FLUCTUATIONS IN THE ATMOSPHERIC DUST CONTENT ABOVE

FLUCTUATIONS IN THE ATMOSPHERIC DUST CONTENT ABOVE

O ceanology No. 12 (1980)
PL ISSN 0078-3234
CZESŁA W G A RBA LEW SK I
I n s titu te of M eteorology a n d W a te r M an ag em en t
M a ritim e B ran c h — G dynia
FLUCTUATIONS IN THE ATMOSPHERIC DUST CONTENT
ABOVE THE CENTRAL ATLANTIC AND TRANSFER OF DUSTS
FROM THE SAHARA *
Contents:
1. In tro d u c tio n , 2. M ethod, 3. T h e circ u m sta n ce s of th e e x p e rim e n t,
4. R esults, 5. C onclusions; Streszczenie; R eferences.
1. INTRODUCTION
F luctuations in th e dust content above th e central A tlantic are in te­
resting because of th e rem oval of dusts from the Sahara [6, 8— 11], Many
problem s are connected w ith this phenom enon, especially the effect of
optically active aerosols on the radiation budget of th e atm osphere-ocean system. The exceptional cum ulative properties of th e sea as
regards h eat energy and th e large inertia of processes of h eat exchange
w ith the atm osphere probably create a system which causes a „signal
am plifier” effect w ith respect to th e influence of dust content on long-term w eather and clim atic changes. However, th e com plexity of this
system requires above all th e investigation of th e dynam ic characte­
ristics of atm ospheric dust above th e ocean.
These studies w ere u n dertaken during the international GARP
A tlantic Tropical E xperim ent in 1974 (GATE-74). They w ere la ter con­
tinued, not only in order to explain the characteristics of th e conditions
obtaining over th e central A tlantic during th e m easurem ent period, b u t
also to check the m ethods used and to obtain reference dimensions w hich
could be used in the developm ent of sim ilar studies in th e Baltic.
2. METHOD
M easurem ents w ere m ade during GATE-74 on the Soviet research
vessel Musson [7], D uring the m easurem ent period from Ju n e to Sep­
tem ber 1974, the ship was stationed north of the equator in th e central
* T h is stu d y w as accom plished in I n te rd e p a rtm e n ta l P ro g ra m m e I. 15.
A tlantic, m ostly betw een 5° and 12° N and 21° and 23° W (Fig. 1). For
collecting aerosol samples, a four stages cascade im pactor of the au th o r’s
own construction was used. The apparatus contained glass plates w ith
a specially prepared sticky surface, and these w ere exposed to dust for
from one to two hours by pum ping on board an average of 20 m 3 of
fresh air p er sample. (The air was obtained from th e outside of the
ship at a height of about 10 m above sea level). Up to four m easure­
m ents in 24 h w ere made.
The im pactor, specially constructed for these experim ents, was, for
organizational reasons, not fu lly tried out before m easurem ents began.
Its m erits and faults could be estim ated only on the basis of m aterials
collected during GATE-74. Particles of sea-salt, collected together w ith
dust, acted as a kind of standard. The application of th e im pactor on
the A tlantic showed, th a t the nuclei of sea-salt of radius r ^ 1 urn are
alm ost all collected on th e first cascade (gap w idth about 5 mm, distance
from collecting surface 3 mm). On the second cascade from th e inlet
(gap 3 mm, distance 2 mm) no salt crystals could be found w ith a
1000-fold magnification.
Fig. 1. M ap of c e n tra l A tla n tic show ing th e are a s stu d ied
S ta tio n s w h e re m e a su re m e n ts w ere m ade on b oard
Rye.
1.
