Oceanic Aerosols

Figure 1a.

A major oceanic aerosol belt stretches over the Pacific just north of the equator (0-20N) as depicted in the aerosol map for the northern spring season. The aerosol belt is rather uniform in width, in aerosol backscattering, and in seasonality, suggesting a common, homogeneous source. The spatial pattern shows an abrupt decline just south of the equator. The seasonal maps, as well as the monthly time chart in Figure 2c shows that the highest backscattering occurs during the spring season, March and April, while the minimum is in the fall. In fact, during September-November, the tropical aerosol belt is hardly discernible from the background. The region is a known source of marine emissions (34). Also, it should be noted that the height of the aerosol layer in this aerosol belt may be greater (> 5km) than over the mid-latitudes (35). A similar oceanic aerosol belt may also exist over the equatorial Atlantic but it is obscured by the strong West African aerosol signal.

Figure 2c.

A significant aerosol belt is evident over the southern mid-latitudes (30-60S). It is most pronounced in the summer map (December-February) and disappears into the low background during the winter. The belt is clearly detached from South America, Africa, and Australia. It is most intense over the southern Indian Ocean (36) which is consistent with the ship-derived haze frequency maps (18). The seasonality within this belt at New Zealand, South Indian Ocean and Southeastern Pacific as shown in Figure 2d is virtually identical: the highest aerosol backscattering is during the summer, January, while in the winter it is an order of magnitude lower. This degree of spatial homogeneity and a consistent seasonal pattern are characteristics of biogenic aerosol emissions over the southern oceans (36).

Figure 2d.

The identification of oceanic aerosol sources over the North Pacific and North Atlantic is somewhat ambiguous since the aerosol patches appear to be "attached" to Asia and North America, respectively. However, based on previous discussions, marine aerosol sources are also likely (33, 37). The seasonality over the Northwestern and Northeastern Pacific is rather similar as shown in Figure 2b; both peak in May and decline by July and August. The North Atlantic, on the other hand, shows a peak that extends from April to August. This may be due to a superposition of the continental haze plume that has a summer peak (32) and the marine source which has a spring peak.

Figure 2b.

From the point of view of anthropogenic impact of industrial aerosol sources it is worth comparing the mid-latitude oceanic aerosol backscattering pattern for the northern and southern hemispheres. In the northern hemisphere most of the anthropogenic emissions are confined to 30-60 latitudes (28), while the southern mid-latitudes have weak industrial and continental emissions. It is evident, from Figure 2e, that the yearly average oceanic northern hemisphere mid-latitudes have about twice the equivalent optical depth of the southern hemisphere. It is interesting to note, however, that most of the excess optical depth occurs during the spring and early summer months (March-June) which is dictated by the seasonality of the Northern Pacific rather than the Northern Atlantic aerosols.

Figure 2e.

Finally, the global and hemispheric aerosol averages are presented on the seasonal chart in Figure 2f. The global average shows only a slight (< 20%) seasonal amplitude, while both the southern and northern hemispheric modulation exceeds a factor of two. Evidently, the hemispheres are not quite symmetric: the northern hemisphere average peaks in April, May, and June, and the lowest values are in November and December while the southern hemisphere has the highest monthly average in January and lowest in May.

Figure 2f.