Chapter outline for the book
Intercontinental Transport of Air Pollution
A. Stohl, Ed, Springer, 2004?
Intercontinental Transport of Dust
Rudolf B. Husar
Center for Air Pollution Impact and Trend Analysis (CAPITA)
Washington University, St. Louis, MO, rhusar@me.wustl.edu
Introduction
This chapter is a brief review of intercontinental dust transport with emphasis on recent
observational evidence. Long-range transport of dust has rich observational history.
Homer – Iliad
Dust plays a major role in several aspects of the Earth System. In geology, dust transport
is responsible in denudation of rocks, soil migration and soil mixing. Dust transport
delivers many of the nutrients to biota on both land and in the ocean. While in the air,
dust changes the weather and climate, both directly by its participation in radiative energy
exchange as well mas indirectly by influencing cloud formation. Atmospheric dust can
also impair human and electronic vision. In extreme dust storms, the airborne dust is a
hazard to human activities, including transportation, human health. With the advance of
monitoring technologies, atmospheric dust can also be used as an biogeochemical
indicator for the Earth System. Droughts, trends in land use and even atmospheric
transport pattern can be inferred from the spatio-temporal pattern of dust.
The 1990s were the beginning of the global observational revolution. Satellite remote
sensing, along with the growth of surface monitoring networks now allow daily globalscale monitoring of dust emissions, transport and spatio-temporal pattern. In fact, over
the recent years real-time observation on the onset, evolution, transport and long-range
impact of dust events has become major topic connecting the global atmospheric science
communities (Husar 2001; Prospero xxx). The combination of source-transport oriented
observations using satellites and backtrajectories with chemical fingerprinting at receptor
provide powerful and compelling tools documenting intercontinental dust transport.
Detailed dust chemical composition data of collected dust samples now allow the
‘fingerprinting’ of dust sources thousands of kilometers away. These developments also
lead to a dramatic improvement in the performance of global dust simulation models.
Still, a full quantitative source-receptor relationship for transcontinental transport of dust
is not in hand. A particularly nagging problem is the estimation of the dust emission rate.
A prudent combination of observation and model-based approaches and global-scale
scientific collabot\ration may provide ‘the answer’. It is hoped that over the next decade,
both the climatological and short-term characterization of long-range dust transport will
be completed by the global atmospheric research community.
Global Dust Emissions
Spatial emission pattern: Arid regions of Africa, Asia, Australia, N.
America
Temporal emission pattern: Long-term, seasonal, synoptic and
diurnal
Compositional pattern
Long Range Transport and Transportation Mechanisms
Trans-continental source receptor relationship is governed by the same processes as local
pollution relationship. The emissions
Fig
Observations show that near the source, dust clouds have two distinct pattern: dust
plumes resemble pollution or smoke plumes where the particulate matter is emitted from
a small point source and dispersed horizontally and vertically as a cone-shape plume by
the intense turbulence of the dust storm. The second near-source dust pattern is the dust
front. At the leading edge of the advancing dust ‘wall’, the dust immediately rises to high
elevations, ready for long –range transport. The synoptic, micrometeorological and dustsuspension mechanisms associated with the two dust storm pattern are not well
understood (does anyone know).
Figure……
Dust transport within the boundary layer is subject to dry and wet removal processes.
This is particularly the case for transport in marine boundary layers. Hence, it is unlikely
that intercontinental dust transport can occur at elevations below 1-2 km. More likely is
that trans-continental transport occurs in free troposphere at 2-10 km elevations where the
transport is swift and cloud removal processes are weak.
The LITE lidar data from the Space Shuttle clearly illustrates that the dust transport from
the Sahara to the Americas occurs in an elevated layer above the scavenging boundary
layer. (LITE, Tratt, Matsouka)
The main transformation process affective dust particles during link range transport is the
loss of large particles by gravitational settling. Since the settling velocity is proportional
to the square large particles are preferentially removed within the first day of transport.
The mass of long-range transported dust is virtually all in the sub-10 nm diameter range.
On the other end, the emitted dust mass is all above about 1 um since strong molecular
forces prevent the mechanical dispersion of sub-micron particles. As a consequence of
settling and mechanical dispersion, the long-range transported dust mass distribution is in
the 1-10 um diameter range, with a mass mode in the 3-5 um range.
Figure ? Typical size distribution of local and long-range transported dust.
The chemical composition of the particles measured thousands of 1000 km from the
source changes very little during multi-day transport. However, some measurements
show that dust particles are often coated with sulfate, particularly in polluted regions such
as the Mediterranean. Cloud processes (gas scavenging and liquid phase oxidation of SO2
is responsible for the sulfate coating. However, sulfate is also found on the dust particles
that have not passed through clouds.
Fig Water Residence time
SRR Emission, Teansport Removal acting together
Ambient Concentration Pattern of Dust
Spatial Pattern
Temporal Pattern
Chemical Patter
Receptor Modeling of Intercontinental Dust Transport
Chemical tracer methods
Inverse chemical-transport modeling
Local and global dust apportionment for North America
Documented Intercontinental Dust Transport Events
Recorded historical dust transport events
Sahara dust transport events to Europe and the Americas
Asian dust transport to N. America
Australian dust transport
Summary and Conclusions