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Intercontinental PM Transport to North America: Sahara Dust


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Title: Intercontinental PM Transport to North America: Sahara Dust

Intercontinental PM Transport to North America
Sahara Dust

Direct questions to Rudolf B. Husar
Aerosol Types Dust, Smoke and Haze
  • Aerosol are composed of multiple types including
    urban-industrial sulfates, nitrates and organics
    (industrial haze), biomass smoke and windblown
  • Each type may be considered a different pollutant
    since it has its own class of sources, aerosol
    properties and associated with different effects.
  • In this sense dust, smoke and haze are aerosol
    equivalents of the gaseous pollutants, SO2, NOx
    and CO but under the current regulations they are
    lumped under PM2.5 and PM10.
  • This section focuses on the dust portion of the
    North American aerosol.

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 June 2001

Applications of the Study
  • NARSTO-PM Assessment. NARSTO is conducting a PM
    Assessment for North America. This work supports
    the NARSTO PM Assessment process.
  • Monitoring Network Design/Evaluation. EPA is
    implementing an extensive monitoring network for
    speciated PM sampling. This work supports the
    design and performance-evaluation of the new
  • MODELS-3 Evaluation. EPAs MODELS-3/CMAQ is a
    sophisticated high resolution, regional-scale
    modeling system designed to simulate and
    investigate gaseous and fine pattern over the US.
    This work supports the evaluation and further
    development of the model.
  • Regional Haze Management. In response to the new
    haze regulations, Regional Planning Organizations
    (RPOs, Central States, Northeast OTC, Western
    States ) have been set up for haze management.
    This work is to provide background information to
    be used by the RPOs.

Dust Physical, Chemical and Optical Properties
  • Physical - size distribution and shape
  • Determines the atmospheric residence time,
    optical properties
  • Chemical elemental and molecular composition
  • Influences optical properties, fertilizing and
    other effects
  • Serves as source fingerprint
  • Optical refractive index
  • Influences effects on visibility and climate
  • Allows detection by remote sensing

Dust Particle Size and Shape
  • Dust particles are irregularly shaped crystals
  • Virtually all the dust mass is over 1 mm in size
  • The mass mean diameter (MMD) of dust near the
    source is over 5-10 mm
  • However, long range transported dust (3-10 days
    old) has MMD of 2-5 mm
  • Hence local dust is virtually all in the coarse
    mode (gt2.5 mm) while long-range dust has 30-50
    of the mass in the PM2.5 range.

Atmospheric Residence Time of Dust
PM2.5 Residence Time Increase with Height
  • Within the atmospheric boundary layer (the lowest
    1-2 km), the residence time is 3-5 days.
  • Aerosols lifted to 3-10 km (e.g. by deep
    convection, convergence) reside in the atmosphere
    for transported for weeks and many thousand miles
    before removal.
  • Coarse dust particles with 10 and and 100 mm
    size, settle out within 1 day and 15 minutes,
  • Fine dust particles are removed by clouds and

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 Properties
  • To be reviewed (concisely) from the literature

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.

Chemical Differences Between Local Sahara
Dust, Big Bend
  • Differences exist in Al/Si (Sahara- 0.66 Big
    bend 0.55) and in K/Si (Sahara- 0.15 Big Bend
  • Potassium at Big Bend is significantly influenced
    by biomass smoke.
  • For the identification of Sahara dust fraction,
    the Al/Si was used (Sahara Al/Si 0.66 Local
    Al/Si 0.4)

Dust Optical Properties
  • To be reviewed (concisely) from the literature

Dust Transport
  • Atmospheric residence time of dust
  • Transport climatology of North America
  • Dust transport pathways to North America

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.

Transport Pathways to North America
  • Low level westerly winds impinging on the
    Cordillera barrier are mostly deflected, some
    pass through to the Plains.
  • At about 500 N the low-level westerly zonal flow
    divides into northerly and southerly branches
    along the western slopes.
  • There are three main routes for the low-level
    westerlies to cross the Cordillera the most
    notable is the Columbia-Snake-Wyoming Channel.
  • In Mexico, the southward deflected westerlies
    usually do not cross the Sierras.
  • Over the Gulf of Mexico, the low level easterly
    the trade winds are usually deflected northward.
  • South of the Yucatan, the trade winds cross the
    continent and turn southward.

