JOURNAL
OFGEOPHYSICAL
RESEARCH,
VOL.100,NO.D9,PAGES
18,707-18,726,
SEPTEMBER
20,1995
Contribution to the atmospheric mineral aerosol load
from land surface modification
Ina Tegen
Department
of AppliedPhysics,
Columbia
University,
NewYork
Inez Fung•
NASA GoddardInstitutefor SpaceStudies,NewYork
Abstract. An estimationof the contributionof mineraldust from disturbed
soils
(i.e.,soils
affected
byhuman
activity
and/orchinate
variability)
to thetotal
atmospheric
mineral
aerosol
loadis presented.
A three-dimensional
atmospheric
dusttransport
model
wasusedto simulate
thedistribution
ofdustoptical
thickness
in response
to individualdust sources,
whichincludenatural soilsknownto have
been•ffected
bytheS•h•r•n/S•heli•n
boundary
shift,cultivation,
deforestation,
andwinderosion.
Thedistributions
extracted
fromadvanced
veryhighresolution
radiometer
(AVHRR)optical
thickness
retrievals
wereusedto constrain
likely
sourcecombinations.
The resultsindicatethat observedfeatureslike the seasonal
shiftof maximumopticalthickness
caused
by Saharandustoverthe Atlanticocean
arebestreproduced
if disturbed
sources
contribute
30-50%
ofthetotalatmospheric
dust loading.
1.
Introduction
nentson the Earth'sradiationbudget.In estimatesof
the radiativeimpactof aerosoltypes,mineraldustis
usuallyclassified
as "natural"aerosol
togetherwithsea
The magnitudeof radiativeforcingand climateimpact of aerosolsis still largely uncertain. For an improvedunderstandingof theseprocesses,
sourcesand
saltandvolcanic
aerosol,
incontrast
to "anthropogenic"
aerosollike sulfate and nitrate aerosolfrom industrial
andsootfrombiomass
burning.[e.g.,Anatmospheric
distributionof the differentaerosolspecies emissions
dreae,1994]. The classification
of mineral dust as "natismisleading
if "natural"isinterpreted
as
Mineraldustis an aerosoltype with oneof the high- uralaerosol"
"not
influenced
by
human
activity",
because
the
proest atmosphericmassloadingson a globalscalebesides
sea-saltaerosol.The globalsourcestrengthof mineral ductionof atmosphericmineral dust without doubt inhave to be quantified.
in areaswherethe soilsurfaceis disrupted
by
aerosolis currently estimated to a value between 1000 creases
activities
ornewsoilsurfaces
areexposed
to
and5000Mt yr-• [e.g.,Schiitz,1980;Duceet al., 1991; agricultural
throughdeforestation
andshiftingdesert
Andreae,1994],comparedto severalhundredMtyr -• winderosion
Forexample,
in theUnitedStates
thehighfor sulfateaerosol[e.g.,Andreae,1994].Mineraldustis boundaries.
est dust production occurs in the area known in the
aerosolopticalthickness[Rao et al., 1988]wheredust 1930sand 1950sas "dustbowl", an area wherethe soil
plumes from the North African deserts over the North was disrupted by cultivation. The relative contribution
also a predominant feature in satellite observationsof
dustloadis unAtlantic oceanand overthe ArabianSeacan clearlybe of eachsourcetype to the atmospheric
do not giveinformation
recognized.In thoseareasthe retrieveddust optical known,as dustmeasurements
thicknesses can reach values of 0.6 to 1. The radiative
aboutthesource
type. Penneret al. [1994]statesthat
impactof desertdustcanbe expectedto be significant anthropogenicmineral aerosolexists due to desertification and deforestation,
but givesno estimateaboutthe
in areaswith high dust loadings.
order
of
magnitude
of
the anthropogenic
contribution
The term "radiativeforcing"is generallyusedto deto
the
atmospheric
dust
load.
scribetheimpactof anthropogenic
atmospheric
compoThe assumption
that the non-natural
sources,
which
arelocatedat thesemi-arid
desertfringesratherthanin
the deserts
themselves,
contribute
a majorpart of the
1Alsoat Schoolof EarthandOceanSciences,
University
of
atmospheric
dustloadis supportedby variousobservaVictoria British Columbia, Canada.
tions.Satellite
observations
[Raoetal.,1988],shipmea-
Copyright 1995 by the American GeophysicalUnion.
Paper number 95JD02051.
0148-0227/95/95 JD-02051$05.00
surements
[McDonald,1938],and observations
of dust
concentrations
in Barbados
andFrenchGuyana[Prosperoet al., 1981;Prospero
andNees,1986]showthat
the locationof the Saharandustplume,whichextends
18,707
18,708
TEGEN
AND
FUNG:
MINERAL
over the North Atlantic ocean and transports dust over
far distances toward South America, is subject to a seasonalshift following the Innertropical ConvergenceZone
DUST
FROM
DISTURBED
SOILS
where qa is the dust flux from the surface, u is the surface wind speed, and utr is a threshold velocity. The
thresholdsurfacewind speedat 10-m height is taken to
to Kalmaet al. [1988].We
(ITCZ). In the northern hemispherewinter, this dust be 6.5ms-•, corresponding
plume is located around 5ø-15øN, that is, south of the
Sahara, while in the northern hemisphere summer, it is
located around 15-25øN. These observationsimply that
this dust seemsto originate from the Sahelregionrather
assumed the dimensional
factor
C to be constant
for all
size classes. To resolve high-frequency events in space
and time, we used European Centre for Medium Range
WeatherForecasts(ECMWF) (TropicalOcean-Global
Atmosphere(TOGA) Analysis)wind products(10mport a strong increase in Saharan dust concentrations surfacewinds)with a spatialresolutionof 1.125øx 1.1250
from the Saharandesert. Prosperoand Nees[1986]remeasured
at Barbados
from
the mid-1960s
to the mid-
and 6-hour time resolution. With this parameterization
the seasonalvariation of dust uplift is described by the
the Sahelsincethe late 1960s[Lambet al., 1986],while seasonality of surface winds and soil wetness.
many authors find that dust production is only weakly
The dust transport in the atmosphere was calcucorrelatedwith meanrainfall in the sourceregions[e.g., lated using the three-dimensional Goddard Institute for
1980s, which is well correlated with the rainfall deficit in
Pye, 1987]. This finding alsoindicatesthat the Sahel, SpaceStudies(GISS) tracer model (8øx10ø horizontal resolution,nine vertical atmosphericlayers) [e.g.,
cultivation,
deforestation
andovergrazing
[Middelton,Prather e! al., 1987; Fung et al., 1983], where source
1992; World Resources
Institute, 1992],contributessig- and sink terms were included. The model transports
which is in contrast to the Saharan desert affected by
nificantly to the atmospheric dust load. Furthermore,
Australia does not seem to export mineral dust into the
Pacific oceanin a comparableorder of magnitude as the
North African deserts to the Atlantic Ocean, although
the main vegetation types of Australia are desert and
tracers by wind fields and subgrid-scalemixing statistics extracted from the GISS general circulation model
(GCM).
