Intercontinental PM Transport to North America: Sahara Dust Rudolf Husar, Washington University

Sahara Dust Transport Across the Atlantic Sahara and Local Dust Identification at Big Bend, TX

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Chemical Characteristics of Asian & Saharan Dust

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Average Elemental

E a s t A s i a A l/S i F e /S i C a /S i K /S i T i /S i

Iron/Silico n

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Potass./Silico Aluminum/Silico n n

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.

Seasonal Average Fine Soil

(VIEWS database, 1992-2002) • • • • • • • 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.

Dust Back Trajectories, July 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.

• • • • 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.

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 Seasonal Pattern of Dust Baseline and Events 1.6

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Events Baseline Total 0.4

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0 01/01/92 02/20/92 04/10/92 05/30/92 07/19/92 09/07/92 10/27/92 12/16/92 • The dust baseline concentration is has a 5x seasonal amplitude from 0.2 to 1 ug/m3 • The dust events (determined by the spike filter) occur in April/May and in July • The two April/May and the July peak in avg. dust is due to the events

Sahara and Local Dust Apportionment: Annual and July

The Sahara and Local dust was apportioned based on their respective Al/Si ratios.

Annual July

• • • Fine soil concentration is highest in the summer over Mississippi Valley, lowest in the winter In the spring, high concentrations also exists in the arid Southwest (Arizona and Texas) Evidently, the summer Mississippi Valley peak is Sahara dust while the Spring peak is from local sources • • 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%. • • 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)