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Global and Local Dust over North America


Global and Local Dust over North America R. B. Husar Center for Air Pollution Impact and Trend Analysis (CAPITA) Washington University, St. Louis, MO – PowerPoint PPT presentation

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Title: Global and Local Dust over North America

Global and Local Dust over North America
  • R. B. Husar   Center for Air Pollution Impact
    and Trend Analysis (CAPITA)
  • Washington University, St. Louis, MO
  • Prepared for Presentation at
  • 8th Int. Assoc. of Meteorology and Atm. Sciences,
    IAMAS Assembly
  • Innsbruck, Austria, 10-18 July, 2001

Objectives and Approach to the Study
  • Background
  • There is considerable research literature on the
    dust aerosol pattern and characteristics over
    North America.
  • However, both the recent satellite and previous
    research on North American Dust is fragmented,
    and uneven in spatial, temporal an compositional
  • An integrated assessment of the North American
    dust using the rich literature and the most
    recent data would be most desirable.
  • Objectives
  • Establish the spatio-temporal and chemical
    pattern of the airborne dust over North America
  • Characterize the features of dust from the
    different sources
  • Attribute the dust over NAM to the major source
  • Approach
  • Integrate data from surface and satellite
  • Combine spatial, temporal and compositional
  • Invite the community to actively particulate in
    conducting this open, integrative analysis
  • Status (May 2001)
  • Recent data from several satellite and surface
    sensors were analyzed and presented graphically
  • The data and knowledge from the literature has
    not yet been incorporated
  • An open discussion and interaction with the
    community is to begin in September 2001

Dust Particle Size and Shape
  • Dust particles are irregularly shaped
  • All the dust mass is over 1 mm in size
  • The near source dust MMD is gt5 mm
  • Long range tr. (5-10 days) is 2-5 mm

The Characteristic Dust Size Inferred from
PM2.5/PM10 Ratio
  • If most of the total aerosol mass (90) is due
    to dust, the PM2.5/PM10 ratio is indicative of
    the characteristic size.
  • In then Virgin Islands during the high
    concentration dust events, the PM2.5 accounts for
    38 of the Pm10 mass.
  • In Washington State during the April 1998 Asian
    Dust Event, the PM2.5/PM10 ration was 0.38.
  • Assuming a log-normal dust size distribution,
    these ratios correspond to a mass median diameter
    of about 3-4 mm
  • This is consistent with a variety of literature
    sources the well aged (5-10 days) Sahara and
    Asian dust is in the characteristic 3-4 mm size

Chemical Characteristics of Asian Saharan Dust
Average Elemental Rations
  • Comparison of dust elemental composition at
    Denali NP, AK (Asian dust) and at Virgin Islands
    NP. (Sahara dust).
  • Major differences exist in Al/Si (Sahara- 0.66
    Asian 0.4) and in K/Si (Sahara- 0.15 Asian
  • Potassium (at Denali and other locations) is also
    contributed by other sources, most notably
    biomass smoke.

Local, Sahara and Gobi Dust over N. America
  • The dust over N. America originates from local
    sources as well as from the Sahara and Gobi
  • Each dust source region has distinct chemical
    signature in the crustal elements.
  • The pattern of different dust contributions
    varies in space as well as by season, episodicity
    and vertical distribution
  • New satellite sensors allow monitoring the
    spatial and temporal pattern of dust events on a
    daily basis.

Transport Climatology of North America
  • The main transport winds are zonal westerlies at
    mid-latitudes, zonal tropical winds and
    north-south excursions
  • The dominant geographic features of N. America
    are the high Cordillera and the eastern Lowlands
  • The Cordillera, extensive system of mountain
    ranges stretches from Alaska to Mexico. It is a
    significant obstacle to the zonal westerly and to
    the easterly trade winds.

( Based on Bryson and Hare, 1974)
  • East of the mountains, the plains allow
    unobstructed path to great meridional excursions
    air sweeps southward from the Arctic and
    northward from the tropics.
  • Cold and dry Arctic air, traveling always near
    the surface, may reach central Mexico in a few
    days, arriving there much colder than the normal
    tropical air.
  • Warm and moist tropical air masses penetrate
    northward to S. Canada, generally rising over the
    cooler Arctic or Pacific air layers.