Dust is removed from the atmosphere by wet and
dry deposition. Wet deposition is parameterized using
shrubland [Matthews,1983].
a scavenging
ratio (the ratio of aerosolmassper unit
In a transport model of mineral dust [Tegen and rain massand aerosolmassper unit air mass)Z = 700
Fung, 1994], where only natural mineral dust sources with high-frequency precipitation statistics applied to
were considered, the seasonal shift of the Saharan dust monthly mean climatologicalprecipitationdata [Shea,
plume could not be reproduced, and the export of min- 1986]. Dry depositionwas describedby gravitational
eral dust from Australia was by far overestimated. This settlingand turbulent mixing as in Genthon[1992].
supports further the hypothesis that the atmospheric
dust load cannot be explained by natural sourcesalone.
To obtain an estimate about which part of the dust
load is caused by wind deflation from undisturbed soil
and and which part originates from source areas influenced by human activity, we derived dust distributions
for several different possiblesourcetypes with the trans-
port model describedby Tegenand Fung [1994].
2.
Mineral
Dust
Model
From this calculated atmospheric dust distribution,
opticalthicknesses
r werederived,usingr = Q•rSUN
andM - •rp• a, whereN isthenumber
of particles,
p is the particle density, 5 is a representative particle
size, and Q is a extinction geometry factor with Q • 2
for large particles [Fouquar!et al., 1987]. We calculated the optical thickness with "typical" particle sizes
of r -- 4/•m for small silt, and r = 0.6/•m for clay, assuming a lognormal clay particle size distribution and
a minimum size of 0.5/•m for individually uplifted clay
particles in the atmosphere. Becauseof increasedadhesive and cohesive forces due to their
increased
surface
The desert dust transport model is describedin detail
area, smaller particles tend to agglomerateand stick to
by Tegenand Fung [1994]. It usesfour differentparti-
larger particles[Gillette et al., 1974]. Thereforewe as-
cle size classeswhich were treated as separate tracers.
sumed that particles smaller than 0.5 pm are not avail-
Particle
able for wind
sizes were derived from the 1øxl ø data set of
erosion.
soil types and particle sizes[Zobler, 1986; Webbet al.,
1991], where the fractions of three major size classes
3. Dust Source Types
are given as percentagesof total soil massin each grid
box. The size classesare sand (particle radius larger
Given sufficientlylow soil moistureand high surface
than 25-pm diameter),silt (particleradiusbetween1 wind speed,dust productioncan originatefrom various
and 25pm), and clay(particlessmallerthan lpm).
sources.Dust deflationcan occurin desertsor sparsely
We assumed dust mobilization to be possiblewhen vegetatedareas,in cultivatedland outsideof the growthe soilmatricpotentialis higherthan 104J/kg, using ing season,or in areaswherethe soil surfaceis freshly
monthly mean soil water contentsas derivedby Bouw- exposedto wind erosionafter the vegetationcoveris
man et al. [1993].
removed. The sourcestrengthfor dust production,as
Several empirical studies show that the amount of well as the size distributionof the dust uplifted, may
uplifteddustfollows[Gillette,1978],
vary for different sourcetypes.
In the following, we separate dust sourceareasinto
qa: C(U- utr)u2,
(1)
two main classes:
"natural"
and "disturbed".
"Nat-
TEGEN AND FUNG:MINERAL DUST FROM DISTURBEDSOILS
ural" sources are assumed to have been acting for a
long time, like deserts,whereas"disturbed"sourcesare
thoseinfluencedby humanactivity (like cultivatedor
deforested
areas)or by climatevariability(like the Saharan/Sahelianboundaryshifts).
Dust flux from disturbed sources may be stronger
than
dust flux from
natural
sources for two main
rea-
sons: freshly exposedsoil can contain more fine material
18,709
The contemporary distribution was obtained by sub-
tracting the percentageof cultivated area [Matthews,
1983]from each løx 10 grid box designatedas a likely
natural source. In grid boxes where no cultivation is
recorded, three caseswere investigated: the fraction of
soil area that
can act as dust source was assumed to be
10%(NS1), 50% (NS2), and 100%for Asiansourcesand
50% for other continents(NS3). Satellite observations
like silt (which contributesthe major part of mineral that were used for constraining the dust export from
aerosol)than "aged"soil surfaces,where fine material the arid regionsgive only information about aerosolopis likely to have been blown out already. For exam- tical thicknesses over the ocean, that is, dust export
ple, stabilized sand dunes do not act as dust sources from North Africa and Australia. In Asia, large loess
[P•/e, 1987].Also, in cultivatedareasthe soilsurfaceis deposits exist that may act as stronger dust sources
in general disrupted by agricultural practices, in which
casea lowerthresholdsurfacewind speedis suflqcient
to
start dust deflation compared to undisturbed soil sur-
faces[Gillette, 1978;Gillette et al., 1980].
Because
of the
short
lifetime
of dust
in the
atmo-
sphere, we further divided "disturbed" sourcesaccording to their age into "old" and "recent" sources. By
old sources,we mean areas where erosion is reported in
the last 50 years, while recent sources are areas where
changesin soil surface conditions occurred during the
last decade. "Old, disturbed" areas are further subdivided into two: those where erosionwas, or was not, as-
compared to the sandy soils of the Sahara, this differencemight be underestimatedby the soil texture data
set [Zobler, 1986]which was usedto describethe ini-
tial soil particle size distribution. Condition NS3 was
chosento take the possibility into account that Asian
desertsmight be more productivedust sourcesthan the
North
Old
African
or Australian
disturbed
deserts.
sources
Middelton[1992]and WorldResources
Institute[1992]
describeregionswhere the soil is degradedas a result
of deforestation,overgrazing,agricultural activities and
sociated with cultivation.
"Recent disturbed"
areas are
overexploitationof vegetation for domesticuse. They
subdivided
into three: those associated with the Sahaalso report the percentage of area on each continent
ran/Sahelianboundaryshift.,recentlycultivatedareas, where the topsoil is affected by wind erosion. From the
and recentlydeforestedareas(but where cultivationis mapsgivenin Middelton[1992],we identifiedcountries
where deflation of topsoil is reported. The percentage
not reported).
The above classification
results in six different dust
of area per grid box affectedby erosionwas chosenin a
source types, which are described in detail below. All
way that for each continent, the sum of affected areas
source data sets were calculated in løx 1 ø resolution.
per grid box agreeswith the total area per continent.
Natural
The latitudes
Sources
NS Deserts and sparselyvegetated soils;
Old
Disturbed
Sources
Natural sources (NS)
Desert, grassland, and shrub lands are assumed to
obtain
areas.