M apa
środkow ej
K rzy ży k am i oznaczono
m iejsca
części
A tla n ty k u
obszarów
sta cjo n o w a n ia s ta tk u
pom iarów
r/v M usson a re in d icated by crosses
z
lo k alizacją
r/v
M usson
o b ję ty ch
w
okresie
b ad a n ie m
prow adzenia
For the single-cascade im pactor, the value r] — the coefficient
of capture on th e plate surface, was established on the basis of auto­
radiographic m easurem ents of Rn-222 and Po-214 in the air ju st above
the w ater on the Baltic [3], On average, it was 2 X 10—4 cm-2 for Po-214
and 2,6 X 10—5 cm-2 for Rn-222. In the first case, the resu lt refers to
the collection of larger particles under th e influence of in ertia forces,
in the second, to th e capture by diffusion of much sm aller particles
in the th in layer of air in contact w ith th e plate and th eir adsorption
onto the surface of th e plate. The value of rj for radon refers ra th e r to
prim ary aerosol particles (if not to free atoms of this gas), for which
the efficiency of capture in th e im pactor should not exactly depend on
the geom etry of th e apparatus, though at least to the degree to which
it is dependent in th e case of tipical aerosol particles collected in the
same way. Taking th e above into consideration, th e value of r] could
have been estim ated from th e analysis and th e extrapolation shown in
Fig. 2. By w ay of a basis, it was accepted th a t th e n atu ral radioactivity
of th e atm osphere is linked m ainly w ith particles 0,015 — 0,5 M-m in
size and w ith th e modal value of 0,1 mri for a spectrum of these m agni­
tudes, also th a t Rn-222 corresponds to th e low er lim it of this interval,
w hile Po-214 corresponds to the characteristic value of 0,1 M-m [5],
The dushed line has been introduced into th e figure 2 so as to
com pare the value of r| which could have been obtained assum ing a
non-linear dependence of rj on r. A lthough such a dependence results
Fig. 2. A nalysis of th e d e p e n ­
dence of th e coefficient of
c a p tu re (tj) in th e im p a cto r
on th e ra d iu s (r) of aerosol
p a rtic le s
Ryc. 2. A naliza zależności
w spółczynnika w y ch w y tu (rj)
od w ielkości p ro m ie n ia (r)
cząstek aerozolu, p o z w a la ją ­
ca
oszacow ać
efektyw ność
d ziała n ia
zastosow anego
w
p ra c y im p a k to ra
from the analysis of one of th e im pactor exam ples described by G reen
and Lane [4], in our case, analysis of m easurem ent m aterials confirm ed
a generally linear dependence w ithin th e lim its of r covered by the
present studies. In any case, th a t a linear dependence exists w ithin the
range of particle sizes from 10 to 18 nm is shown by the registration
data of the im pactor described by G reen and Lane [4].
The low efficiency of particle capture in th e im pactor caused the
collected samples to be far from saturated. The concentration of p ar­
ticles on th e plates covered quite a wide range of extinction differences.
The increase in capture efficiency w ith the increase in radius was suf­
ficiently great to ensure th e predom inance of coarse — grained p ar­
ticles on the first cascade. The particles collected on th e second cascade
w ere a t least one order of m agnitude sm aller than those on the first
cascade; crystals of sea salt of radius r ^ 1 |im w ere not noted on the
second cascade.
Traces of the dust samples collected on th e glass plates, in the
form of rectangular m a tt spots, w ere then analysed photom etrically.
This analysis was done by m easuring th e reduction of light intensity
after having passed it through th e dusty p arts of th e plates. A P ulfrich
photom eter w ith an Elpho photoelectric attachm ent and an S-53 filter
w ere used. The background correction was carried out by m easuring
the light reduction through unexposed plates prepared in the same way.
As a check for this, parallel light rays w ere passed through th e non-dusty parts of th e plates n ex t to th e spots of collected dust. Only sam •
pies collected on th e first and second cascade w ere suitable for such
analysis. The samples collected on the following two cascades appeared
as very thin threads because of the narrow gaps. Since th e beam of
light in th e P u lfrich photom eter was relatively wide, it was impossible
to take exact m easurem ents of these samples.
The data obtained from th e photom eter readings enabled th e light
extinction value (E) for sam ples of opaque or sem itransparent dusts to
be determ ined. The sea salt particles, being relatively transparent, had
practically no effect on these readings. This is confirm ed by a comparision of th e results of th e photom etric studies of th e aerosol samples
w ith those obtained from microscopic exam ination of th e num ber and
size of th e sea salt crystals collected (Fig. 3). T hat th e aerosol causing
fluctuations in extinction is not derived from th e sea is proved by the
lack of any kind of dependence of these fluctuations on wind velocity.
This is one of th e differences betw een aerosols of m arine and non-m arine origin. The influence of w ind velocity on th e increase in sea
salt concentration has already been confirm ed [5], Thus, w e can say
th a t the m ethod applied here perm itted the registration of concentration
fluctuations of dusts of non-m arine origin, m ostly those removed from
above land. The cosmic com ponent is negligibly small.