Airmass Transport to Big Bend, TX
During winter and into spring, airmasses come
from the west
During the summer, the airmasses come from the
Sahara Dust Transport Across the Atlantic
  • The transport of Sahara dust across the Atlantic
    to N. America has been studied systematically
    since the late 1800s. More recently it has been
    documented extensively by Prospero and co-workers
  • Currently, the daily pattern of global dust,
    smoke and sulfate is being simulated by dynamic
    aerosol transport models, most notably by
    Westphal at the Naval Research Laboratory. The
    NRL model indicates that the dust layer is
    highest over Africa and subsides as it approaches
    N. America.
  • Data from the LITE space-born lidar instrument
    (above) show that a large fraction of the Sahara
    dust travels across the Atlantic in elevated
    layers (up to 5km).
  • However, surface measurements along the dust
    track also show ground-level dust throughout the
    dust path.

Satellite Observation of Sahara Dust (SeaWiFS)
  • The SeaWiFS satellite provides truecolor images
    of the Sahara dust as it approaches (July 21,
    1998) and covers part of the continent (July 24).
  • Such SeaWiFS and other satellite data allow daily
    dust tracking as well as climatological dust
  • Sahara dust has also been frequently photographed
    over the Caribbean by the astronauts.

Sahara Events over the Eastern US
  • Based on PM10 data in EPAs AIRS. Previous work
    by Prospero, Cahill, Malm
  • Scanning the AIRS PM10 database several
    regional-scale PM10 episodes over the Gulf Coast
    (gt 80 ug/m3)
  • Three such episodes are shown on the right for
    July 5, 1992, June 30, 1993 and June 21, 1997.
  • Speciation data (IMPROVE) show that during the
    events, the fine particle dust exceeds 20 ug/m3.
  • The Sahara dust impact on PM10 is not confined to
    fluke events. In fact, the regional PM10
    concentrations over the entire Eastern US (90th
    percentile) occur in July over the Gulf Coast
  • Hence, Sahara dust is the dominant contributor to
    peak PM10 levels over the Gulf Coast (and over
    the EUS NW Mexico?).
  • The Sisler Malm analysis also shows that Fine
    Dust over the entire US is highest over the
    Sahara impact region.

Fine Dust Concentration based on IMPROVE
  • Ref Sisler Malm
  • The year average Fine Dust is highest over Texas
    and the Gulf States (gt 1 mg/m3).
  • In Texas and the West, Fine Dust accounts for
    10-25 of the Fine Mass.
  • However, in the Northeast, dust account for lt 5
    of the Fine Mass

Sahara Dust Transport to N America, July
  • Based on TOMS Satellite. Work of Herman,

Sahara Dust Plume
Sahara Dust Plume
  • In July (1998) elevated levels of absorbing
    aerosol (Sahara Dust) reaches the Gulf of Mexico
    and evidently, enters the continent .
  • High TOMS dust levels are seen along the
    US-Mexican borders, reaching New Mexico. Higher
    levels also cover the Caribbean Islands and S.
  • Another patch of absorbing aerosol (local dust?)
    is seen over the Colorado Plateau, well separated
    from the Sahara dust.

Sahara and Local Dust Identification at Big Bend,
  • The two dust peeks at Big Bend have different
    Al/Si ratios
  • During the year, Al/Si 0.4
  • In July, Al/Si reaches 0.55, closer to the Al/Si
    of the Sahara dust (0.65-0.7)
  • The spring peak is identified as as Local Dust,
    while the July peak is dominated by Sahara dust.
  • If most of the Coarse Mass (PM10-PM2.5) is dust,
    the CM/FM ratio is indicative of the dust size.
  • In the winter, CM/FM 20, which implies large
    characteristic dust size (gt10 mm). The spring
    ratio is 8 which corresponds to smaller size
    (8-10 mm?)
  • In July, CM/FM dips to 4
  • The July ratio approaches the Sahara dust ratio
    of CM/FM 3.

Attribution of Fine Particle Dust Local and
  • In Florida, virtually all the Fine Particle Dust
    appears to originate from Sahara throughout the
  • At other sites over the Southeast, Sahara
    dominates in July
  • The Spring and Fall dust is evidently of local

Sahara and Local Dust Apportionment Annual and
The Sahara and Local dust was apportioned based
on their respective Al/Si ratios.
  • The maximum annual Sahara dust contribution is
    about 1 mg.m3
  • In Florida, the local and Sahara dust
    contributions are about equal but at Big Bend,
    the Sahara contribution is lt 25.
  • In July the Sahara dust contributions are 4-8
  • Throughout the Southeast, the Sahara dust exceeds
    the local source contributions by w wide margin
    (factor of 2-4)

Seasonal and Secular Trends of Sahara Dust over
the US
  • Daily dust levels at 6 IMPROVE sites over the SE
    US were averaged to indicate regional values.
  • Regional Sahara Dust events occur several times
    each summer (as shown by Prospero, Cahill,
  • Seasonal pattern peaks sharply in July when the
    Sahara plume swings to ne North into the
  • The July average dust declines from 7 ug/m3 in S
    Florida to about 1 ug/m3 in Shenandoah.