The fraction of
cultivated land per 1øx 1ø grid box is given in Matthews
The sourcestrength of each sourcetype is expressedas
the percentageof area of each 1øxl ø grid box is given
that may act as a sourceof mineral dust, given sufficient
low soil moisture and high surfacewind speed.
To
and uncultivated
Cultivated eroded soils (OA).
recently deforested areas.
sources.
soil degradationdue to human activities is reported in
vided into cultivated
RC recently cultivated areas; and
dust
con-
tal sourcestrengths"of this sourcetype with additional
regionalinformation. The erodedsoil areaswere subdi-
RB Saharan/Sahelianboundaryshift;
be natural
180 and 28øN of the African
the Saharan desert. In this way we obtained "continen-
OA cultivated eroded soils; and
OE uncultivated eroded soils;
Recently Disturbed Sources
RD
between
tinent were not included for this dust source type, as no
the
[1983] as five classesof cultivation intensity. As the
most recent information used to compile the cultivation
intensity data set was dated in the 1970s, we assumed
that the regionsidentifiedin this data set havebeen disturbed for over 20 years. The cultivated areas that are
colocatedwith erosionaccordingto Middelton [1992]
and World Resources
Institute [1992]were includedin
this dust source
class.
distribution
No dust deflation is assumedto take place during the
of the natural source regions, the vegetation data of growing season,when the root systemsare supposedly
Matthews[1983]was usedto excluderegionswith tall holding the soil in place. For an estimation of the time
vegetation(e.g.,forests)aspossibledustsourceregions. periodwhenthe soil in cultivatedareasis bare, the norLithosols, as extracted from the soil type data set of malized
difference
vegetation
index(NDVI) wasused.
Zobler [1986], were also excludedas possiblesources. During photosynthesis,green leavesabsorb incident soThis soil type is defined as consistingof hard rock with lar radiation at visiblewavelengthsstrongly,but absorb
less than 10-cm soil cover, and occur in mountainous
less than 10% radiation
regions(e.g., Himalayas),wheredustproductionis unlikely [Littmann, 1991].
the differencein radiancesin thesespectral regionscan
be used as a measurefor density of green vegetation.
in the near infrared.
Therefore
18,710
NDVI
TEGEN
is defined
AND FUNG: MINERAL
DUST FROM DISTURBED
The decadalincreasein cultivated area (croplandand
pastures)per countryis givenbetweenthe years1979
and 1989in WorldResources
Institute [1992],and is expressed,for our study, as the percentageof newly culti-
as
N•V•:
where Cysts and C•v•
C•v•R - Cv•s
C•v•R + Cv•s
are reflected radiances at visible
and in the near infrared wavelengths.The NDVI usually rangesfi'om < 0.02 for desertareasto > 0.5 for fully
developedgreencanopies[Funget al., 1987].The NDVI
data were calculated using radiancesmeasuredby the
advancedvery high resolutionradiometer(AVHRR) instrument on board the NOAA series of polar-orbiting
satellites. The mean monthly NDVI values were calculated for the years 1983 to 1987.
A thresholdof NDVI < 0.07 was chosen,abovewhich
the groundcoverby vegetationwasassumedto be dense
enoughto inhibit dust deflation. This value agreeswith
the NDVI valuefor the 200mmyr- • precipitationisoline givenby Tuckeret al. [1991]
Uncultivated
eroded soils (OE).
SOILS
vated land per country between 1979 and 1989. In contrast to the "old" cultivated sources,not only cropland
but also pasture was considered to be cultivated land
in this case. The newly cultivated lands were assumed
to be uniformly distributed in each country, excluding
desertsas identifiedby Matthews[1983]and areaswith
populationdensities
lessthan 1000/kin2.
Recently deforested areas (RD).
Deforestation,
occurring mostly as a result of population pressures,
leavesfreshly exposedsoilsthat may act as dust sources.
The decrease in the area covered by forest per country is given between the years 1979 and 1989 in World
Resources
Institute [1992].As for RC, we obtainedthe
percentageof deforestedland area per country from the
statisticsin World ResourcesInstitute [1992] for the
Uncultivated years 1979-1989. Deforestation was assumed to take
eroded soils are those areas where erosion,but no cultivation, is reported. In these areas, the erosionis likely
place in those areas described as forest or woodland by
Matthews [1983] and where the populationdensityis
causedby overgrazing. Becauselivestockis not exhigherthan 1000/kin2.
plicitly reported in the cultivation data set [Matthews,
As the source types RC and RD are most likely not
1983], we obtained the percentageof affectedarea per independent of each other, we combined them into one
continent for this source type by subtracting the per-
centageof cultivatedarea (OA) from the total eroded
source
type(RL) whichdescribes
landusechange.
We
assumed
that
in countries
where
deforestation
and in-
area.
crease in cultivated area were reported simultaneously
during 1979 and 1989, the deforestedarea is completely
Recently Disturbed Sources
converted into cultivated land. This may lead to an unWe assume that recent dust sources to be those with
derestimation of the recent dust source area, but this
origin in the past 20 years.
error is probably smaller than the error that would be
Saharan/Sahelianboundary shift (RB). Tuck- introduced by assumingthese source types to be inde-
er et al. [1991]describethe Saharan/Sahelianboundary pendentof eachother.
Plate i summarizes the geographicaldistribution of
the
source types described above. The colors indicate
averageshift of the 200mmyr-1 precipitationisoline
the
areas
where the different dust sourcetypes are posfrom about 16øN to 15øN during that time period. It is
shift between the years 1980 and 1990. They report an
unclear whether the Saharan/Sahelianboundary shift
is caused by human activity like overgrazing or a con-
sible.
In the case of the recent sources it is indicated
whether deforestation or conversion into cultivated land
is predominant in the respective country. No indication
We assumed this area to be a potential dust source, about sourcestrengths is given in the map. Overlap of
where 100% of the areas in those 1øxl ø grid boxes different disturbed sourcesoccurs only in a few areas,
such as the Southern United States, Sahel, and Near
which lie within the affected area are assumed to be
dust,sources.In grid boxeswhere alsoerosion(OA and East where both erosion and recent land use changeis
sequence
of natural variability[Tuckeret al., 1991].
