 1972; Karyampudi and Carlson, 1988] as substantiated by satellite
imagery [Ott et al., 1991]. The monthly mean dust concentrations at Barbados (Fig. 5a)
show a pronounced summer dust maximum with concentrations about ten times those in
winter; the summer maximum at Barbados occurs when the core of the EAOT plume
(Fig. 1, JJA) lies along the latitude of Barbados. The aerosol data are consistent with the
seasonality shown in the Caribbean block (Fig. 3k) which also matches that of the West
Africa block (Fig. 3b) although the seasonal amplitude is much greater in the former than
the latter.
In addition to carrying large quantities of dust, the winds over the NAO often bring high
concentrations of pollutants. The pulses of increased dust concentrations are
accompanied by sharply increased concentrations of nss-SO4= (Fig. 4b) and NO3-. At
Barbados, on an annual basis about half of the nss-SO4= is attributed to pollutants, most
of which appear to be derived from sources in Europe [Savoie et al, 1989a; Savoie et al.,
1992]. Biomass burning is widespread in tropical Africa [Delmas et al, 1991; Andreae,
1993; Brustet et al., 1991] during the NH late winter and spring, peaking in March-April
[Hao and Liu, 1994]; biomass burning products are observed in aerosols over the tropical
NAO [Andrea, 1983] and at Barbados [Savoie et al., 1992]. Nonetheless, on a mass basis,
mineral dust is the major aerosol component in this region [Prospero, 1996a,b]. At
Barbados during the past decade, the mean concentration of mineral dust is about ten
times greater than the combined total of the other important non-sea-salt aerosol
components (i.e., NO3-, nss-SO4=, and NH4+) [Prospero, 1996a; Li et al., 1996]. Aerosol
light-scatter measurements on Barbados [Li et al., 1996] show that on an annual basis,
light scatter by mineral dust is four times greater than that by nss-SO4=.
The seasonal cycle of aerosol concentrations in the Canary Islands is similar to that at
Barbados and consistent with the AVHRR seasonal patterns. Studies made at an
observatory at Izana, Tenerife (28.30ºN, 16.50ºW), at an altitude of 2360m, above the
mean top of the marine boundary layer, provide data in the free troposphere within the
Saharan air layer [Arimoto et al., 1995; Prospero et al., 1995a]. Tenerife is located on the
northern edge of the main transport plume during much of the year (Fig. 1). As a result,
aerosol concentrations are highly variable (Fig. 6a), depending on the day-to-day
synoptic conditions. Although the aerosol concentrations on some days can be extremely
high, the annual mean concentrations at this site are similar to those on Barbados
[Prospero, 1996a,b]. The similarity in concentrations is largely due to the fact that
Barbados lies in the path of the North African plume a larger fraction of the time than
does Tenerife, in agreement with the depiction of the plume in Fig. 1. While dust
transport to the Canary Islands can take place any time of year, depending on specific
synoptic conditions, the frequency of events becomes much greater in mid summer when
the islands lie on the north edge of the plume.
The EAOT values in the Caribbean and Central America blocks (Fig. 3k) show a
maximum in the late spring and the summer. As suggested earlier, the seasonal cycle in
the Caribbean is largely driven by the advection of African dust into the region. In
contrast the Central American block values peak in the late spring because of emissions
from regional sources, as suggested by the high EAOT values and steep gradients along
the coast in this region (Fig. 1). Some of the Central American aerosol is probably
transported from large urban regions (e.g., Mexico city). However, as discussed more
fully below, there is also extensive biomass burning in this region in the spring.
The SE US block has a modest annual average EAOT=0.17. The monthly averages (Fig.
3l) show a very strong peak in spring and summer. The spring peak appears to be largely
due to the impact of transport from Central American sources to the Gulf of Mexico
region. The summer peak is due in part to the effects of North American pollutants; note
the similarity in timing and peak values with that in the E US block (Fig. 3l, see below).
In addition, the SE US block is strongly affected by the advection of African dust into the
region as suggested by the similarity to the Caribbean block trend (Fig. 3k) and by dust
measurements made in Miami (25.75oN, 80.25oW) [Prospero et al., 1987, 1993] and
Bermuda (32.27oN, 64.87oW) [Arimoto et al., 1992, 1995], all of which show a very
strong summer maximum.
During the NH winter the large scale circulation systems shift southward. In North
Africa, winter is the season of the Harmattan [see Morales, 1979], when large quantities
of dust are carried southward out of the dust source regions in the Sahara and the Sahel,
producing dense hazes in the countries bordering the Gulf of Guinea [see Morales, 1979;
Prospero, 1981]. Dust is lifted by convection to altitudes of several km and carried out
over the Gulf of Guinea, above the southwesterly monsoon winds, consistent with the
DJF EAOT distribution shown in Fig. 1. The EAOT is highest in February and March
and lowest in September and October. In the Guinea block (Fig. 3c) the average EAOT
during the peak months February and March is 0.48 - 0.49, a value comparable to the
peak monthly EAOT's for West Africa (Fig. 3b) in June and July. However, as noted
below, biomass burning products could contribute to these high values.
Thus, all evidence suggests that the AVHRR plume over the tropical NAO and Caribbean
is largely attributable to African dust. This conclusion is supported by the TOMS data
(Herman et al., this volume) wherein the plume has the same shape and dimensions and
shows the same seasonal variability as that observed in AVHRR.
