Intercontinental PM Transport to North America: Sahara Dust

1
Intercontinental PM Transport to North America
Sahara Dust

Direct questions to Rudolf B. Husar
rhusar_at_me.wustl.edu
2
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
  dust.
• 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.

3
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 June 2001

4
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
  network.
• 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.

5
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

6
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.

7
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,
  respectively.
• Fine dust particles are removed by clouds and
  rain

8
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.

9
Chemical Properties

• To be reviewed (concisely) from the literature

10
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.

11
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
  0.08).
• 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)

12
Dust Optical Properties

• To be reviewed (concisely) from the literature

13
Dust Transport

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

14
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.

15
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.

16
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.

17
5. Seattle, WA
April
January
July
October
Seattle, WA is affected by air masses coming
mainly from the west throughout the year.
18
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.
19
Spatio-Temporal Pattern of Dust Over N. America

• Annual and seasonal dust map
• Seasonal pattern at specific stations

20
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

21
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.
22
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
  episodicity.
• Big Band NP, TX, shows two distinct fine dust
  peaks (April and July) and a

23
Central EUS Fine Soil Percentiles

• Fine dust

24
Mid-Atlantic Fine Soil Percentiles

• Fine dust

25
New England Fine Soil Percentiles

• Fine Dust

26
Sahara Dust over North America

• TOMS and AVHRR Sahara Dust Plume

27
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.

28

• 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
  winds.
• 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
  wide.

• s

29
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.

30
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.

31
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

32
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)

33
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

34
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.

35
East Asian Dust over North America

• TOMS and AVHRR East Asian Dust Plume

36
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.

37
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

38
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.

39
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).

40
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.

41
Local Dust over North America

• TOMS Dust Signal over NAM

42
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)

43
Seasonality of the TOMS Dust Signal

• The dust signal is most pronounced in the hot
  summer season

44
EPA FRM
IMPROVE

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

PM10
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
PM2.5
PMCoarse
45
Colorado Plateau Fine Soil Percentiles

• Fine dust

46
Peripheral Sites Fine Soil Percentiles

• Fine dust

47
Locally Generated Dust Clouds over N. America

• Dust clouds are visible from the SeaWiFS satellite

• Dust clouds emanating from Owens Lake in
  California

48
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?)