OE) is reported,the fractionof erodedsoilper grid box
reported(types 5 and 6 in the map). In thoseareas
is subtracted from this source type so that the source the sourcestrengths of the different types were added,
area can not exceed100%of the grid box if both source assumingthat in those casesthe different sourcetypes
were independent. The error that is introduced by this
types are considered.
combination of sources is likely to be small, because
As the area affectedby the Saharan/Sahelianboundin
general the percentageof areas affected by land use
ary shift is only 6% of the total area of the Sahara, no
changes
per grid box are about an order of magnitude
noticeable
difference in dust distributions
would be obsmaller than the percentageof area affected by erosion.
tained by adding this area to the natural dust sources
Table 1 summarizes for each source type the area
(NS), if the emissionfactorsfor the disturbedand natper continent that may act as a dust source when low
ural sources were the same.
soil moisture and high surfacewind speedoccur. With
Recently cultivated areas (RC). Soil surface this parameterization of the different source types, 10newly disrupted for cultivation may act as dust sources. 67% of area per continent may act as a natural dust
TEGEN AND FUNG: MINERAL DUST FROM DISTURBED SOILS
HIl
18,711
EROSION SOIIIICE TYPES
3O
-3O
I
,I
-6O
-180
I
I
6O
LAND USE CHANGE:
CULTIVATION ß DEFORESTATION
EROSION, NO "OLD" AGRICULTURE
LAND USE •[ANGE:
DEFORESTATION > CULTIVATION
EROSION + "OLD" 3J•IIICULTURE
(OV[JIGRA•ING)
180
SAHARA BOUNDARY SHIFT
NATURAL SOURCES
EROSION, NO "OLD" AGRICULTURE
+ LAND USE CHANGE
EROSION + "OLD" AGRICULTURE
+ LAND USE CHANGE
Plate 1. Geographical distribution of the different sourcetypes of mineral aerosol.
source,this sourcetype is predominantwith respectto 4.
the area. Old sourcescover0.4-4.1% of the continents,
while recent sources cover 0.01 to 2.5 of the continaental
Requirements for Contributions
From Dust Source Types
area. Thesevaluesare upper limits for areasthat might
act as dust source.
Observeddust concentrationsand optical thicknesses
Figures la-le show the annual mean optical thick- originate from several different source types. We atnessesthat are calculated with the transport model tempted to combine the model results obtained from
for eachindividual sourcetype. The sourcestrength the individual source types in a way that calculated
of eachsourcetype was scaledwith an annualglobal dust concentrationsand optical thicknessesmatch obdustsourcestrengthof 3000Mt yr-1 whichis the "best served features. This would yield an estimate of the
guess"global estimatefrom Tegenand Fung [1994] contributionfrom disturbedsourcesto the atmospheric
Figuresla-lc all showrelatively high dust optical thick- dust load.
nessesoverthe North Atlantic oceanwestof the Sahara,
There are only a few estimatesof the production of
over the Pacific ocean east of China, and over the Ara- mineral dust massper continent [e.g., Goudie, 1983;
bian Sea; thesefeaturesshowgeneralagreementwith Duce et al., 1991], and most of them are derivedfrom
satelliteretrievalsof opticalthickness
[Raoe! al., 1988]. sedimentationrates into the ocean [e.g., Sirokko and
18,712
TEGEN
AND FUNG:
MINERAL
DUST FROM
DISTURBED
SOILS
Sarntheim, 1989]. These estimationsdo not include
dust deposited on land relatively close to the source areas. Another problem in the estimation of dust source
strengths is the high variability of dust production in
space and time. Therefore to obtain an estimate of the
contribution of the various source types to the atmospheric mineral dust load, we compared regional seasonal variations resulting from different source distributions
rather
than
total
dust amounts.
We chosethe following requirements that should be
met by the calculated dust distribution.
1.
A well-observed
feature
is the seasonal
shift
of
the Saharan dust plume over the North Atlantic, following the ITCZ. The maximum dust concentrationin
the northern hemisphere summer is observed at 20øN
and at 10øN in northern hemisphere winter. This sea-
sonalshift is observedin ship measurements[McDonald, 1938],satelliteobservations
[Rao et al., 1988],and
measurements
of dust
concentration
at Barbados
and
French Guyana [Prosperoet al., 1981; Prosperoand
Nees, 1986]. Figure 2 showsa latitudinal crosssection
at 20øW of AVHRR optical thicknessretrievals[Ignatoy et al., 1995; Rao et al., 1988]for summer(JJA)
and winter (DJF) 1990. A shift of the maximumoptical
thickness
between
20øN in JJA
and 9øN in DJF
can be seen clearly. The usual explanation for this shift
in the location
of maximum
dust
concentration
is the
changingwind patterns by season[0oft, 1983]. Nevertheless, results from transport models of desert dust
[Joussaume,1990; Wcfers and Jaenicke,1990; Te#en
and Fung, 1994],while showinga certainseasonalshift
in the Saharan dust plume, all show maximum dust
concentrations
around
20 oN rather
than
10 øN in the
northern hemisphere winter, which is in disagreement
with
the observations.
A requirement for our modeled source distribution
was chosen that
the modeled
dust concentration
should
reproduce this seasonal shift. It turns out that dust
distribution from each source type reproduces the dust
maximum at 20øN in northern hemisphere summer.
Therefore the following requirement should be fulfilled
by the modeled dust concentrations:The optical thickness over the North
Atlantic
Ocean
west of the Sahara
in the northern hemisphere winter should be lower at
20øN (r20)than at 10øN (rx0). We calculatedminimum amount of anthropogenic contribution to the atmospheric dust load that is necessaryto meet the following requirements,respectively:
5-
r•o-r2o
•.5(r•0+r20)
T1 o •
> 0
(Conditionla),
T20
5- 0.•(•0+•20)
> 0.7
The
value
of 6 m 1 of condition
(Condition
lb).
lb
is taken
from
the
AVHRR opticalthicknessretrievalsby Rao et al. [1988],
allowing for an optical thicknessof 0.06 due to biomass
burning at 10øN. This value is estimated by using estimations for smoke emissionsfrom burning of forests,
savannahs,and grasslands
by Andreae[1991],and calculating smoke distribution for 1 year with the GISS
TEGEN
AND FUNG-
MINERAL
DUST
FROM
DISTURBED
SOILS
18,713
(a)
,3O
-3O
-9O
-180
-120
-60
0
60
120
180
Longitude
0.05
0.1
0.15
0.2
OpticalThickness
0.25
0.3
(b)
-3O
-9O
-180
-120
-60
0
60
120
180
Longitude
0.05
0.1
0.15
0.2
0.25
0.3
OpticalThickness
Figure 1. Annual meanopticalthicknesses
calculatedwith the transportmodelfor eachindividualsourcetype. (a) Naturalsources
(NS), (b) eroded,cultivatedsoil(OE) (c) eroded,uncultivated
soil(OA) (d) landusechange(deforestation,
newcultivatedareas)(RL) (e) SaharanBoundary
shift (RB).
reportedthere are 15dyr-1, comtracer model. Optical thicknessesof biomass burning storm frequencies
smoke were calculated with the assumption of a parti- paredto 30-60daysyear-1 in variousregionsof Asia
cle densityof i g/cm3 and a mass-averaged
radiusof [Py½,1987]. Optical thicknessretrievals[Reo et el.,
1988]overthe Pacificcloseto Australia usuallydo not
0.3ttm [Penneret el., 1992].