Central North Atlantic Ocean. Plume-like EAOT features are clearly evident over the
mid-latitude NAO in Fig. 1. During the MAM and JJA, a large EAOT plume emerges
from the middle-Atlantic states of the U.S. and extends to the central NAO, consistent
with the prevailing westerly winds in this region. In spring the plume passes over
Bermuda (32.27oN, 64.87oW). Aerosol nss-SO4= concentrations measured on Bermuda
(Fig. 7a) are relatively high, reflecting the proximity of Bermuda to North American
pollution sources [Galloway and Whelpdale, 1987; Arimoto et al., 1992; Arimoto et al.,
1995; Ellis et al., 1993]. Tracer studies [D. L. Savoie, unpublished data] suggest that on
an annual basis, 70% of the nss-SO4= aerosol at Bermuda is anthropogenic. Monthly
mean nss-SO4= concentrations (Fig. 7b) are substantially higher in the spring when
increased pollutant concentrations are transported to Bermuda behind fronts [Moody et
al., 1995]. Concentrations can be relatively high in the summer as well (Fig. 7b) but
transport from the west is much more variable because of the influence of the BermudaAzores high pressure center which frequently brings southerly winds to the region. This
is consistent with Fig. 1, JJA, which shows the core of the EAOT plume passing
somewhat to the north of Bermuda.
The aerosol measurements on Bermuda conform to aerosol trends implied by the seasonal
EAOT pattern in the SE US block (Fig. 3l) where the monthly values peak in April-May
(0.24-0.26) and in the E US block in June - July (0.24-0.25); the minimum is in the
November - February. Note that the annual average EAOT in the E US block is rather
modest, 0.15, compared to the W Africa block, 0.26, and the Caribbean, 0.20. The
increased EAOT values during the summer (Fig. 3l) and the long plume over the NAO in
JJA are consistent with seasonal trends in pollution emissions and haze [Husar and
Wilson, 1993] and with AOT trends obtained from sun photometers in the eastern US
[Husar et al., 1981; Trijonis et al., 1990]; recent studies of aerosol light extinction [Malm
et al., 1994] show a summertime maximum in the region of the middle Atlantic states
with over 50% of the extinction attributable to ammonium sulfate.
African dust is also a major aerosol constituent at Bermuda especially during the summer
months in association with trajectories that come from the tropical NAO [Arimoto et al.,
1995] and Africa, a feature that is consistent with the strong circulation associated with
the Bermuda-Azores high. Dust transport to the SE coast of the US is suggested in the
AVHRR/JJA EAOT distribution and it is clearly shown in the TOMS absorbing aerosol
data [Herman et al., this volume]. At Bermuda there are dust events in the spring and fall
but they appear to come from sources in North America; however the dust concentrations
associated trajectories from the US are a factor of ten lower than those from Africa
[Arimoto et al., 1995].
In the eastern NAO, the large area of increased EAOT in the spring and summer appears
to be largely attributable to European pollution events, usually associated with the
presence of a high pressure center over western Europe [Doddridge et al., 1994].
Pollutants are carried westward, around the high, and into the high latitudes. At Mace
Head (53.32oN, 9.85oW), on the west coast of Ireland, nss-SO4= (and NO3-) aerosol
concentrations peak sharply in the spring and summer (Fig. 8); 80-90% of the nss-SO4= is
attributed to pollution sources [McArdle and Liss, 1995]. Measurements of aerosol size
distributions and chemical characteristics at Mace Head [Jennings et al., 1991, O'Dowd et
al., 1993] show that when the site is impacted by air masses from Europe the aerosol
properties are typical of pollutant products; in contrast, the aerosols carried in westerly
trajectories show minimal anthropogenic influences. Similarly, in the higher latitudes of
the NAO, sharply increased concentrations of nss-SO4= and NO3- are observed at
Hiemaey, Iceland (64.40oN, 20.30oW) due to pollutant transport events from Europe
[Prospero et al., 1995b]; in the absence of such events, aerosol concentrations are usually
quite low, comparable to values measured in the remote SH oceans.
During MAM and JJA, the North American plume merges with that from Europe,
effectively bridging the NAO. The monthly mean EAOT values for the N. Atlantic block
(Fig. 3n) are relatively constant (between 0.16 to 0.18) from April through August. The
large area of relatively low EAOT values in the central NAO coincides with the mean
position of the Bermuda-Azores high pressure center. On the basis of the quarterly
composites, it is not possible to judge which of the two continental source regions is
having the greatest impact on the central NAO. However, monthly composite images of
the AVHRR EAOT data (not presented here) show a clear separation between these
plumes. Based on the gradients in EAOT distributions, it would appear that transport
from Europe is dominant during MAM. In contrast, during JJA, the North American
sources seem to be more important. This interpretation is supported by the trend in
monthly average EAOT values for the NW Europe and W Europe blocks (Fig. 3h); the
annual cycles for both regions are essentially identical with a maximum in April-May
(0.23-0.24) and a minimum in November-January (0.04-0.05).
Also during the spring and summer large quantities of pollutants are carried out of
Europe southward across the Mediterranean [Bergametti et al., 1989a,b] and over the
coastal waters off the Iberian peninsula and the North Africa. The average EAOT for the
Mediterranean is 0.18. There is a strong seasonality in EAOT (Fig. 3i) with a summer
maximum (August EAOT=0.29) and a winter minimum (December, 0.07). There is also
evidence of a secondary peak in April that is comparable in timing and magnitude to
those for the NW Europe and W Europe blocks (Fig. h). The seasonal composites (Fig. 1)
show a gradient in EAOT values that increases toward the coast of North Africa; this
gradient (which is more sharply defined in monthly AVHRR composites) is consistent
with the observation [Bergametti et al., 1989a,b] that the transport of dust from North
Africa to Europe is greatest during the summer and with the TOMS absorbing aerosol
distributions [Herman et al., this volume].
Tropical and Subtropical South Atlantic. In DJF and MAM (Fig. 1) large amounts of
North African dust are transported over the tropical NAO to the NE coast of South
America [Prospero et al., 1981; Talbot et al., 1986]. At this time of year, African dust is
an important aerosol constituent in the atmosphere over the Amazon basin where the dust
is believed to serve as an important source of nutrients for the soils [Swap et al., 1992].
