The Transport of African Dust to North America: The Link between African Climate and Air
Quality
Joseph M. Prospero
Rosenstiel School of Marine and Atmospheric Science, University of Miami
4600 Rickenbacker Causeway, Miami, FL, 33149
tel: 305-361-4159
fax: 305-361-4457
jprospero@rsmas.miami.edu
Long-term measurements of aerosols made in the Caribbean, Miami, and Bermuda show that African
mineral dust is a prominent, often dominant, aerosol component over the western Atlantic during much of
the year. These aerosol data, coupled with satellite and meteorological data, suggest that African dust
affects a huge area. Indeed, in the southeastern United States in summer, dust concentrations often exceed
by far those of typical pollutant aerosols such as sulfate and nitrate. A large fraction (about half) of the
dust mass is under 2.5 m diameter, a size class defined by the US EPA as “respirable” particles. For
these reasons there is great interest in possible health impacts and the linkage to African dust source
processes. In this presentation I review these long term measurements and I show how dust transport
across the Atlantic undergoes large year-to-year changes and that these changes are linked to rainfall in
the Soudano-Sahel region of North Africa. Thus climate change in Africa will have an impact on the air
quality in the US. Dust is also believed to play an important role in climate forcing processes. Thus the
variability of dust emissions as a result of changes in climate could provide a mechanism for climatefeedback forcing.
Satellite images often show dense clouds of dust emerging from the west coast of North Africa almost all
year long. Figure 1 shows a typical dust outbreak. African dust is subsequently carried across the Atlantic
in the trade winds. Figure 2 shows a dust outbreak along the coast of Africa; dust from an outbreak that
occurred several days earlier can be seen extending across the Atlantic almost to the coast of South
America. It takes about a week for dust to transit from the coast of Africa to the Caribbean, a distance of
over 5000 km.
The Univ. Miami Aerosol Group has made aerosol measurements on Barbados, the easternmost of the
Caribbean islands, since 1965. We see a strong seasonal cycle in dust concentrations with maximum dust
in the summer months. Figure 3 shows the daily dust concentration over the years 1996-1997. Starting in
the Spring, we see sharp pulses of dust that correspond to the passage of the dust clouds from Africa.
Note that during the summer, dust concentrations are typically a few tens of micrograms per cubic meter
of air but usually every year there are a few dust events that exceed 100 ug/m3. In an urban context, these
would be regarded as extremely high.
Over longer time scales, decades, we see large year-to-year changes in dust concentration. Figure 4 shows
the monthly mean dust concentration over the period 1965-1998. (The Barbados record is the longest
continuous aerosol record in existence.) In Fig. 4 the annual seasonal cycle stands out clearly. We also
note, however, that there are longer-term trends in the record. In general dust concentrations after about
1970 are markedly higher than those in the 1960’s although the record in the 60’s is relatively short. After
1970 we see that there are periods when dust concentrations increase sharply, for example, in the early
70’s, the early 80’s, the late 90’s. These dusty periods correspond to period of intense drought in North
Africa, a drought that began in the late 60’s and early 70’s. More will be said about this later.
Large concentrations of African dust are measured at other sites in the Univ. of Miami Atlantic network.
Measurements in Miami made since 1974 (Fig. 5) show a strong seasonal cycle similar to that in
Barbados, although the dusty period is much shorter, usually only July and August. Also similar to
Barbados, we see decadal changes in dust concentrations. The peak years in Miami in most cases match
those in Barbados. Thus, to the extent that climate processes in Africa affect dust transport, the effects are
seen over a large latitudinal band in the western Atlantic.
In 1997 the EPA established a standard for suspended particles less than 2.5 µm diameter which are
regarded as highly “respirable”. This standard specifies an annual mean not to exceed 15 µg m-3 and a 24hour mean of 65 µg m-3. About half of the dust mass in Barbados and in Miami is in the “respirable” size
range. Thus it would appear that the dust concentrations over the Caribbean and Miami fall in a range that
could be of concern on health issues, especially during the more intense dust transport years.
To further explore the relationship of dust to climate, we looked at the dust concentrations in the trade
winds at Barbados in comparison to rainfall in Africa [Prospero and Lamb, 2003]. We find that Barbados
dust is correlated with rainfall in the Soudano-Sahel region of North Africa, an arid region to the south of
the Sahara. The relationship was best with rainfall in the previous year to the dust collections in Barbados
as seen in Fig. 5. The correlation (R2 = 0.56) is remarkably strong for climate-related processes.
Unfortunately our dust record only extends back to 1965 on Barbados. Rainfall records go back further.
To get an idea of what dust transport was like in the earlier decades of the 20th century, we used the
regression equation shown in Fig. 6 and the longer-term rainfall data. Figure 7 shows the complete Lamb
rainfall data record for the Soudano-Sahel. Also shown are the estimated Barbados dust concentrations
based on the rainfall record. Rainfall was much more plentiful in the 1950’s, a relatively “wet” phase.