2. Australia is probably not a strongsourceof desert exceed 0.1, compared with more than 0.4 at the locadust compared to other arid regions. Maximum dust tion of the Saharan dust plume over the North Atlantic
18,714
TEGEN AND FUNG- MINERAL DUST FROM DISTURBED SOILS
(c)
60
30
-3O
-60
-9O
-120
-180
-60
0
60
180
120
Longitude
0.05
0.1
0.15
0.2
OpticalThickness
0.25
0.3
(d)
..
60
....
'
.........
.... ..........
o..
.......
...5
.....
3O
:::::::::::::::::::::::::::::::::::::::::::::::::::::::::
!•i•!!!:!
.............-....
:11:.::::::::•::::::::::::::•11•::•:.}..:•:.•i•::.•.
':•!:•:!•i::•ii:::'-'11:.:'1111}:.i•i::
•: ii:::::!::::::i!•:.•::•-:i::i::::::•::11-..:."
--:-:.:-:.-.:
:-:.:.:.:25-:25:-:::•v•'5?•57:'•:j.:.:.:..
:555-55:::•5•-.-':-'=::5=:•22:
.'::.:
.....::::::::::::::::::::::::::::
:.:55::5:-::55:2-5:•5:2:::-:.
' :!?iii•!•i:::
::-::':::
::!:i:ili?..:::::?'-:
....
•!?-•ii::?:?
......
•:;•:•i':•?::
•:'ti
:::•:-•:':""
•:•::-:.:.•:::•:.-.
:---....:•½•:::•:?...-:-::•
......
':::::5•::-:::::?
-.........::-•5.•-':".
'::::2:.:•-
-,•i•o-. '
-
:'•
::::::::::::::::::::::
......
-i•:•i•i::!•::::•::::'!::i::•
....
•iii.?:'::'
:::::::::::::::::::::::::::::
'i%•...:!•.:-•...[; ;'••_
' ',-•
....
i'::.!i:::..:.-:..,:.
.:::::!i::!i•!:•:".
....
...-:-?:!::•!iiii::::::::::ii!:..:'•i:•!i•::
:..,
-3O
-60
-180
-120
-60
0
60
120
180
Longitude
0.05
0.1
0.15
0.2
0.25
0.3
OpticalThickness
lVigure!. (continued)
or the maximum optical thickness over the Arabian sea
in July and August of up to 0.6 and 1. As the absolute
value of dust optical thickness retrieval from satellite
data is uncertain, we applied the following requirement
to the calculated dust optical thicknesses.
tralia, and far is the maximum dust optical thickness
modeled
over the Arabian
Sea.
3. The seasonalvariation of dust storm frequencies
is well
sonal
observed
variations
at several
of the
locations.
mineral
dust
Modeled
load
should
seabe
in agreement with these observations. Observed seap- rar/raus > 3
(Condition2),.
sonal variations in dust storm frequenciesreported by
where ra• is the maximum dust optical thickness over Liftmann [1991]in China (maximum: April), Sahel
the South Pacific in the model grid boxes next to Aus- (maximum:February-April), India (maximum:June),
TEGEN AND FUNG: MINERAL DUST FROM DISTURBED
SOILS
18,715
(e)
60
30
o
-30
-60
-90
-120
-180
-60
0
60
180
120
Longitude
ß
0.05
0.1
0.15
.
0.2
0.25
0.3
OpticalThickness
Figure 1. (continued)
andMiddle
East(maximum:
July-August)
were
com- The
pared to the modeled seasonalva;riationsof dust concen-
trations for the different sourcetypes at thoselocations.
For conditions la and lb to be useful, we must first
establish that the reasons for the model's failure to re-
produce the seasonalshift in maximum optical thick-
nessbetween
10ø'and20øNin i•heAtlanticis caused
by
model winds underestimate the north-south advection and overestimate the east-west advection over the
Sahara. It might be possiblethat more realisticmodel
winds would reproduce the seasonalshift of the Saharan dust plume for desertsources.On the other hand,
the dust model of Wefers and Jaenicke[1990] which
is basedon monthlymean wind fieldsfrom Oort [1983]
an unrealistic sourcedistribution and not by an unre-
alsoshowsa maximum in dust optical thicknessat 20øN
rather than at 10øN in winter for a desertsource.Also,
ures3a and 3b the wind vectorsof the first modellayer the locationof the grid box we usedto calculate6 is acfor the northernhemisphere
winterin the Sahara/Sahel tually located between 8o and 16øN, slightly northward
regionare.comparedwith the corresponding
wind vec- of the maximumopticalthicknesslocatedbetween6o
torsfromobservations
[0oft, 1983].Additionallyshown and 12øNin the AVHRR retrieval.Takingthe weaker
is the isoline .of the optical thickness of 0.3 from the
north-south advection for the model winds into account,
AVHRR satelliteretrievalof aerosolopticalthickness. it can be argued that the air massescarrying maximum
alistic
windfieldin thetransport
calculation.
In Fig-
dust optical thickness have crossed approximately the
same areas in the model
o4
as in the real world.
Therefore
, ß i ß ß , ß ß ! • , i ß ß i ß ß
we assume
--
our transport model. For the month of January, the
dust transport has been calculated additionally with
the conditions
la and
lb can be used with
ß
JJA
ß
the new 4o x 5o versionof the GISS GCM [Druyan
et al., 1994]wherenorth/south advectionof the model
winds over North
O
.1
-90
Figure 2.
-30
0
30
is more realistic.
The results of
that experiment agreedwith the resultsfor the 8o x 10o
tracer model winds. Nevertheless, this point has to be
further addressed in the future, for example, by using
more realistic model winds or validating the assumptions about the origin of the air massesby means of
,
-60
Africa
60
other
tracers.
5.
Experiments
90
Latitude
Latitudinalcrosssectionat 20øW of
AVHRR opticalthickness
retrievalsfor summer(JJA)
and winter (DJF) 1990.
Table
2 summarizes
and Results
results
of the tracer
model
for
the individual
sources.
It givesthe arealextentof the
18,716
TEGEN AND FUNG: MINERAL DUST FROM DISTURBED SOILS
(a)
-40
-20
0
20
40
Longitude
--•
= 7.000e+00
(b)
40
,_•
v
.• 20
-40
-20
0
20
40
Longitude
--•
= 1.200e+01
Figure 3. Wind vectors
of the firstmodellayer(a) for the northernhemisphere
winterin the
Sahara/Sahel
regionandcorresponding
windvectors
fromobservations
[0o•t, 1983](b). The
contourline describes
the isolineof opticalthickness
of 0.3 from the AVHRR satelliteretrievalof
optical thickhess.
"potential"sourceareais an "active"sourceareain the
percentageof total land area. The differencebetween model. In the caseof "old" sources,about 20-25% of
thesepercentages
and the percentage
valuesin Table the "potential"sourcearea is active. includingthe Sa1 is that these values describe the fraction of the area
haran/Sahelianboundaryshift, the recentsourceareas
where the surface wind speed is sufficiently high and are only about20%of the old sourceareas.