Asian Dust over North America
  • Multi-year satellite data from the AVHRR sensor
    shows the springtime Asian aerosol plume
  • In the middle of the Pacific, the Equivalent
    Aerosol Optical Thickness (EAOT) in the plume is
    about 0.3
  • Dust is a contributor to the EAOT plume along
    with biomass smoke and industrial (sulfate haze)
  • Asian dust is generated over the Gobi desert and
    its surrounding.
  • The dust storms are most frequent in the spring
  • The Gobi dust clouds frequently traverse the
    Pacific and and a fraction reaches North America

Global Scale Dust Transport The April 1998 Asian
Dust Event
Approximate location of the April 19 dust cloud
over the Pacific Ocean based on daily SeaWiFS,
GMS5/GOES9/GOES10 and TOMS satellite data. Over
the Pacific Ocean, the dust cloud followed the
path of the springtime East-Asian aerosol plume
shown by the optical thickness derived from AVHRR
  • a. GOES 10 geostationary satellite image of the
    dust taken on the evening of April 27.
  • The dust cloud, marked by the brighter
    reflectance covers the entire northwestern US and
    adjacent portions of Canada.
  • A dust stream is also seen crossing the Rocky
    Mountains toward the east.
  • b. Contour map of the PM10 concentration on April
    29, 1998. Note the coincidence of high PM10 and
    satellite reflectance over Washington
  • c. Regional average daily PM10 concentration over
    the West Coast. The sharp peak on April 27-30 is
    due to the Asian dust.

The Asian Dust Event over NAM A Spatial
The PM2.5 dust concentration data from the
IMPROVE speciated aerosol network show virtually
no dust on April 25th, high values over the West
Coast on April 29th and dust further inland on
May 2. Evidently, on April 25th the dust layer
seen by the sun photometers was still elevated
since the surface dust concentration was low.
  • The average PM2.5 dust concentration at three
    IMPROVE monitoring sites over the 1988-98 period
    was well below 1 mg/m3
  • On April 29, 1998 the sites show simultaneous
    sharp rise to 3-11 mg/m3.
  • It would be interesting to perform a long-term
    apportioning the Asian and local dust
    contributions over the West Coast (similar to the
    Sahara impact on the Southeaster US).

The Asian Dust Event over NAM A Long-Term
  • Evidently, the April 1998 Asian dust event caused
    2-3 times higher dust concentrations then any
    other event during 1988-1998.

Dust Emitted over over the North America
  • TOMS satellite data indicate elevated Aerosol
    Absorbing Index over the Southwestern US
  • The TOMS signal is believed to be due to
    absorbing dust rather then absorbing smoke.
  • The dust signal is present East and West of the
    Rocky Mountains
  • The source of the dust in the intermountain
    plateau is not known. (Daily thermal mixing?)

TOMS Absorbing Aerosol Index
  • Western US Dust Bowl.
  • Spatially homogeneous dust concentration
  • Temporally homogeneous pattern (summer peak,
    small episodicity)
  • Relatively low coarse dust concentration - small
    particle size (4-8 um?)
  • Dust is mixing to high elevation (visible in TOMS
    satellite data)

Summary of Dust Pattern over North America
  • NOT a summary just notes
  • The Sahara and Gobi dusts are quite uniform but
    the NAM dust sources vary in composition.
  • The characteristic size dust from Sahara and Gobi
    to NAM is 2-4 mm mmd while the local dust is much
  • Western US Dust Bowl.
  • Spatially homogeneous dust concentration
  • Temporally homogeneous pattern (summer peak,
    small episodicity)
  • Relatively low coarse dust concentration - small
    particle size (4-8 um?)
  • Dust is mixing to high elevation (visible in TOMS
    satellite data)
  • So what is the source of the dust in the
    intermountain plateau? (Daily thermal mixing?)

Data Sharing through a Global Aerosol Information
  • The Internet allows the linking of data producers
    and consumers
  • A catalog the aerosol-related resources would
    allow finding the resources
  • Rudimentary standards for data sharing would
    facilitate data exchange
  • Open, nonexclusive virtual communities would
    foster the production of new aerosol knowledge
  • A Global Aerosol Information Network, GAIN ?