During the 50’s dust transport was dramatically lower than in recent decades, only about 25% of that in
the 80’s, a “dry” period.
The variability of dust transport is much higher than the long-term trends in pollution emissions. Figure 8
shows the estimated dust concentration record for Barbados (the same as in Fig. 7) along with the
emissions of SO2 from US sources over the same time period. US emissions show a gradual rise from
1940 to the mid-80’s, roughly doubling, then stabilizing and slowly decreasing. In contrast, dust transport
shows a much larger shorter-term variability and a long term trend that quadrupled concentrations of dust
over the time period that SO2 pollution emissions doubled.
Thus, to the extent that air-quality and health effects can be traced to African dust, the year-to-year impact
will vary according to climate conditions in Africa. The question arises as to how much of the African
dust is “natural” and how much is “anthropogenic”. Various lines of evidence suggest that the large
variability that we observe is not related to direct human impacts (e.g., agriculture carried out in marginal
lands, overgrazing, etc.). A recent study [Prospero et al., 2002] of major dust sources shows that dust is
emitted from specific types of environments: topographical low lands in arid regions that were flooded in
early geological history. Figure 9 shows the regions in North Africa that are the major sources of dust.
One characteristic of these sources is that most are located in extremely remote, harsh environments
where there are very few inhabitants.
On the other hand, if the unusually long (30 year) drought is due to human-induced climate change, then
the increased dust observed in recent decades must be attributed to anthropogenic causes. Air quality
models must be developed which can anticipate the consequences of intermediate term climate variability
and consequent air quality impacts at great distances.
Prospero, Joseph M., Lamb, Peter J., African droughts and dust transport to the Caribbean: Climate
change implications. Science 302: 1024 - 1027, 2003.
Prospero, J.M., et al. Environmental characterization of global sources of atmospheric soil dust identified
with the NIMBUS 7 Total Ozone Mapping Spectrometer (TOMS) absorbing aerosol product. Rev.
Geophys. 10.1029/2000RG000095, 04 September 2002
Barbados Daily Dust
200
180
Dust
160
Dust (ug/m3)
140
120
100
80
60
40
20
Ja
n9
M 6
ar
-9
M 6
ay
-9
Ju 6
l-9
S 6
ep
-9
N 6
ov
-9
Ja 6
n9
M 7
ar
-9
M 7
ay
-9
Ju 7
lS 97
ep
-9
N 7
ov
-9
7
0
Fig. 3
Fig. 1
Fig. 2
50
Mineral Dust (ug/m3)
8
0
2
4
6
8
0
2
4
6
8
0
Ju
l-8
Ju
l-8
Ju
l-8
Ju
l-8
Ju
l-9
Ju
l-9
Ju
l-9
Ju
l-9
Ju
l-9
Ju
l-0
5
J-65
J-66
J-67
J-68
J-69
J-70
J-71
J-72
J-73
J-74
J-75
J-76
J-77
J-78
J-79
J-80
J-81
J-82
J-83
J-84
J-85
J-86
J-87
J-88
J-89
J-90
J-91
J-92
J-93
J-94
J-95
J-96
J-97
J-98
Fig. 4
Jun-Aug: -10x+14
Rainfall
Fig. 7
90
85
95
19
19
80
75
Fig. 8
19
19
19
70
0
19
2000
65
1990
19
1980
60
1970
19
1960
55
-1.5
1950
2
0
19
-5
1940
4
5
50
-1.0
0
6
10
45
5
8
15
19
-0.5
10
20
19
10
D ust con cen tration (ug /m 3 )
0.0
12
25
40
15
14
0.8
Soudano-Sahel Precipitation Index
16
R eg re sse d dus t
U S em ission s
30
19
0.5
Lamb's Sahel - Soudano Rainfall Index
20
previous year
R egre ssed B arb ado s D u st a nd U S Em iss io n of S O2
35
1.0
10
Fig. 6
Fig. 5
1.5
25
15
U S Em ission s of S O2 (Tg S )
30
20
0
-2.0 -1.6 -1.2 -0.8 -0.4 0.0 0.4
Ju
l-8
10
y = -9.77x + 12.80
R2 = 0.56
25
5
6
15
Ju
l-7
20
Ju
l-7
Dust (ug/m3)
25
30
4
30
40
35
30
25
20
15
10
5
0
Ju
l-7
35
June-August Dust Conc (ug/m3)
Mineral Dust (ug/m3)
40
0
35
Miami Mineral
Dust: Monthly
Mean Dust
Miami
Monthly
Mean
Barbados Monthly Mean Dust
45
Fig. 9