-r•o
and p -- Tar/Taus
the soil moisture is sufficiently low to allow dust deThevalues
for 5 - 0.s}•ø•o+T2o)
flation. For most sourcetypes about 10% of the global are also given in Table 2 for the individual sources.
sourcesin the model as well as the areas expressedas
TEGEN AND FUNG- MINERAL DUST FROM DISTURBED SOILS
18,717
Condition la (5 > 0) is not satisfiedfor the natural
sources NS1-NS3, but for all disturbed source types
(0.05 < 5 < 0.53). No sourcetype alonefulfills condition lb (5 > 0.7). This may be due to incomplete
representationof aerosolsfrom biomassburning to the
total aerosol optical thickness: we included the burning of forests, savannas, and grasslandsand did not
considersmokefrom fuel wood burning due to lack of
information.
Therefore
no mixture
of natural
and an-
thropogenic dust will fulfill condition lb. Condition 2
(p > 3) cannotbe fulfilledfor sourcetypesRD (deforestation)or RL (land usechange).Thesesourcetypes
are unlikely to be the only dust sources. For the natural
dust sources,p is 1.9-2.9, slightly below the observed
value of > 3 for (condition2). Thus natural sources
alone cannot meet the required conditions l a and 2.
On the other hand, with exception of RD and RL, all
anthropogenicsourcesfulfill those conditions.
We estimated
the minimum
contribution
from
dis-
turbed dust sourcesto the global atmosphericdust load
necessaryto fulfill the conditions la and 2 for nine scenarios that
are summarized
in Table
3. As we are con-
sideringthe seasonaland regional variation in dust distributions rather than the total dust mass, the source
strengths of experiments were scaled to a total global
dust sourcestrength of 3000Mt/yr for the sum of all
source types.
Experiments 1-3 are combinations of natural sources
NS2 (50% of the areais possibledustsource)and "old"
sourcesOA and OE. Experiments 4-6 are combinations
of NS2 and "recent" sources RD, RL, and RB. Experiments
7-9 are combinations
of natural
with
old and
recentsources.In experiment7 (combinationC1) the
disturbedsourcesincludecultivated(OA) and uncultivated erodedsoil (OE) and land usechange(RL), but
not the Saharan/Sahelianboundary shift. In experiment 8 (C2) the disturbedsoilsincludealsothe Saharan/Sahelianboundaryshift (RB). Experiment9 (C3)
is a combination of disturbed sourcescorrespondingto
experiment8 and natural sourcesNS3 (Asian deserts
being twice as productive as desertsfrom other continents)
o
u•
o4:1
In the experiments with more than one disturbed
source type, these sources were assumed to have an
identicalemissionfactor (1) for eachsourcetype (i.e.,
1-m2 of cultivatedarea producesthe same amount of
dustas 1-m2 deforested
areaat the samewindspeed).
The total sourcestrengths(disturbedplusundisturbed
sources)are scaledto a total annualemissionof 3000Mt
dust.
Figures 4a-4c show the dependencyof the ratio 5 =
TlO -- T20
0.•(•0+•20)
onthepercentage
of dustmassfromdis•,?.o •
turbed sourcesfor each experiment, respectively. From
these curves the necessary minimum contribution from
disturbedsourcesto fulfill conditionla (5 > 0) can be
obtained.
These results are summarized
minimum
disturbance
contribution
in Table 3. The
to fulfill
condition
la liesbetween8 and 88%. The conditionlb (5 > 0.7)
cannot be fulfilled for any source combination.
18,718
TEGEN AND FUNG' MINERAL DUST FROM DISTURBED SOILS
(a)
.0
....
! ....
(b)
! ....
i ....
! ....
,
ß
8'
OA
OF_,
--
.
--
.6
OA+OE
--
.O
.4
RL+RB
.4
.2
0
-.2
-.2
-.4
o.... •'o.... •'o.... o'o
.... do"'ioo
Percent Disturbance
-.4
....
0
Dust
i
....
20
i
....
40
i
i
60
Percent Disturbance
(c)
.0
' ' ß * ! ....
i
.
,
!
,
,
.
80
100
Dust
(d)
! ....
10
! ' ' ' ' ! ....
........................
ß
OA
c1
.8- _ c2
OE
ß
.6
--
OA+OE
!2/2t
.4
.2
-.2
ß
-.4
.
ß
.
,
.
i
.
i
i
20
i
i
i
i
i
4O
i
i
....
60
Percent Disturbance
i
80
.
.
.
.
100
Percent Disturbance
Dust
(e)
10
Dust
(f)
........................
--
RL+RB
4
øo.... 2'o'"'4:'o""do""do"'ioo
Percent Disturbance
o.... 2'o'"'4'o
.... 6'o""•'o'"ioo
Percent Disturbance Dust
Dust
TJ.Q-- T20
Figure4. (a)-(c)Dependency
oftheratio(5- o.•(•-•o+•-2o)
onthepercentage
ofdustmass
from
disturbedsources
for eachexperiment.(d)-(e) Dependency
of p- rar/ra•, on the percentage
of
dust mass from disturbed sourcesfor each experiment.
TEGEN
AND
FUNG'
MINERAL
DUST
FROM
DISTURBED
SOILS
18,719
To fulfill condition2 (p > 3) (Figures4d-4e) at least
5-73%hasto originatefrom disturbedsources
(Table
3). In the caseof experiments3 and 4, the maximum
value for p is below 3, therefore condition 2 cannot
be fulfilled for these experiments. For the experiment
wherewe assumedAsian desertsto be twice as strong
dustsourcesthan desertsfrom other continents(experiment 9), the required minimum contributionof disturbed
dust mass is 33% to meet both conditions
la
and 2; for all other source type the minimum contri-
bution is more than 40%. Although the resultsfor the
different sourcesdiffer widely, they indicate that at least
30% of the atmosphericdust load shouldoriginate from
disturbed sourceswith the chosen source type parameterization.
For the same global emission,the dimensional "emis-
sion factor" C from (1) would be different for each
source type, as each source type has different areal coverage. Table 3 shows the emissionfactors for experiments 1-9 for a contribution
of 50% from disturbed
sourcesto the atmospheric dust load, that is, the natural and the sum of disturbed sourceseach have a global
sourcestrengthof 1500Mtyr- • From measurements
reported by Gillette [1978]and Jaenicke[1988], emission factorsof about 0.5-2/•gs2m-s for undisturbed
soils can be deduced. For a 50% contribution
AAA
;•
¸
AAAA
The emission factors thus obtained
o
AAAAAAAAA
from dis-
turbedto the total dustload,the valueof 0.6/•gs2m-s
for NS2 (50% of area act as dust source)appearsto be
realistic.(If 100%of the undisturbedsourceareaswere
a potential dust source(NS1), this the emissionfactor wouldbe 0.3/•gs2m-s, whichis probablytoolow).
for disturbed
sources
are higher than the natural sourcesby factors of 4-300.
