Continental Aerosols over the Oceans

The most prominent aerosol plume is seen over the equatorial Atlantic, originating from West Africa and crossing the tropical Atlantic. It is the well known Sahara dust plume (4, 22). Its westward decay is gradual compared to other plumes and it clearly reaches the Caribbean Sea and Amazon delta, 6,000 km away. The plume is most intense during the warm season (March-August) as shown in Figure 2a. The seasonal map in Figure 1a also shows that in the winter, the plume is shifted south toward the Gulf of Guinea. This points to biomass burning as a further possible aerosol source contributing to this plume (23, 24).

Figure 2a.

Figure 1a.

Another strong aerosol backscattering region is evident over the Arabian Sea with the highest values in the Gulf of Oman. In fact, the Arabian Sea with summer average equivalent backscattering optical depth of 0.4 is the haziest oceanic region of the world. Arabian aerosol shows strong and uniform values during June, July and August in Figure 2a above with abrupt changes prior and after the summer peak. The region is known for intense dust storms (4) and shows up prominently in the global haze maps from ship observations (18).

Significant aerosol plumes are also evident in the southern hemisphere, particularly in the spring season, September, October, November, when the Southwest African plume is clearly separated from the West African plume to the north of the equator. However, the length of this oceanic aerosol plume is about 2,500 km compared to the 5,000-8,000 km long West African plume. Africa south of the equator is known for intense biomass burning (24) and it is a bio-geochemically active region (5, 25). Another major aerosol plume is anchored at the islands of Borneo and Java. During September-November it extends as a coherent plume across the Indian Ocean to East Africa. Biomass burning is the presumed source (26). High aerosol backscattering is also evident over the coastal waters of China during the spring season. In fact, the aerosol belt anchored at China extends across the entire Northern Pacific. However, it is a rather peculiar "plume" in that it shows a strong decline adjacent to the China coast, but virtually no reduction over a 8,000 km range between Japan and the Northeast Pacific. It is not clear whether this is a single continental plume from eastern Asia or if there is an independent oceanic aerosol source over the Northern Pacific. The entire "plume" from China to the Northeastern Pacific exhibits the same seasonal pattern: strong peak in May and a decline toward the cold season in Figure 2b. Hence, the seasonality can not be used to separate the source regions. Continental Eastern Asia is known for its springtime dust sources (4, 27), sulfur and other emissions from industrial sources (28), and for occasional major biomass burning (29).

Figure 2b.

Other aerosols of continental origin are evident off the coasts of Central America, from Mexico to Venezuela. The backscatter signal is most significant during the spring and summer season and virtually disappears during fall and winter. This territory is also known for strong seasonal biomass burning (1, 5, 23). However, over the Caribbean and northeastern South America, the tail of the West African plume is also present (30). A narrow north-south aerosol belt is visible over the northwest coast of South America, adjacent to Peru. This aerosol gradient steeply declines westward. Recognizing the intense biomass burning occurring within South America the absence of aerosol plumes over the surrounding oceans (23, 29, 31) is most conspicuous.

A distinct aerosol plume is apparent emanating from the east coast of North America, extending across the North Atlantic during the spring and summer seasons. The aerosol backscattering shows a summer peak similar to the haze and optical depth pattern over the eastern U.S. (32) The summertime North American plume declines rapidly near the coast, but continues undepleted over the North Atlantic. Hence, it is likely that the aerosol patch over the central North Atlantic arises from oceanic sources, independently from the continental haze plume (33)

During the spring and summer seasons, elevated aerosol backscattering is also recorded over the waters surrounding Europe. The Mediterranean shows the highest values during the summer and there is a gradient increasing toward North Africa. This suggests that Africa is the primary source of backscattering Mediterranean aerosol. Within the continent, the Black Sea shows the highest aerosol backscattering which also peaks during the summer. Anthropogenic emissions of aerosol precursors (7, 28) are the suspected source. In the spring season (Figure 1a), an elevated aerosol patch is apparent over the Atlantic west of Gibraltar. However, it does not appear to be attached to Europe or North Africa.

Figure 1a.