Fig. 3. C om parison of th e dep en d en ce of th e se a -sa lt p a rtic le co n c en tra tio n (qa)
in th e n e a r w a te r a ir la y e r a n d of th e re d u c tio n in lig h t in te n sity (E) in aero so l
sam p les on w ind velo city u
C rosses in d ic a te d a ta on sa lt p a rtic le s; circles in d ic a te lig h t e x tin c tio n in sam ples collected
on th e f irs t cascade, d o ts — lig h t e x tin c tio n in sam ples fro m th e second cascade
Rye. 3. P o ró w n an ie zależności k o n c e n tra c ji cząstek soli m o rsk ie j w p o w ietrzu
p rzyw odnym i sto p n ia o sła b ie n ia w iązk i św ia tła w p ró b ac h aerozolu od p ręd k o ści
w ia tru
K rzyżyki odpo w iad ają dan y m u z y sk an y m d la cząstek soli, p u n k ty p u ste — d la e k sty n k c ji
św ia tła w p ró b a ch z e b ra n y ch n a p ierw szej k ask ad zie, pełne — d la e k sty n k c ji w p ró b a ch
z d ru g iej k a sk a d y
3 — O ceanologia n r 12
2
Fig. 4. G ra p h ic al an a ly sis of ac­
cid en ta l e rro r on th e b asis of d is­
p ersio n (oE) of m e asu red v alu es
of AE fo r c h a ra c te ris tic tim e in ­
te rv a ls At
Rye. 4. G ra ficz n a an aliza p rz y p a d ­
kow ego b łę d u n a p o d staw ie d y sp e r­
sji (aE) zm ierzonych w arto śc i AE
d la c h a ra k te ry sty c z n y c h p rze d zia­
łów czasu At
Values of the reduction in light in tensity (E) in aerosol samples
w ere obtained from photom etric readings in relative units. The effect
of accidental erro r on th e m easured results has been defined by the
following equation [9],
° e (At) = «2 (At) + 2 <£
w here aE — th e dispersion of m easured differences in values of E for
sections of tim e At,
o and o0 — the dispersion of real and apparent differences respecti­
vely, caused by th e accidental error.
G raphical analysis of the dispersion cE (Fig. 4) for th e characteristic
variation of dust content in the study area showed, th a t the accidental
erro r was sm all, no greater th an 10%.
3. THE CIRCUMSTANCES OF THE EXPERIMENT
The aerosol conditions off th e coast of A frica during GATE-74 cannot
be taken as norm al, not deviating from th e average conditions obtaining
in these areas. The drought, which lasted from 1968-9 to 1974 favoured
a considerable increase in th e atm ospheric dust content; as the drought
continued th e atm ospheric aerosol content increased th ree fold. Accor­
ding to Savoie and Prosipero [10], during GATE-74 th e average dust
concentration going a t tropical latitudes from th e A frican Coast to the
Sal Island and fu rth er, to th e w est of Barbados, i.e. in an area p racti­
cally free from industrial pollution fl], am ounted to 29.8, 20.5 and
8.1 (Ag • m ~3. Tim ofiejev’s findings [12] on the transparency of th e atm o­
sphere at such latitudes also agree w ith these results. In A ugust and
Septem ber 1974, th e transparency of th e atm osphere was below its
average value. W hile the size and concentration of aerosol particles
generally increased, th ere w ere also considerable variations in th e tra n ­
sparency of the air in time. These variations w ere due to th e rem oval
of dust by the NE Trade Wind blowing from th e A frican continent.
H owever it has already been shown w ith the aid of satellite photo­
graphs [11], th a t clouds of dust carried by the NE Trade W ind from
the Sahara bring about th e greatest increase in atm ospheric dust con­
te n t above the A tlantic. The conditions described w ere reflected in the
results presented in this paper, results obtained using an original photo­
m etric research method. It is difficult to equate them w ith results
achieved using other methods, nevertheless such a com parison g reatly
helps to explain facts and can be im portant for the broadening of
research into the dynam ic of atm ospheric dust content.