This agreeswith the resultsof Gillette [1978],who observed in wind tunnel experiments that the amount of
deflated dust is about 1 order of magnitude higher for
disturbed soils than for undisturbed soils. The high
valuesof the emissionfactorfor the recentsources(ex-
periments
4-6) of 43-179/•gs2m-s wouldcertainlybe
¸
¸
¸
upper limits, as it is highly unlikely that these sources
are the only disturbed source types.
Consideration
ditional
of the emission
constraint
factor
on the estimate
leads to an ad-
of the disturbance
contributionto the dust load. Westphal[1987] states
that only 10% of the Saharais a potential dust source.If
this condition is applied globally for all natural sources,
emission
factorsbetween0.5 and 2/•gs2m-s wouldresult for global source strengths between 300 and 1000
Mtyr -z Assuminga total globalsourcestrengthof
3000Mt yr-z, 2000-2700Mt dust per year wouldhave
to originate
fromdisturbed'
soil.Thesecalculations
suggest that 60-90% of the atmosphericdust load would
have to originate from other sources,that is, disturbed
soils.
The
seasonal
variations
of modeled
dust
concentra-
tion are shownin Figures 5a-5d at the locations China,
India, Middle East (Ire[n),and Sahel,comparedto observedseasonalvariationsreportedby œittmann[1991].
All source types reproduce the observed seasonal variation reasonably well. This shows that at these sites
18,720
TEGEN
AND FUNG: MINERAL
DUST FROM DISTURBED
SOILS
(a) China
.3O
.3O
.25
' .....
Observations
-
NS
.25
.---.--- OE
o^
g.o
'
RL
.•.
/:!
/ •!/
/ •[/
/ •/
•. \\
•.•
•%
.
•
i
NS
•
[
i
i
i
i
4
i
I
6
i
B
i
10
i
',
i
12
!
ß
' .....
!
....
2
30
ß
4
6
,.----
/.½
d
/",.
I
--'
=.10
•
.0,5
/1' \
I//•
• /'JD
]
,'h
'•
/
."'•.•/i ', i
....
,2:
2
/L
•
4
12
(d) Sahel
./•
.
6
-- OE/
'
•.•. 20 ---
E.20
ß
10
--- Observations
NS.t' ,
.25
¸E
•.16
8
ß••' ii' ' , ß, ß, ß.
Observations
- NS
.25
", \5
Month
(c) Iran
ß
/•',
//•.",
,,,,, /x/,;Z-
Month
.3O
......
',
t
,- -- OE
2O '.--- OA
.06
i
2
-
(b)
India
;,:,
Observations
•.16
• •
•
V'.....,,//
.05
.....
r½t5[ i;F"'--"'
;
,,
'
,-,
/
•\ ',\
\• :,,,•,
"',.,"i
B
10
\
051!tf
0
12
2
,
x%',</,,
.,.
<.
•
4
Month
6
8
I0
12
Month
Figure 5: Seasonalvariationsof modeleddust concentration
at the locations(a) China, (b)
India, (c) Middle East (Iran), and (d) Sahel,comparedto observed
seasonal
variationsreported
by Liftmann [1991]in relative units.
seasonalvariations in dust concentration are causedby Hanson,1989]. Gillette and Sinclair [1990]find that
changingpatterns of surface wind speedand soil mois- about 60% of the dust emission in the United States
ture rather than by the differencein the geographicdis- is causedby local convective
circulations(dust devils).
a dustfluxof about6g/(m2yr)forpartribution of the various sourcetypes. The seasonalvari- Theyestimate
ticles smaller than 215t•m causedby dust devils. This is
ation at those sites therefore cannot be used as an adin agreement with the modeled dust flux from natural
ditional constraint on the contributions
from individual
source types.
sources,
whichis about10g/(m2yr)in NorthAmerica
bined sources,experiment8), respectively.The global
annual dust emissionfor each of both source types is
set to to 1500Mt yr-1 The modeledmajor dustsource
areasin North America, Asia, and Australia are in good
agreement with areas of high dust storm activities reported by for example, Pye [1987]. For the United
for the experiment8 (bestguess),comparedto the seasonalvariationsfor only natural sourcesNS (Figure6c
and 6d). As in all experimentswith dust contribution
in
Plate 2a and 2b shows the resulting modeled dust (only particlessmallerthan 50t•m were considered
the
dust
transport
model).
sourcestrengthsfor the caseof 50% contributionto the
Figures 6a and 6b show the seasonal variation of
dust load from natural soilsand disturbedsoils(commodeled dust optical thicknessesfor the case of 50%
contribution
from disturbed
soils to the total dust load
from disturbed soils, the seasonalshift of the Saharan
dust plume followingthe ITCZ is better reproducedfor
States, the areas of modeled maximum dust emission the best guesscasecomparedto the caseof only natural
from disturbedsoils (parts of Oklahoma,Texas and dust. All experiments reproduce the maximum in dust
Kansas) agree with areas of maximum dust produc- optical thicknessover the Arabian sea during northern
tion between100ø and 105øWreportedby [Gillette and hemisphere summer.
TEGEN
AND FUNG:
MINERAL
ANNUAL DUST EIISSI
I
I
ANNfilL
DUST
FROM
ON FROl
I
I
DIJST ElqI SS I 0N FR0
DISTURBED
SOILS
18,721
NATURAL SO ILS
I
I
D ISTIIRBED S01LS
TEOEN
&}7IN{I,,
JOR
1995
ß00
.05
.10
.15
.20
.25
>0.25
Plate 2. Modeledannualdustemission
from (a) natural and (b) disturbedsoilsfor the caseof
50% contribution from disturbed soils to the total dust load. The global annual dust emission
for eachof both sourcetypesis 1500Mt yr- •.
Figure 7 compares the difference in optical thickness between
winter
and summer
month
at a latitude
of 20øW, as estimated from the AVHRR data product, the model results for only natural sources,and the
bestguessresult (experiment8 with 50% contribution
of dust from disturbedsources). Obviously,the seasonalshift of the Saharan/Saheliandust plume is better reproducedfor the caseof 50% dust from disturbed
sources.
encedby dust aerosol:(30øN, 30øW), (20øN, 30øW),
(10øN,60øE), and (10øS,5øE)in northernhemisphere
summer,(40øN, 150øE)in northernhemisphere
spring,
and (10øN, 30øW) in northernhemispherewinter. The
comparisonof the modeled optical thicknessratios with
the AVHRR optical thicknessratios is shownin Figure
8 for the best guesscase and for only natural sources.