5. Seattle, WA
Seattle, WA is affected by air masses coming
mainly from the west throughout the year.
10. Big Bend, TX
There are large seasonal differences in the
directions that air masses arriving in Big Bend,
TX have taken. During winter and into spring,
they come from the west and the northwest,while
during the summer, they come mainly from the east.
Spatio-Temporal Pattern of Dust Over N. America
  • Annual and seasonal dust map
  • Seasonal pattern at specific stations

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

Seasonal Percentiles Method to Characterize
Source Behavior
  • The seasonal and synoptic (daily) variation of
    the dust concentration can be used to identify
    different dust sources.
  • Each dust source has a unique seasonality, but
    the resulting concentrations are modulated by
    transport and removal processes.
  • The charts depict the magnitude of seasonal and
    synoptic variation as measured by the 20-80
    percentile spread

Dirty days, 80-90
Dirty days, 80-90
Clean days (20)
Clean days (20)
  • At Lye Brook, VT, the clean days (20 percentile)
    corresponds to 4 ug/m3 throughout the year
  • The dirty days are (80-90-ile) have 2-5 times
    higher concentration than the clean days.

At the Smoky Mtn, the clean days in the winter
are also 4 ug/m3. However in the summer, even
the clean days have 14 ug/m3 PM2.5. The dirty
days are have 2-3 times higher than the clean
days through out the year.
Peripheral Sites Fine Soil Percentiles
  • The Fine Dust concentrations show a unique
    seasonality and episodicity (80-20 percentile
    ratio) for each site.
  • At the Everglades NP, FL, the dust concentration
    shows a sharp peak in July (8 ug/m3) and high
  • Big Band NP, TX, shows two distinct fine dust
    peaks (April and July) and a

Central EUS Fine Soil Percentiles
  • Fine dust

Mid-Atlantic Fine Soil Percentiles
  • Fine dust

New England Fine Soil Percentiles
  • Fine Dust

Sahara Dust over North America
  • TOMS and AVHRR Sahara Dust Plume

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.

  • III. Desert Winds
  • Deserts are typically windy places because of
    intense solar heating. The heating goes more to
    sensible heating of the ground than to latent
    heating because of the lack of moisture.
  • These hot surfaces produce superadiabatic lapse
    rates in the lower layers of air. Hence, desert
    regions are areas where great instability occurs,
    leading to vigorous convection, and gusty surface
  • A hot spot on the surface, such as a plowed
    field, may be accompanied by a horizontal wind
    shear caused by a nearby obstacle, leading to the
    ubiquitous dust devil. ( pics)
  • These are microscale systems that rarely cause
    damage, but larger dust devils can have wind
    speeds in excess of 73 km/hr and can cause damage
    to structures.
  • A common storm found in the Southwestern US is
    called a haboob or sand storm. These are
    associated with thunderstorms with gusty
    downdrafts but no rain because it evaporates
    before it hits the ground. These can be 100 km
  • s

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.

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)

Sahara Events over the Eastern US
PM10 in Sahara Dust Events
  • 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.
  • Issue Can this be true????RBH

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.

East Asian Dust over North America
  • TOMS and AVHRR East Asian Dust Plume

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.

Local Dust over North America
  • TOMS Dust Signal over NAM

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)

Seasonality of the TOMS Dust Signal
  • The dust signal is most pronounced in the hot
    summer season

  • Measured Annual PM10 Concentration based on FRM
    PM10 data in AIRS.

Measured Annual PM2.5 Concentration based on the
1999-2000 FRM network. Estimated Annual PMCoarse
Concentration PMCoarse PM10
PM2.5 Difference of the aggregated PM10 and
PM2.5 Note Sampling methods differ estimate
uncertain. The PMCoarse concentration based on
the EPA FRM methods is highest (gt 20 mg/m3) in
the arid Southwest (California, Arizona), in
Colorado/Wyoming and
Colorado Plateau Fine Soil Percentiles
  • Fine dust

Peripheral Sites Fine Soil Percentiles
  • Fine dust

Locally Generated Dust Clouds over N. America
  • Dust clouds are visible from the SeaWiFS satellite
  • Dust clouds emanating from Owens Lake in

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?)