4. RESULTS
The m aterial obtained from m easurem ents of th e reduction in light
intensity for samples from th e first cascade is distinguished by large
differences in extinction. The graph (Fig. 5) of th e m easurem ents illu ­
strates the wide range of variation of extinction. The graphical com­
parison of the data for th e central A tlantic w ith those obtained in th e
same w ay for th e Baltic during A pril — Ju n e 1971 serves as a check
for the method. The m ean am plitudes and periods of th e day-to-day
fluctuations in dust content for the two areas differ considerably, w hich
is, of course, th e resu lt of different geographical and circulational con­
ditions. Analysis of the periodicity of th e day-to-day dust content fluc­
tuations for th e central A tlantic has shown, th a t a tim e in terv al of
fluctuations predom inates w hich is a period lasting about 40 hours. In
the spectrum of variations of meteorological values, th e m axim um
frequency is usually around 11.57 m-Hz, which corresponds to th e 24 h
period. The m axim um in th e energy spectrum falls on th e frequency
2.78 nHz, which corresponds to a period of 4.2 X 24 h, i.e. approxim ately
to the variations characteristic for n atu ral synoptic periods. A nalysis
of the day-to day dust content fluctuations thus shows th a t such flu ctu a­
tions of a frequency of approxim ately 6.9 i^Hz which corresponds to a
period of about 1 . 7 X 2 4 h are characteristic for th e central A tlantic.
If the extinction variations during m easurem ents for th e aerosol
sam ples collected on th e first cascade (Fig. 6) are followed, it m ay be
seen th a t the m axim um values are periodically repeated a few tim es
each month. These values w ere com pared w ith those of M artin [6] con-
A £ / £ max
Fig. 5. A n aly sis of d u st c o n ten t flu c tu a tio n s re la tiv e to p erio d s At se p a ra tin g its
e x tre m e v alu es
D ots in d icate d a ta for th e c e n tra l A tlan tic, circles — d a ta fo r th e B altic
Rye. 5. A naliza w a h a ń zapylenia w sto su n k u do o k resó w At d zielących jego
w arto ści e k stre m a ln e
P u n k ty
petne
o d pow iadają
danym
uzyskanym dla
dla B a łty k u
środkow ej
części A tla n ty k u , p u ste
—
cerning th e rem oval of dust from the Sahara to over the A tlantic. These
investigations w ere carried out as p art of GATE-74 on th e basis of
visible light and infrared photographs taken from a satellite. These
m easurem ents covered th e period from 3rd to 8th A ugust 1974. The
occurence of a large dust cloud in the Sahara, to th e w est of th e A haggar
massif, and moving in a w esterly direction, was confirmed.
According to the analysis, th e first discharge of dust, which pro­
bably took place during th e early hours of 3rd August, caused the cloud
to cross the coastal zone of th e A tlantic Ocean w ith th e trad e wind
as a tongue extending tow ards the west. The cloud was particularly
easily visible on the visible light photograph, w hen is was crossing the
coastal zone on 6th August. The analysis of satellite data helped to
establish, th a t by th e following day th e cloud had already spread out
over a large area of the ocean, reaching its central part. The satellite
photographs also confirmed, th a t a second discharge of dust had taken
place in the same region of th e Sahara during th e m orning of 5th August.
If the results of M artin’s analysis are com pared w ith th e results
of the direct m easurem ents made in this study (Fig. 6a), th e following
conclusions m ay be drawn.
1. The two discharges of dust on the Sahara on 3rd and 5th A ugust,
Fig. 6. F lu ctu a tio n s in atm o sp h eric d u st co n ten t o v er th e c e n tra l A tla n tic d u rin g
th e A tla n tic T ropical E x p e rim e n t GATE-74
G rap h s re p re se n t th e re su lts of m ea su re m e n ts m ade on th e basis of lig h t e x tin c tio n values
in sam ples fro m th e f irs t (a) amd second (b) cascade. A t th e top of g ra p h (a), on a n
e n la rg ed tim e scale, d a ta of S a h a ra n d u st d ischarges o b tain e d by D.W. M artin (1975) d u rin g
GATE-74 on th e basis of sa tellite observations (salid line) com pared w ith m ea su re m e n ts
m ade on th e ocean w ith an im p a c to r (dotted lines). T he d ates of observed d ischarges are
m ark e d by arro w s
Kyc. 6. W ah an ia zapylenia po w ietrza nad obszaram i środkow ej części A tla n ty k u
w okresie A tlantyckiego E k sp ery m e n tu T rop ik aln eg o GATE-74
Na p odstaw ie pom iarów e k sty n k c ji św iatła w p ró b a ch z pierw szej (a) oraz z d ru g ie j (b)
ka sk a d y . U góry na w ykresie (a) w zw iększonej sk a li czasu dane z rzu tu p yłu sah ary jsk ieg o ,
uzyskane przez D. W / M artin a (1975) podczas GATE-74 n a podstaw ie obserw acji sa te lita rn y c h
(łam ana linia ciągła), p orów nano z d an y m i pom iarów d o konanych n a oceanie za pom ocą
im p a k to ra (kropkow ane k re sk i poziom e i łączące je proste). D aty obserw ow anych zrzutów
oznaczono strz ałk am i
which w ere discovered after analysing the satellite photographs, may
explain the extinction m axim a (peaks No. 6 and 7 on Fig. 6a) in aerosol
samples collected on 5th and 7th A ugust in th e Atlantic.