If the optical thicknessratios for the best guessmodel
case are corrected for biomass burning optical thick-
ratio (p) of dustopticalthicknessoverSaudiArabia to
ness, the results are almost linear with the AVHRR
optical thickness ratios. In the case of the model re-
that over Australia, but also the ratios of optical thicknessat several other locations over the ocean to the optical thicknessat Australia during northern hemisphere
winter. The locations chosenwere regions where the
optical thicknesscan be expected to be strongly infiu-
sults for only natural sources,the ratios are generally
smaller than the observations,as the dust optical thicknessover Australia is relatively high for this case. Also
they are not linear with the AVHRR optical thickness
ratios. This supports further the assumptionof a con-
For the best guesscase, we calculated not only the
18,722
TEGEN
AND
FUNG-
MINERAL
DUST
FROM
DISTURBED
SOILS
(a)
90
60
30
o
-30
-60
-90
-180
-120
-60
0
60
120
180
Longitude
0.05
0.1
0.15
0.2
0.25
0.3
OpticalThickness
(b)
30
-30
-60
-90
-180
-120
-60
0
60
120
180
Longitude
0.05
0.1
0.15
0.2
0.25
0.3
OpticalThickness
Figure 6. Modeled dust optical thicknessesfor the case of 150%contribution from disturbed
soilsto the dustloadfor the experiment
8 (bestguess)for (a) December-February
and(b) JuneAugust,andseasonal
variationsfor onlynaturalsources
NS for (c) December-February
and (d)
June-August.
siderable
contribution
from dust from disturbed
sources
to the atmospheric dust load.
Figures 9a and 9b show the annual mean optical
thickness for the best guess case in comparison with
the annual mean of the AVHRR optical thickness data
product(for the year 1990). The comparison
showsthat
the calculated optical thickness correspond well to the
optical thicknessretrievals over the ocean, but alsoemphazises the need of satellite measurements of aerosols
over land. Although few attempts of satellite optical
thickness retrievals
of dust over land have been made
[e.g.,Fraser,1993],theseare limitedto well-knowndust
events and are only possible over dark surfaces.
TEGEN AND FUNG: MINERAL DUST FROM DISTURBED SOILS
18,723
(c)
9.0
30
-30
-90
-180
-120
-60
0
60
120
180
Longitude
0.05
0.1
0.15
0.2
0.25
0.3
OpticalThickness
(d)
30
-30
-180
-120
-60
0
60
120
180
Longitude
:i>2•:..';.'.'.a•'[.'::-x->:.:.:.ii•:.;.a'•:•.::::::
0.05
0.1
0.15
0.2
OpticalThickness
0.25
0.3
Figure 6. (continued)
6.
Conclusions
likely at least 50% of the dust is of disturbedorigin.
Even a higher contribution of disturbed dust would be
in agreementwith the model results. This part of the
ferent source indicate that certain observed seasonal
atmosphericdust load that originates from disturbed
dust distributions can be reproducedonly if disturbed soils,can be interpretedas "anthropogenic"
dust.
sourcesare taken into account. Furthermore, with the
One problem of the dust model experimentsis the as,chosendust sourceparameterizationsthe requirements sumption that dust production in all natural source arfor dust sourcedistributions are fulfilled only if at least eas like desertsand sparselyvegetatedareasis modeled
30%of the dustoriginatesfrom disturbedsources.More usingthe same dependencyon surfacewind speedand
The results from the model experiments with dif-
18,724
TEGEN
AND
FUNG:
MINERAL
DUST
FROM
DISTURBED
SOILS
Difference DJF- JJA
,
ß
i
•
•
'
.
.
i
!
.
!
.
.
i
.
.
i
.
q- Natural sources
•Best guess
AVHRR
!-1
Best
guess,
corrected
-- Model,
natural
source
only
q-
d
q-
i
•
--90 --60 -30
o
0 ' '30' '60' '90
i
I
2
,
i
,
3
i
4
.
i
5
i
i
6
,
i
7
,
i
8
AVHRR Dust Optic.a!Thickness(re!.)
Latitude
Figure 7.
I
1
Comparisonof the differencein optical Figure 8. Comparisonof the optical thicknessratios
(ratiosof optical thicknesses
at severaloceanlocations
thickness between winter and summer months at a lat-
itude of 20øW, as estimated from the AVHRR data
and the optical thickness in the months December-
product,the modelresultsfor only naturalsources,and Februaryabovethe oceannear Australia) of modeled
the bes.tguessresult (experiment8 with 50% contribu- optical thicknessesfor the best guesscase (with and
tion of dustfrom disturbedsources).
without correctionfor biomassburning) and for only
natural sourceswith the AVHRR optical thickness ratios.
soilmoisture.In the real world,areaswith, say,Wadi
sediments
or loessdeposits,act as strongersources
of
dustcompared
to crusteddesertsurface.This regional give a first-order estimate of contributionsfrom differvariability wasnot includedin the parameterizationof
ent large-scaledust sources.
naturaldustsources.Also,dustproductioncausedby
The radiative effect of dust is not well known. As it
humaftactivitieslikeroadbuildingor militarytrafficin isabsorbing
in thermalwavelengths
andabsorbing
and
arid regionswasnot includedin the parameterization
of reflecting
at solarwavelengths,
it willsupposedly
infludisturbeddust sources.Nevertheless,
the calculations enceatmospheric
heatingratesin areaswithhighdust
(a)
90
30
-30
-90
-180
-120
-60
0
60
120
180
Longitude
0.05
0:1
0.15
0.2
0.25
0.3
Figure9. Annual
meanoptical
thickness
for(a)thebestguess
model
result,
and(b)theannual
meanoftheAVHRRopticalthickness
retrievals
fortheyear1990[I#natov
et al., 1995;Raoet al.,
1988].
TEGEN
AND FUNG: MINERAL
DUST FROM DISTURBED
SOILS
18,725
(b)
90
30
-30
-90
-180
-120
-60
0
60
120
180
Longitude
0.05
0.1
0.15
0.2
0.25
0.3
Optical Thickness
Figure 9. (continued)
loading.
If, asourcalculations
indicate,
a largepartof
sensitivityto SST forcing., Int. J. Clim., 1994, in
press.
the dust load is anthropogenic, this effect needs to be
taken into account in estimates of the human impact on Duce, R. A., et al., The atmosphericinput of trace
the Earth's climate. On the other hand, climate change
speciesinto the worldocean,GlobalBiogeochem.
Cywill in turn influence the dust production. Feedback
cles, 5, 193-259, 1991.
mechanismscan be anticipated between climate condi- Fouquart, Y., B. Bonnel, M. Chaoui Roquai, R. Santions and dust production.
ter, and A. Cerf, Observations of Saharan aerosols:
Acknowledgments.
This study grew out of a discussion with J. Prospero at the 1994 Dahlem Conference. The
support of the NASA Office of the Mission to Planet Earth is
acknowledged. This paper is a contribution to the Climate
System History and Dynamics Programme that is jointly
sponsoredby the Natural Sciencesand Engineering Research
Council of Canada and the Atmospheric Environment Service of Canada.
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(ReceivedDecember26, 1994;revisedMay 15, 1995;
acceptedMay 15, t995.)