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 8 th 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  m in size The near source dust MMD is >5  m Long range tr. (5-10 days) is 2-5  m

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  m 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  m size range.

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

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.Cordillera 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. ( Based on Bryson and Hare, 1974)

8. Airmass Transport to Big Bend, TX JulyApril 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.Westphal 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. climatological Sahara dust has also been frequently photographed over the Caribbean by the astronauts.

11. Sahara Events over the Eastern US The Sahara dust impact on PM10 is not confined to fluke events. In fact, the regional PM10 concentrations over the entire Eastern US (90 th 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’. Based on PM10 data in EPA’s AIRS. Previous work by Prospero, Cahill, Malm … 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.

12. Fine Dust Concentration based on IMPROVEIMPROVE Ref: Sisler & MalmSisler & Malm The year average Fine Dust is highest over Texas and the Gulf States (> 1  g/m3). In Texas and the West, Fine Dust accounts for 10-25% of the Fine Mass. However, in the Northeast, dust account for < 5% of the Fine Mass

13. Sahara Dust Transport to N America, July Based on TOMS Satellite. Work of Herman, Prospero….TOMS 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. Sahara Dust Plume

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 (>10  m). The spring ratio is ~8 which corresponds to smaller size (8-10  m?) 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 maximum annual Sahara dust contribution is about 1  g.m 3 In Florida, the local and Sahara dust contributions are about equal but at Big Bend, the Sahara contribution is < 25%. The Sahara and Local dust was apportioned based on their respective Al/Si ratios. In July the Sahara dust contributions are 4-8  g.m 3 Throughout the Southeast, the Sahara dust exceeds the local source contributions by w wide margin (factor of 2-4) AnnualJuly

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 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 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)

19. Global Scale Dust Transport: The April 1998 Asian Dust EventThe 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 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  g/m 3 On April 29, 1998 the sites show simultaneous sharp rise to 3-11  g/m 3. 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 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) TOMS Absorbing Aerosol Index 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?)

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  m 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 ?