2. Analysis of visible light and infrared satellite photographs ena­
bles the boundary of the m ain body of the dust cloud to be draw n
(w here the dust is dense enough). On the other hand the peripheral parts
of the cloud, w here th e dust concentration is less, are impossible to
trace photographically, and m ay be for m ore extensive than th e central
part of the cloud observed on the photographs.
W hen analysing th e variations of contrast of dust cloud spots on
the satellite photographs (Fig. 6a, from 0 to m ax on the upper graph)
alternation is draw n to the effect of cloudiness on th e variations and
on the reduction of th a t contrast during the day time, a link betw een
the m axim um contrast on 5th A ugust and the discharge of dust on the
Sahara noted on the same day nonetheless probably exists. The extin­
ction m axim um corresponding to this (Fig. 6a, m axim um 7) shows, th at
the increase in dust content over the central A tlantic followed two
days after th e discharge on 5th August. The same applies to the dis­
charge on 3rd August, for which the extinction m axim um over the
central areas of ocean was noted on the 5th A ugust (Fig. 6a, m axim um
6). Therefore, the result of photom etric analysis has found definite
confirm ation. It characterises the rate at which the dust cloud spread
under the influence of the trad e wind. The distance from A haggar to
the m easuring station on board r/v Musson in the A tlantic (about 5° N,
23° W) was roughly 3500 km. Taking into account the average tim e of
air filtration on the ocean (15°°h on 5th and ll°°h on 7th August) it is
easy to calculate th a t in both cases the dust cloud moved w ith an
approxim ate speed of 65 km • h ~ 1. This is a speed characteristic for
zonal winds in the free atm osphere, blowing in the north ern p art of
the equatorial zone in the low er stratosphere from east to west. The
duration and distance of spreading of the dust suggests the possibility
th a t the equatorial jet had a hand in its movement.
It appears th a t som ewhat sm aller discharges of dust w ere noted
by M artin [6] at two points in th e Sahara on 29th July. The author
assumes th a t th e sm aller contrastive effect of th e tongues and bands
of dust was in this case caused by greater cloudiness. This is w hy it
was impossible to carry out a m ore precise analysis of the dust clouds
and to reach more certain conclusions about th eir characteristics. N ever­
theless, our m easurem ents showed a sm aller dust content m axim um
relative to the situation described above (Fig. 6a, m axim um 5). This
identification is supported by th e fact that, like th e above two increases
in dust concentration, the m axim um also occured two days after th e
discharge registered on the satellite photographs. Some doubt has arisen
over the presence of two other parallel dust content peaks beside this
m axim um . This is not satisfactorily explained by the analysis of satellite
photographs for the dust discharge in the Sahara of 29th July.
Since we did not have any satellite photographs of the period and
place investigated ap art from those used in M artin’s paper [6], we w ere
unable to verify the rem aining maxima. One can only assum e th a t it
was not so much the cloudiness as w eaker discharges of dust th a t made
a closer study of the event of 19th Ju ly impossible. It took place, appa­
rently, not only in two parts of the desert, b u t also w ith differing in ten ­
sity and during a relatively short time.
From a comparison of the results obtained for the first cascade
of the im pactor (Fig. 6a) w ith those for the second cascade (Fig. 6b)
the following statem ents m ay be made:
— in the second case all peaks are distinctly sm aller,
— the peaks of extinction identified for dust discharges in the Sahara
in samples from the first cascade correspond to the m inim a in th e
samples from th e second cascade.
From the above it can be concluded, th a t coarse-grained dust of
eolitic and Saharan origin over the A tlantic was collected m ainly on the
first cascade. Thus, apart from m axim a 5, 6 and 7, a desert origin m ay
also be indicated by the other peaks in concentration m arked 1 and 10,
because m inim um values on th e second cascade correspond to them . A t
the same time, the possibility can not be excluded, th a t the m axim um
concentrations 3, 4, 8 and 9 shoving a distinct fall in extinction on the
second cascade, are also linked w ith cases of dust discharge from the
Sahara. The reverse case is interesting — the distinct m axim um 2 on
the second cascade corresponding to a m inim um on th e first one. This
indicates an increase in concentration of particles of radius r <C 1 nm
in the second half of June. W hat connection this m ay have the rem oval
of dust from the A frican continent and, possibly w ith the extrem e
northerly position of the intertropical convergence zone at this timfe
of year, is at present difficult to say.
