Transcript Intercontinental PM Transport to North America: Sahara Dust

Intercontinental PM Transport to North America:
Sahara Dust
Direct questions to Rudolf B. Husar [email protected]
Aerosol Types: ‘Dust’, ‘Smoke’ and ‘Haze’
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Aerosol are composed of multiple types including urban-industrial sulfates, nitrates
and organics (industrial haze), biomass smoke and windblown dust.
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Each type may be considered a different pollutant since it has its own class of
sources, aerosol properties and associated with different effects.
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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.
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This section focuses on the dust portion of the North American aerosol.
Objectives and Approach to the Study
Background
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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
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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
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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)
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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
network.
MODELS-3 Evaluation. EPA’s 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
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Physical - size distribution and shape
– Determines the atmospheric residence time, optical properties
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Chemical – elemental and molecular composition
– Influences optical properties, fertilizing and other effects
– Serves as source fingerprint
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Optical – refractive index
– Influences effects on visibility and climate
– Allows detection by remote sensing
Dust Particle Size and Shape
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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 (>2.5 mm) while long-range dust has 3050% of the mass in the PM2.5 range.
Atmospheric Residence Time of Dust
PM2.5 Residence Time Increase with Height
Residence Time in the Atmosphere
(Jaenicke)
1 mm ~ 15 days
10 mm ~ 1 day
100 mm ~ 15 min
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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
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Within the atmospheric boundary layer (the
lowest 1-2 km), the residence time is 3-5 days.
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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.
The Characteristic Dust Size:
Inferred from PM2.5/PM10 Ratio
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If most of the total aerosol mass (90+%) is due
to dust, the PM2.5/PM10 ratio is indicative of
the characteristic size.
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In then Virgin Islands during the high
concentration dust events, the PM2.5 accounts
for 38% of the Pm10 mass.
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In Washington State during the April 1998
Asian Dust Event, the PM2.5/PM10 ration was
0.38.
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Assuming a log-normal dust size distribution,
these ratios correspond to a mass median
diameter of about 3-4 mm
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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.
Chemical Properties
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To be reviewed (concisely) from the literature
Chemical Characteristics of Asian & Saharan Dust
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Average Elemental Rations
Sahara
East Asia
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Iron/Silicon
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Al/Si
Fe/Si
Ca/Si
K/Si
Ti/Si
Potass./Silicon
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Aluminum/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.
Chemical Differences Between Local & Sahara Dust, Big Bend
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Sahara
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SW US
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Al/Si
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Fe/Si
Ca/Si
K/Si
Ti/Si
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)
Dust Optical Properties
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To be reviewed (concisely) from the literature
Dust Transport
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Atmospheric residence time of dust
Transport climatology of North America
Dust transport pathways to North America
Local, Sahara and Gobi Dust over N. America
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The dust over N. America originates from local sources as well as from the Sahara and Gobi
Deserts
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Each dust source region has distinct chemical signature in the crustal elements.
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The pattern of different dust contributions varies in space as well as by season, episodicity and
vertical distribution
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New satellite sensors allow monitoring the spatial and temporal pattern of dust events on a
daily basis.
Transport Climatology of North America
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The main transport winds are zonal westerlies at mid-latitudes, zonal tropical winds and north-south excursions
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The dominant geographic features of N. America are the high Cordillera and the eastern Lowlands
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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)
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East of the mountains, the plains allow
unobstructed path to great meridional
excursions: air sweeps southward from the
Arctic and northward from the tropics.
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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.
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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
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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
January
April
July
October
Seattle, WA is affected by air masses coming mainly from the west throughout the
year.
10. Big Bend, TX
January
April
July
October
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
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Annual and seasonal dust map
Seasonal pattern at specific stations
Fine Dust Concentration based on IMPROVE
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Ref: Sisler & Malm
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The year average Fine
Dust is highest over
Texas and the Gulf States
(> 1 mg/m3).