The relatively high EAOT values observed in the NE Brazil block (Fig. 3c) in January
through March appear to be largely a consequence of this transport. EAOT values are
relatively low during the later half of the year, reflecting the fact that this region lies in
the SH circulation; nonetheless the EAOT values remain high (monthly means, 0.160.19). The AVHRR distributions suggest that this region is under the influence of
transport from Africa all year long.
Biomass burning products are a major contributor to some of the low-latitude plumes
seen over the SAO in Fig. 1. There is extensive burning in the savanna that covers much
of the low latitudes in central and west Africa [Hao and Liu, 1994] as indicated by
astronaut observations of fire (Fig. 2) and by TOMS [Herman et al., this volume]. The
extensive EAOT distribution over the Gulf of Guinea during DJF is probably associated
with both dust and biomass burning products. Modeling studies of CO2 transport from
biomass fires [Iacobellis et al., 1994] shows a very prominent plume in January and
February that fits very well with the DJF EAOT in Fig. 1.
In central Africa, savanna and grassland fires are most frequent in June while in southern
Africa they occur mostly in July-September [Andreae et al., 1994]. The reported location
and seasonality of the burning in southern Africa is consistent with the EAOT
distributions shown in Fig. 1. The monthly average EAOT data for the SW Africa block
show the maximum in August-September (0.29-0.32). Because of the position of the
SAO high pressure center, in conjunction with meteorological processes over southern
Africa, transport to the west is strongly favored during most of the burning season as
shown in Fig. 1; transport to the east, into the Indian Ocean (IO), becomes significant in
October but it is relatively minor compared to the transport over the SAO as indicated by
the monthly EAOT data for the SE Africa block (Fig. 3d). During biomass burning
studies in southern Africa in August to October 1992 [Andreae et al., 1994], it was noted
that westward-moving smoke-laden air masses tended to exit over Angola, consistent
with the EAOT distribution in Fig. 1; these parcels followed trajectories that crossed the
coast of South America over central and northern Natal, Brazil, in agreement with the
distribution in Fig. 1. These conclusions are all supported by TOMS [Herman et al., this
volume].
During SON there is evidence of a weak equatorial plume that extends from the states
along the Gulf of Guinea towards the NE coast of Brazil (Fig. 1). This plume appears to
be distinct from the dust plume that emerges further to the north along the African coast
and the prominent biomass-burning plume to the south, along the coast of Angola and
Namibia. Aircraft measurements made in the low latitudes off the Brazilian coast during
September 1989 [Andrea et al., 1994b] showed that haze layers were frequently present
at altitudes of 1 to 5.2 km; the layers contained enhanced concentrations of aerosols and
chemical constituents (e.g. O3, CO and oxides of nitrogen) that are characteristic of
biomass burning products. These observations are consistent with the seasonal cycle of
EAOT observed in the NE Brazil block (Fig. 3c).
Some of the EAOT in SON (Fig. 1) could be associated with dust transported out of arid
regions in Angola and South Africa (e.g., the Namib and Kalahari deserts); however, the
axis of the plume is somewhat further north than would be expected for dust from these
regions.
The ocean region off the SE coast of Brazil is generally characterized by surprisingly low
EAOT values (Fig. 3j) in light of the intense biomass burning that takes place throughout
this region [Andreae, 1993; Cahoon et al., 1991; Artaxo et al., 1994]. In the Amazon
basin, south of the equator, burning takes place during much of the dry season, JulySeptember, but it is most intense in the late season, August-September [Artaxo et al.,
1994; Hao and Liu, 1994; Herman et al., this volume]. Models [Iacobellis et al., 1994]
show a major plume extending SE from central South America, the axis crossing the
coast of Brazil between about 20o-30oS; a plume also extends to the NW, crossing the
west coast of South America, roughly between 10oS to 10oN. Yet there is no evidence of
any plume along the east coast in either JJA or SON in Fig. 1. The absence of strong
evidence for biomass burning plumes is attributed to the fact that the export of smoke to
the SE often appears to be associated with meteorological conditions that are
accompanied by extensive cloud cover which would preclude detection by AVHRR. This
scenario is consistent with TOMS [Hsu et al., 1996; Herman et al., this volume] which
shows a marked peak in emissions in August-September; smoke from the northern
Amazon basin is transported over the NW coast near Ecuador while smoke from the
southern region is transported over the SE coast in association with cloudy conditions.
Nonetheless, in agreement with AVHRR, the TOMS data [Herman et al., this volume]
suggest that the export of biomass burning products from South America is much less
extensive than from south Africa.
3.3 Indian Ocean, Arabian Sea and the Bay of Bengal
The annual mean EAOT for the Arabian Sea is 0.32, the highest of all regions in Table 1.
EAOT in the Arabian Sea block (Fig. 3b) peaks sharply in JJA (0.61-0.65); the seasonal
cycle is similar to that for the West Africa block, suggesting that the entire region is
dominated by similar dust-forcing meteorological and climatological processes. Soils in
the Tigris and Euphrates basin appear to be a major source of dust that is transported to
the Arabian Sea [Ackerman and Cox, 1989; Prospero, 1981; Khalaf et al., 1985] although
soils in the arid regions of NW India and East Africa could also contribute [Ackerman
and Cox, 1989]. The seasonality of dust storms and haze conditions in this region
[Ackerman and Cox, 1989] is consistent with the seasonality of the EAOT distributions
in Fig. 1. During JJA, much of the transport takes place in deep atmospheric layers that
extend to 4-7 km and have properties similar to those observed with Saharan dust
outbreaks [Ackerman and Cox, 1989]. It is important to note that the SW summer
monsoon is well established over the Arabian Sea in June; the fact that high values of
EAOT in JJA (Fig. 1) extend so far to the south is attributed to the transport of dust over
the top of the monsoon inversion [Ackerman and Cox, 1989; Sirocko and Sarnthein,
1989]. In the low latitudes and at low altitudes, the SW monsoon winds carry dust from
the Horn of Africa into the Arabian Sea [Sirocko and Sarnthein, 1989].