5. CONCLUSIONS
1. The use of a four-cascade im pactor in th e A tlantic has shown,
th a t particles of sea salt of radius r ^ 1 urn are collected almost entirely
on the first cascade. A 1000-fold magnification of th e second cascade
from the inlet did not reveal any sea-salt crystals of radius r ^ 1 ixm.
By analysing photom etrically the light extinction in aerosol samples
it was possible to obtain results referring exclusively to dusts of non-m arine origin for both the first and the second cascades.
2. D uring Ju n e and Ju ly th e increase in extinction for dust dis­
charges in the Sahara identified in samples from th e first cascade
usually correspond to m inim um values m easured irn samples from the
second cascade; this confirm s th e presence of coarse-grained dusts of
eolitic and Saharan origin in the sam ple composition. The inverse case,
w here the reduction in light intensity was due to particles of radius
r < 1 i^m, was valid only for the second half of June.
3. D uring m easurem ents above the central A tlantic, the most fre­
quently occurring period of fluctuation was about 1.7 X 24 hours. In­
creases in dust content took place two days afte r storm y dust discharges
in the A haggar area of th e Sahara and had been recorded on satellite
photographs. The dust cloud w as carried to above th e central A tlantic
at a speed of 65 km p er hour, w hich indicates th e possibility of the
equatorial je t helping in th e m ovem ent of the cloud.
4. A nalysis of th e visible light and in fra-red satellite photographs
perm its only th e boundaries of th e m ain body of th e dust cloud to be
draw n. A m uch m ore extensive p a rt of th e cloud, w here the dust con­
centration is less, can not be discovered photographically using cu rren t
techniques of satellite observation of the E a rth ’s atm osphere.
The au th o r wishes to express his thanks to D r H enryka B erek and
m gr Jozef Kowalewski for th e ir help w ith th e m easurem ents.
O ceanologia n r 12 (1980)
PL ISSN 0078-3234
CZESŁA W G A R B A LEW SK I
I n s ty tu t M eteorologii i G o sp o d ark i W odnej
O ddział M orski — G d y n ia
WAHANIA STOPNIA ZAPYLENIA ATMOSFERY NAD ŚRODKOWYM
ATLANTYKIEM I PRZENOSZENIE PYŁÓW Z SAHARY
S treszczenie
B adano m iędzydobow e w a h a n ia sto p n ia za p y len ia p o w ietrz a przyw odnego
w c e n tra ln e j części A tla n ty k u . P o m ia ry p rzep ro w ad zo n o z rad zieck ieg o s ta tk u
r/v M usson w o k resie A tla n ty ck ieg o E k sp e ry m e n tu T ro p ik aln eg o GATE-74. Z a ­
stosow ano c z te ro k a sk ad o w y im p a k to r oraz m eto d ę fo to m e try c zn e j an a liz y z e b ra ­
n y ch próbek. S tw ierdzono d om inow anie o k resu w a h a ń za p y len ia około 1,7 doby.
Z anotow ano w zro sty za p y len ia n a s tę p u ją c e po u p ły w ie dw óch d ni od o b serw o ­
w an y ch n a zd jęciach s a te lita rn y c h p rzy p a d k ó w b u rzliw eg o zrz u tu p y łu eolicznego
w okolicy A h ag g a r n a S aharze. O błoki p yłu b y ły przenoszone n a odległość około
3500 km do re jo n u p o m ia ró w z p ręd k o śc ią ( ~ 65 km h — c h a ra k te ry sty c z n ą dla
w ia tró w strefo w y ch , w iejąc y ch w północnej części s tre fy ró w n ik o w ej n a pozio­
m ie dolnej stra to sfe ry .
S tw ierdzono, że w spółczesna te c h n ik a zdjęć s a te lita rn y c h w p ro m ie n io w an iu
w id zialn y m i podczerw onym p ozw ala n a u zy sk an ie zary su g ran ic ty lk o głów nej
m a sy obłoku pyłow ego. Część obłoku o m niejszy ch , choć m ierzaln y ch , stężen iach
p y łu przenoszonego n a d ocean z S a h a ry je s t znacznie b ard z iej rozległa.
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