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In Texas and the West,
Fine Dust accounts for
10-25% of the Fine Mass.
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However, in the
Northeast, dust account
for < 5% of the Fine
Mass
Seasonal Percentiles: Method to Characterize Source Behavior
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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
Lye Brook
Great Smoky Mtn.
Dirty days, 80-90%
Dirty days, 80-90%
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.
‘Clean’ days (20%)
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
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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
Badlands
Big Bend
Voyageurs
Acadia
Everglades
Central EUS: Fine Soil Percentiles
Upper Buffalo
Sipsy
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Fine dust
Mammoth Cave
Shining Rock
G.Smoky Mtn.
Mid-Atlantic: Fine Soil Percentiles
Shenandoah
Dolly Sods
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Fine dust
Washington DC
Brigantine
Jefferson
New England: Fine Soil Percentiles
Acadia
Lye Brook
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Fine Dust
Sahara Dust over North America
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TOMS and AVHRR Sahara Dust Plume
Sahara Dust Transport to N America, July
Based on TOMS Satellite. Work of Herman, Prospero….
Sahara Dust Plume
Sahara Dust Plume
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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.
III. Desert Winds
1.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.
2.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.
3.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)
4.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.
5.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.
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Sahara Dust Transport Across the Atlantic
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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, TX
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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.
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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 (>10 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 Sahara
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In Florida, virtually all
the Fine Particle Dust
appears to originate from
Sahara throughout the
year
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At other sites over the
Southeast, Sahara
dominates in July
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The Spring and Fall dust
is evidently of local
origin
Sahara and Local Dust Apportionment: Annual and July
The Sahara and Local dust was apportioned based on their respective Al/Si ratios.
Annual
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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 < 25%.
July
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)
Sahara Events over the Eastern US
Based on PM10 data in EPA’s AIRS. Previous work by Prospero, Cahill,
Malm …
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Scanning the AIRS PM10 database several regional-scale PM10 episodes over the
Gulf Coast (> 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
PM10 in Sahara Dust Events
Satellite Observation of Sahara Dust (SeaWiFS)
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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.
East Asian Dust over North America
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TOMS and AVHRR East Asian Dust Plume
Seasonal and Secular Trends of Sahara Dust over the US
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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.
Asian Dust over North America
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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
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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)
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 EastAsian 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.
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 29 th 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 Perspective
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
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TOMS Dust Signal over NAM
Dust Emitted over over the North America
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TOMS satellite data indicate
elevated Aerosol Absorbing
Index over the Southwestern US
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The TOMS signal is believed to
be due to absorbing dust rather
then absorbing smoke.
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The dust signal is present East
and West of the Rocky
Mountains
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The source of the dust in the
intermountain plateau is not
known. (Daily thermal mixing?)
TOMS Absorbing Aerosol Index
Western US Dust ‘Bowl’.
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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)
Seasonality of the TOMS Dust Signal
Dec
Jun
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Jan
Jul
The dust signal is most pronounced in the hot summer season
Feb
Aug
EPA FRM
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.
PM2.5
The PMCoarse concentration based
on the EPA FRM methods is
highest (> 20 mg/m3) in the arid
Southwest (California,
Arizona), in
Colorado/Wyoming and
PMCoarse
IMPROVE
Colorado Plateau: Fine Soil Percentiles
Tonto
Hopi Point
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Fine dust
Petrified F.
Bandelier
Chiricahua
Peripheral Sites: Fine Soil Percentiles
Sequoia
San Gorgonio
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Fine dust
Glacier NP
Guadalupe
Big Bend
Locally Generated Dust Clouds over N. America
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Dust clouds are visible from the
SeaWiFS satellite
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Dust clouds emanating from
Owens Lake in California
Summary of Dust Pattern over North America
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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.
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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)
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So what is the source of the dust in the intermountain plateau? (Daily thermal mixing?)