Global and Local Dust over North America

1
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

2
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
  coverage.
• 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
  regions
• Approach
• Integrate data from surface and satellite
  observations
• Combine spatial, temporal and compositional
  analysis
• 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

3
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

4
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
  range.

5
Chemical Characteristics of Asian Saharan Dust
Average Elemental Rations
Iron/Silicon
Aluminum/Silicon
Potass./Silicon

• 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
  0.08).
• Potassium (at Denali and other locations) is also
  contributed by other sources, most notably
  biomass smoke.

6
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
  Deserts
• 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.

7
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.

8
Airmass Transport to Big Bend, TX
July
April
During winter and into spring, airmasses come
from the west
During the summer, the airmasses come from the
east
9
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.

10
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
  studies.
• Sahara dust has also been frequently photographed
  over the Caribbean by the astronauts.

11
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.

12
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

13
Sahara Dust Transport to N America, July

• Based on TOMS Satellite. Work of Herman,
  Prospero.

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.
  Florida.
• Another patch of absorbing aerosol (local dust?)
  is seen over the Colorado Plateau, well separated
  from the Sahara dust.

14
Sahara and Local Dust Identification at Big Bend,
TX

• 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.

15
Attribution of Fine Particle Dust Local and
Sahara

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

16
Sahara and Local Dust Apportionment Annual and
July
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
  mg.m3
• Throughout the Southeast, the Sahara dust exceeds
  the local source contributions by w wide margin
  (factor of 2-4)

17
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,
  Malm.)
• Seasonal pattern peaks sharply in July when the
  Sahara plume swings to ne North into the
  Caribbean.
• The July average dust declines from 7 ug/m3 in S
  Florida to about 1 ug/m3 in Shenandoah.

18
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
  season.
• The Gobi dust clouds frequently traverse the
  Pacific and and a fraction reaches North America

19
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
data.

• 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.

20
The Asian Dust Event over NAM A Spatial
Perspective
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).

21
The Asian Dust Event over NAM A Long-Term
Perspective

• Evidently, the April 1998 Asian dust event caused
  2-3 times higher dust concentrations then any
  other event during 1988-1998.

22
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
  (Arizona-Idaho)
• 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)

23
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
  larger.
• Western US Dust Bowl.
• Spatially homogeneous dust concentration
  (Arizona-Idaho)
• 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?)

24
Data Sharing through a Global Aerosol Information
Network

• 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 ?