Although dust storm statistics suggest that there are major sources of mineral dust in
northern India [Ackerman and Cox, 1989], there is very little dust storm activity in
southern India below 15oN at any time of year. Thus the prominent bulge in the EAOT
distribution (Fig.1) located off the SW coast of India in MAM is most likely due to
pollution aerosol, not dust.
EAOT values over the Bay of Bengal are substantially lower than over the Arabian Sea
(annual mean, 0.22). EAOT values are highest in JJA, similar to the Arabian Sea, with
the maximum monthly average in June (Fig. 3f, 0.39). The gradients in Fig. 1 suggest that
the sources lie mostly in India , but transport of dust across the subcontinent from the
Arabian Sea is a possibility. In addition, monthly EAOT composites (not shown here)
suggest that there are substantial sources in Bangladesh and northern Burma; although
biomass burning is common in these regions, the timing of the enhanced EAOT does not
match the peak burning period, March through May [Hao and Liu, 1994], which is also
shown in TOMS [Herman et al., this volume].
There is very little aerosol data for the IO with which to compare the AVHRR
distributions; the data that is available has been largely obtained over relatively short
periods during ship cruises. Nonetheless, these clearly show that dust concentrations are
very great over the Arabian Sea and the NW IO close to Africa, comparable to those
measured along the west coast of Africa and the Mediterranean [Savoie et al., 1987;
Prodi et al., 1983]. These values and the seasonality of the concentrations are generally
consistent with the dust transport distributions shown in AVHRR (Fig. 1) and with the
monsoon circulation [Ackerman and Cox, 1989]. Although dust sources appear to be
dominant in the Arabian Sea and possibly the Bay of Bengal, substantial concentrations
of pollutant species are also present [Savoie et al., 1987].
The EAOT distributions over the Arabian Sea in JJA show a very strong gradient in the
low latitudes, close to the equator, in the vicinity of the ITCZ . Dust concentrations drop
sharply as one moves from the Arabian Sea southward into the SH circulation [Savoie et
al., 1987; Prodi et al., 1983]. On a recent cruise in the southern and central IO [Dickerson
et al., 1996], nss-SO4= concentrations south of the ITCZ were generally in the range 0.10.5 µg/m3, values that are typical of background ocean values; north of the ITCZ,
concentrations were ten times greater with values as high as 9 µg/m3. The concentration
of CO also showed large temporal variability that was highly correlated with aerosols,
which suggests that both species were derived from pollution sources on the Asian
subcontinent.
Although the ITCZ represents a demarcation line in aerosol distributions, the EAOT
gradients in JJA suggest that substantial amounts of aerosol material are being
transported south wind, perhaps as far as 15o-20oS. During the remainder of the year, the
eastern tropical IO appears to be affected by other aerosol sources in Oceanea, Australia
and southern Africa. The monthly mean EAOT values for the Indonesia block (Fig. 3e)
have a strong seasonal pattern with peak values in September-October, possibly due to
extensive biomass burning in this region (see Fig. 2) [Malingreau et al., 1985]. This
period corresponds to the end of the dry season; the large scale winds would transport
burning products to the west, over the IO. The EAOT map for SON (and to a lesser
extent, DJF) shows a coherent aerosol plume that appears to originate from these islands
and northern Australia and that extends over the IO to East Africa. The New Guinea
block (Fig. 3e) does not reveal a seasonal trend but EAOT values are moderately high all
year long. The aerosol emissions in this block would include materials from northern
Australia which has a long dry season (from April-May through the end of the year), and
from the eastern portion of Indonesia; again, we might assume that biomass burning
could be a major source based on the frequent sightings as indicated in Fig. 2. However
the TOMS data [Herman et al., this volume] do not show a great deal of burning in this
region; the burning that is observed takes place in September-November, consistent with
the trend of EAOT. Six years of atmospheric turbidity measurements at Broome on the
NW coast of Australia (17.97oS and 122.23oE) also show a strong seasonal cycle with a
maximum in September-November and a minimum in April-June [Scott et al., 1992].
The EAOT distributions do not reveal any persistent, substantial, well-defined plumes
emerging from Australia. The absence of dust plumes is particularly noteworthy
considering that Australia has the largest expanse of desert land in the SH [Pye, 1987].
EAOT values over the southern IO are quite low most of the year (annual average, 0.08).
Nonetheless, there is a pronounced seasonal cycle (Fig. 3m) with a minimum in June-July
(0.04) and a maximum in January (0.13). The summer maximum is associated with a
clearly visible band of enhanced EAOT values (Fig. 1) that circles the southern oceans
between 40o-60oS in DJF. This will be discussed in a later section.
3.4 Pacific Ocean
From Fig. 1 it is clear that the NPO is much more heavily impacted by aerosols than the
SPO. This difference is reflected in the annual mean EAOT's for the NE Pacific and EC
Pacific blocks (0.11 and 0.13, respectively) compared to the SE Pacific (0.07) and in the
very different seasonal cycles in the monthly EAOT's (Fig. 3m, 3n, 3o). In the following
sections we discuss the data for various regions in the NPO and SPO.
North Pacific. In Fig. 1 in MAM a plume extends from Asia across the central NPO,
almost to the west coast of Alaska and Canada. High EAOT values are visible along the
Asian coast in all seasons. In contrast, the west coast of North America shows relatively
low EAOT values at all times. Because of the presence of the high pressure center in the
eastern NPO, this region is climatologically dominated by strong westerly and northerly
flow in all seasons; consequently, transport of aerosols from the western US is not
favored. Thus, the aerosol distributions over the mid-latitude NPO seem to be largely
derived from sources in Asia.
There is a strong seasonal cycle which is most pronounced in the monthly EAOT
averages in the NW and NE Pacific blocks (Fig. 3n) but it is also well-defined in the EC
and WC Pacific (Fig. 3o). The seasonal cycle is largely driven by the transport of
aerosols out of Asia as suggested by the plume in Fig. 1 and by the close match between
the seasonal cycle for the NW Pacific block with those for the Japan (Sea of Japan) and
China (Yellow Sea) blocks (Fig. 3g). The high coastal EAOT values are associated with
mineral dust and pollutant aerosols [Arimoto et al., 1996; Mukai et al., 1990; Tsunogai et
al., 1985; Uematsu et al., 1992; Prospero, 1996a; Gao et al., 1996]. The seasonality of the
transport in this region is consistent with the large scale climatology: storms and cold
outbreaks during the winter and spring favor transport out of Asia; the southerlysoutheasterly monsoons bring relatively clean ocean air into the coastal regions in the
mid and low latitudes beginning in late spring and reaching their maximum northerly
extension in July.
The dust maximum corresponds to the seasonal cycle of dust storm activity in Asia
[Goudie and Middleton, 1992; Littmann, 1991; Middleton, 1991; Merrill, 1989; Prospero
et al., 1989; Duce, 1995]. Also, in the spring, strong westerly flow in the mid latitudes
favors the transport of dust over great distances [Merrill, 1989; Duce, 1995; Prospero et
al., 1989; Gao et al., 1992]. In Japan and Korea during the spring, they frequently
experience extensive hazes that are caused by yellow dust (Kosa) that can be traced to
sources in Asia [Takayama and Takashima, 1986; Tsunogai et al., 1985; Mukai, 1990;
Uematsu et al., 1992]. Asia is also a major source of anthropogenic sulfur and nitrogen
emissions [Hameed and Dignon, 1992; Husar and Husar, 1990; Galloway et al., 1994;
Kasibhatla et al., 1993] and biomass burning products [Cahoon et al., 1991]. Coal is a
major energy source and emissions of black carbon appear to be very high making these
sources visible in TOMS in the winter months [Herman et al., this volume].
Aerosol measurements made in a network of surface-based stations in the NPO
substantiate the seasonal character of the aerosol distribution in Fig. 1. In MAM the
concentrations of soil dust, nss-SO4=, and NO3- increase sharply across a large area of the
central NPO [Prospero et al., 1989; Savoie et al., 1989b; Arimoto et al., 1996]. At
Midway (28.22oN, 177.35oW) which lies at the southern edge of the plume in MAM (Fig.
1), there is a strong spring maximum in dust concentration (Fig. 9a). Nss-SO4= also
shows a pronounced maximum in the spring (Fig. 9b), along with the dust, suggesting a
common source region; a similar spring maximum occurs for NO3- aerosol [Savoie et al.,
1989b]. Tracer studies [Savoie et al., 1989b] suggest that during the spring a major
fraction of nss-SO4= is derived from anthropogenic sources. Enhanced concentrations of
these aerosol species are also observed at Shemya, in the Aleutians (52.92oN, 176oE) and
on Oahu, Hawaii (21.33oN, 157.70oW) [Prospero et al., 1989; Savoie et al., 1989b;
Arimoto et al., 1996]. In MAM, sharply increased concentrations of mineral dust
[Holmes and Zoller, 1996] and NO3- [Lee et al., 1994] are also observed in the free
troposphere at Mauna Loa Observatory (19.53oN, 155.57oW) on the island of Hawaii; the
advection of Asian dust clouds into the region is also readily apparent in measurements of
solar spectral irradiance and atmospheric transmission at Mauna Loa [Dutton et al.,
1994].
Equatorial Pacific. major oceanic aerosol belt stretches over the Pacific just north of the
equator (0o-20oN) in MAM (Fig. 1) along the northern edge of the ITCZ. (The belt is also
present, albeit more weakly, in JJA.) The uniform aerosol characteristics across this huge
region in MAM are reflected in the similarity of both the magnitude and the seasonalality
of the EAOT values in the EC (Eastern Central) Pacific and WC (Western Central)
Pacific blocks (Fig. 3o). While this region is occasionally impacted by the transport of
Asian aerosol in the spring as discussed above, the plume, which lies in a band of easterly
trade winds, appears to emanate from Central America between 10o-15oN. This plume is
different from the other plumes discussed thus far in that it shows no gradient "downwind"; indeed, the plume appears to broaden and intensify over the western NPO.
There are no well-quantified sources in Mexico and Central America that could explain
such a prominent plume. Biomass burning is quite intense throughout this region (Fig. 2),
mostly in April-May [Hao and Liu, 1994]. In April-May TOMS [Herman et al., this
volume] shows very extensive areas of absorbing aerosol in precisely the same region
where EAOT values are high in Fig. 1; the TOMS data would seem to implicate biomass
burning sources but TOMS does not show any extensive plumes comparable to those
from burning in south Africa. Nonetheless, it is difficult to see how emissions from
sources in the boundary layer could be transported such a great distance over the ocean
unless there is an efficient mechanism to lift the emissions into the free troposphere.
Mexico City, at an elevation of 2,303 m, might be an effective source for injecting large
quantities of pollutants into the free troposphere but it would seem unlikely to be the
source of such a long plume. There was a major volcanic eruption in Guatemala (Pacaya,
volcanic explosivity index of 3*) in January 1990 and it continued through 1993
[Simpkin and Siebert, 1995]. Perusal of the original weekly AVHRR product reveals that
the plume is visible in both 1991 and 1990 although it is more prominent in 1991.
However the fact that the EAOT plume in Fig. 1 has a distinct seasonality would seem to
preclude a volcanic source.
The EAOT plume is not explained by cirrus cloud contamination. Wylie, et al. [1994]
analyzed thin cirrus with the NOAA/HIRS CO2 slicing technique over global oceans
from 1989 to 1993 and concluded that there is a high frequency of cirrus (greater than
50%) in the ITCZ in all seasons; also there is a modest seasonal movement that tracks the
sun. Thus, if there was an artifact caused by thin cirrus, we would expect to see it in all
seasons, not just in the MAM period.
There is very little aerosol data from the equatorial Pacific. Measurements made
continuously over the period 1981 to 1986 [Savoie et al., 1989b] in the easterly trade
winds at Fanning Island (3.92oN, 159.33oW) yielded moderately high nss SO4=
concentrations (annual mean, 0.76 µg/m3); however, concentrations are relatively
uniform all year long with monthly means falling in a rather narrow range of 0.60-0.74
µg/m3 [Savoie et al., 1989b]. Although transport from Asian sources is occasionally
noted at Fanning in the spring, the impact of continental sources is believed to be small
and sporadic. The absence of strong continental impacts at Fanning suggests that the nssSO4= is largely derived from oceanic sources of DMS, a conclusion that is supported by
the measured concentration of methanesulfonate (MSA) an oxidation product of DMS
[Savoie et al., 1989b]. As was the case with nss-SO4=, MSA shows no seasonal
variability, an observation that is consistent with the absence of any strong seasonality in
the oceanic productivity in this region. Furthermore, extensive measurements along
140oW in the spring of 1992 found that oceanic DMS concentrations were highest
between 3oN to 15oS [Huebert et al., 1994; Bates et al., 1993], considerably south of the
EAOT plume which is located roughly between 8o and 16oN (Fig. 1); the DMS
concentrations south of the equator were as much as twice those to the north. Yet there is
no evidence of any comparably prominent oceanic-nss-SO4= plume to the south of the
equator in this season or in any other season. Thus, the EAOT plume observed in this
region in the spring does not seem to be directly related to oceanic sources of nss-SO4=.
The absence of obvious impacts form oceanic DMS sources is emphasized by that fact
that there is no sign of any prominent source of enhanced EAOT values in the region off
the NW coast of South America where there is very strong upwelling, making these
waters among the most productive in the world. There is a narrow belt of increased
EAOT along the coast, adjacent to Peru (see the Peru block, Fig. 3j), but this appears to
be attributable to sources in South America, including biomass burning (Fig. 2). Sources
in Brazil and Argentina should not have a major impact on this ocean region; transport to
the west is hindered by the Andes. TOMS [Herman et al., this volume] does not show
major transport of burning products to this region. Also, in the eastern SPO near the coast
of South America there is a strong semi-permanent high pressure center at about 25o-30oS
which results in southerly winds that tend to lie parallel to the coast all year long; the
presence of the high is reflected in the position of the low EAOT values in this region in
Fig. 1. Pollution sources could be significant here; high concentrations of nss-SO4= have
been measured in the coastal ocean regions in the Peru block; these are largely attributed
to smelters in Chile [Saltzman et al., 1986].
Mid-latitude South Pacific. In general, EAOT values throughout this region are quite low
with both the SE Pacific and New Zealand blocks (Fig. 3m) having annual means of 0.07.
Furthermore there is a strong seasonal cycle in the New Zealand block that is identical to
that for the SE Pacific and S. Indian Ocean blocks. This suggests that sources in New
Zealand and Australia have no discernible effect on EAOT in the western SPO. Extended
measurements made on Norfolk Island (29.08oS, 167.98oE) and New Caledonia (22.15oS,
167.00oE) [Prospero et al., 1989] and on New Zealand [Arimoto et al., 1990] confirm
that aerosol concentrations, including mineral dust, are generally quite low throughout
this region.
The most prominent feature in Fig. 1 is a band of enhanced EAOT that circles the
southern oceans between 45o-55oS during DJF, the SH summer. It is interesting that
statistics on the distribution of haze at sea compiled from ship meteorological
observations prior to the 1930's [MacDonald, 1938; see also Prospero, 1981] show a band
of increased haze in precisely the same latitudes as that shown by AVHRR in DJF (but
not in other seasons). The similar EAOT signatures across this huge area suggests that the
AOT in this region might be due to natural oceanic sources. As previously stated, it
seems unlikely that this feature could be due to sea-salt aerosols. Sea salt aerosol
concentrations should be at a maximum in the winter when wind velocities are highest. A
global model of sea-salt aerosol distributions (Erickson et al., 1986) shows a well-defined
band of enhanced sea-salt aerosol concentrations in precisely the same latitude band as
the band of enhanced EAOT in Fig. 1 in DJF; however, on an annual basis, the model
shows substantially higher aerosol concentrations in DJF relative to JA.
It might be possible to ascribe the enhanced EAOT values in this band to oceanic nssSO4= sources. The waters in the low-latitude southern oceans are highly productive at this
time of year and the atmospheric concentrations of MSA are at a maximum
[Mihalopoulus et al., 1993]. Measurements made at Cape Grim (40.68oS, 144.68oE) show
a clear annual cycle of MSA and nss-SO4= concentrations with peak values in DJF [Ayers
et al., 1991; Gras, 1995] yielding a seasonal average of about 0.3 µg/m3 for nss-SO4=.
Similarly, measurements at Mawson (67.60oS, 62.50oE) on the Antarctic coast in the IO
and Palmer Station (64.77oS, 64.05oW) show an extremely strong seasonal cycle in both
MSA and nss-SO4= aerosol concentrations that peak at this time of year [Savoie et al.,
1993]; however, even during spring, the mean nss-SO4= concentration is only a little over
0.2 µg/m3. Thus, the mean aerosol nss-SO4= values at remote southern ocean locations are
typically about a factor of ten lower than concentrations in the NAO during the seasons
when the EAOT plumes are clearly visible. Thus we would expect that the EAOT values
due to ocean nss-SO4= would be rather low and perhaps not readily detectable in the
present generation of AVHRR satellites.
4.0 Conclusions
The seasonal distributions of AVHRR EAOT over the oceans are clearly consistent with
the long-term data sets for major aerosol constituents in the marine boundary. Similarly,
the distribution of the EAOT plumes and their seasonal movement are generally
consistent with our understanding of the meteorology and climatology in the source
regions and the large scale circulation systems that transport the aerosol products.
Aerosol measurements, although limited, provide a semiquantitative indication of the
species that are most important in aerosol light scattering processes over the oceans.
These studies clearly show the importance of anthropogenic products such as sulfate
aerosol in light scattering processes. These impacts are most clearly evident over the mid
latitude NAO and NPO where the produce long "plumes"; also, areas with very high
EAOT values are often noted along the coasts, most prominently in Asia.
Nonetheless, the most notable features in the AVHRR images are those that are
attributable to mineral dust and biomass burning products. The impact of dust is
especially evident. The EAOT plumes attributed to dust yield the highest EAOT values
and cover that largest areas. Dust plumes are most prominent over the tropical NAO and
the Arabian Sea, reflecting the impact of sources in North Africa and the Middle East;
AVHRR suggests that, from the standpoint of long range transport, these regions are the
largest dust sources in the world, a conclusion that is substantiated by TOMS [Herman et
al., this volume]. It would appear that the activity of the dust sources is somehow linked
to the strong, deep and persistent convective mixing that takes place over the deserts and
arid regions of North Africa [Karyampudi and Carlson, 1988; Westphal et al., 1987;
1988]. Once the dust is lifted high above the surface, it can be rapidly transported long
distances by the vigorous wind systems that are found in the middle troposphere. Thus,
the strong dust transport that we observe over North Africa and the Middle East is
perhaps unique because of the convergence of these several factors.
While the AVHRR images clearly show that dust is a prominent feature, they also
suggest that deserts are not necessarily good sources of dust. As previously noted, there is
no visible dust plume associated with Australia, a continent that is largely arid.
Furthermore, with regard to African sources, the very large dust plumes in DJF and
MAM are seen to emerge from the sub-Saharan (Sahel) region; the Saharan sources
appear to be dominant only in JJA and SON. It is believed that land use practices in the
Sahel, coupled with drought, have greatly increased the rates of wind erosion [Williams
and Balling, Jr., 1996]. This is supported by the observation of greatly increased dust
transport across the NAO since the onset of drought in the sub-Sahara in the late 1960's
[Prospero and Nees, 1986; Prospero et al., 1993]. Land use is implicated as an important
factor in soil deflation, [Williams and Balling, Jr., 1996]; under such circumstances, dust
might properly be regarded as an anthropogenic pollutant. The complexity of the dust
deflation processes and the importance of land disturbance [Pye, 1989; Williams and
Balling, Jr., 1996] is reflected in the difficulty that models have in replicating large-scale
dust transport distributions compared to AVHRR [Tegen and Fung, 1994, 1995].
Finally, there is mounting evidence that dust can play an important role in global climate
forcing. Early work had shown that dust has a substantial affect on the radiation balance
in dust outbreaks over the NAO [Carlson and Benjamin, 1980] and that this forcing plays
a role in the large-scale dynamics of the outbreaks [Karyampudi and Carlson, 1988].
More recently, models have shown that these effects have important implications for
climate [Lacis and Mishchenko, 1995; Andreae, 1995, 1996; Tegen et al., 1996]. Thus,
AVHRR can play an important role in characterizing the temporal and spatial variability
of dust transport on a yearly basis and long term trends that might result from changes in
climate and land-use.
Biomass burning is also an important source of EAOT plumes. However burning in
southern Africa is the only source that seems to produce a persistent, large, clearly
identifiable plume over the ocean. Other important burning sources are inferred (e.g.
South America) but their impact seems to be much more limited and subtle. It may be
that in many regions the smoke from fires is not lofted into the free troposphere where it
can be transported over great distances. In general, AVHRR suggests that the areal
coverage and EAOT resulting from burning is considerably less than that from mineral
dust, a conclusion supported by TOMS [Herman et al., this volume].
Aerosols from conventional pollution sources (e.g., energy consumption), clearly have a
great impact in the northern hemisphere EAOT distributions, especially in the mid
latitudes. However, as previously stated, the mean EAOT values for these regions are
considerably less than those affected by mineral dust and biomass burning sources.
Nonetheless, it must be remembered that the current AVHRR algorithm may be
discriminating against aerosols present at concentrations that yield low EAOT values;
thus, the effects of conventional pollution aerosols (and other types of aerosols) could be
more widespread (albeit at low concentrations) than indicated in Fig. 1.
The AVHRR images do not show any evidence of strong oceanic sources of aerosol. The
prominent band of high EAOT values in the southern oceans might be attributed to
oceanic sources of nss-SO4= aerosol. However, it is difficult to speculate on DMS-derived
sulfate distributions over the oceans because of the considerable uncertainties about the
relationship of DMS emissions to primary productivity (especially to chlorophyll
distributions either directly measured or based on satellite-sensed values) and the
subsequent conversion rates to sulfate aerosol. These uncertainties are reflected in the
widely varying distributions obtained in global models of the atmospheric sulfur cycle.
For example, Chin et al. [1996 - in press, JGR] show a distinctive band of increased
concentration of DMS, MSA and nss-SO4= in January in the southern oceans, but the
band is centered at about 60o S, whereas the band in AVHRR is centered at about 50oS;
furthermore, there is little overlap between the model distribution and that viewed by
satellite. In contrast, Pham et al. [1995] show no distinctive band of enhanced DMS in
the mid- and high latitude southern ocean; their model yields the highest DMS emissions
in equatorial regions. There is considerable research on role of oceanic sources of
aerosols [e.g., Huebert et al., 1994]; the AVHRR data suggests that the region between
40o-60oS area is a prime candidate for study.
Another puzzling feature is the large plume that extends over much of the equatorial NPO
in MAM. We speculated on possible sources in Central America (i.e., biomass burning
and pollutants); but it is difficult to see how these sources could affect such a large region
of the ocean unless they were transported in the middle and upper troposphere.
Alternatively, we considered the possible role of ocean sources of nss-SO4=; however,
existing data seems to argue against this source.
Taken as a whole, the AVHRR data show in a distinctive way that the NH is much more
heavily impacted by continental sources than the SH. This is consistent with our
understanding about the global distribution of pollution sources. It is also notable that
there is a pronounced seasonality aerosol transport, especially in the mid latitudes; in
general the largest plumes and higher EAOT levels are seen in the spring and summer
hemispheres. This is true for conventional pollutants (e.g., sulfates) and also for dust and
burning, an observation that is consistent with the TOMS product [Herman et al., this
volume].
In order to asses the role of aerosols in climate, models require detailed knowledge about
aerosol properties [Penner et al., 1994; Lacis and Mishchenko, 1995]. AVHRR provides
a global-scale picture of aerosol distributions that can be used to guide the design and
implementation of intensive aerosol campaign-type measurements and the development
of both diagnostic and prognostic models of aerosol sources, transformations, deposition,
and effects. Focused measurements at a few regionally representative sites would suffice
to characterize the important aerosol properties [Stowe et al., 1990]. Future integrated
earth observing satellite systems [King et al., 1992] will be capable of more detailed,
long-term monitoring of global aerosol changes and their gross physical properties
[Kaufman, 1995] as well as their relationship to climatic and other bio-geochemical
variables and processes. Nonetheless, it will still be necessary to obtain detailed in situ
aerosol and radiation data to further improve satellite aerosol retrieval algorithms and to
validate the data that is subsequently obtained. The satellite data could then be used to
extrapolate the in situ observations to larger space and time scales.
Acknowledgments. We thank Craig Long for providing the aerosol data files and
Attila P. Husar for preparing contour maps. This research was partially supported
by the NOAA grant #NA 16RC0517 and the National Science Foundation's
Atmosphere/Ocean Chemistry Experiment (AEROCE) grants - ATM9414808,
ATM9414812, and ATM9414846.
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FIGURE CAPTIONS
Figure 1: Radiatively equivalent aerosol optical thickness (EAOT x 1000) over the
oceans derived from NOAA AVHRR satellites for the four seasons. The figure
incorporates data for the period July 1989 to June 1991.
Figure 2: Aerosol regions over the oceans. The rectangular boxes identify regions
for which monthly average EAOT values were calculated (see Table 1 and Fig. 3).
Within the rectangles that overly land masses, EAOT values were only obtained
over water surfaces.
Figure 3: Monthly mean EAOT for various ocean regions. Note that in 3p, the
southern hemisphere annual cycle is shifted by six months so that the seasons can be
compared directly for both hemispheres. (Note to reviewers: the symbol for sulfur
emissions will be changed in the final version to avoid confusion with the biomass
burning symbol.)
Figure 4: Aerosol concentrations in the trade winds at Barbados, July 1989 - June
1991. Daily samples are collected only during on-shore winds. 4a: Mineral dust. 4b:
nss-SO4=. Data are plotted against time measured as months from 1 January 1989
[Prospero et al., 1993].
Figure 5: Monthly mean aerosol concentrations in the trade winds at Barbados, July
1989 - June 1991. 5a: Mineral dust; 5b: nss-SO4= [Prospero et al., 1993].
Figure 6: Daily aluminum and nss-SO4= concentrations at Izaña, Tenerife, July 1989
- June 1991. Measurements are made only at night during down-slope wind
conditions to ensure that the free troposphere is being sampled. 6a: Aluminum.
Concentrations can be converted to an equivalent dust concentration by multiplying
by 12.5, assuming that the average concentration of Al in soils is 8%. Data provided
by R. Arimoto [see Arimoto et al., 1995]. 6b. nss-SO4= [Unpublished data, D. L.
Savoie and J. M. Prospero; see also Prospero et al., 1995a].
Figure 7: Aerosol nss-SO4= concentrations measured on Bermuda, July 1989-June
1991. 7a: Daily concentrations measured during on-shore winds. Data is composited
from samples collected at two sites, one on the west end of Bermuda [Arimoto et al.,
1995] and a second on the east end; combined, these two stations provide coverage
of winds through 335o. 7b: Monthly means composited from the west end and east
end sites. To demonstrate the variability over longer time periods, monthly means
are shown for two time periods: July 1989 - June 1991 and July 1991 - June 1993.
Note the relative stability of concentrations in the fall-winter time periods and the
large variability in August. [Unpublished data, D. L. Savoie and J. M. Prospero]
Figure 8: Aerosol nss-SO4= concentrations measured at Mace Head, Ireland, July
1989 - June 1991. 8a: Daily concentrations during on-shore winds. 8b: Monthly
mean concentrations for two time periods: July 1989 - June 1991 and July 1991 June 1993. Note the anomalously high concentrations in December. These are
attributed to a few pollution events with unusually high nss-SO4= concentrations;
otherwise, concentrations during the winter are relatively low. The December highpollution events were are not reflected in the AVHRR EAOT distributions in Fig. 1.
[Unpublished data, D. L. Savoie and J. M. Prospero].
Figure 9: Aerosol concentrations measured on Midway, 1981 - 1994. Each data
point is a one-week-long sample collected during on-shore winds. 9a: Al
concentration in aerosols [Prospero et al., 1989; R. Arimoto, personal
communication], dust concentrations can be calculated by multiplying by 12.5,
assuming an average soil concentration of 8%; 9b: nss-SO SO4= [Savoie et al.,
1989b; Savoie and Prospero, unpublished data].
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