Salt aerosol South Atlantic

During a February 1981 oceanographic cruise in the western South Atlantic Ocean, we used our system aboard the USNS Hayes to sample marine aerosols automatically for 16 days at 1.5-h intervals along the cruise track shown in Figure 1. The sea-salt aerosol concentrations were determined for correlation with wind speed and other relevant meteorological parameters. This chapter identifies the most important variables and the extent to which they affect modeling of salt loads in the marine atmosphere. 

Salt Aerosol Observations. The average salt aerosol concentration for the entire cruise was 21.7 30.9 /xg/m (arithmetic mean and standard deviation, respectively, for 253 samples from the ARCAS). Salt aerosol concentrations were also measured by Prospero 16) from ships in the South Atlantic Ocean at an elevation of 15 m. His overall statistics for 35 samples taken in the tropical and central South Atlantic (5° S to 35° S) are 9.1 5.3 jLtg/m. When he includes five additional samples collected near the Cape of Good Hope, these values become 11.3 9.2 /xg/m. A higher salt... 

Our data set definitely shows high variability for any simple relationship between local wind speed and sea-salt aerosol concentration. This occurrence may only reflect unique meteorological conditions during one summer season because frequency plots of our wind-speed data show values outside the ranges of the expected seasonal averages prevailing in this region of the South Atlantic (21). The data set was also filtered (Table II, Case XII) to exclude samples collected at wind speeds less than 8 m/s. This trial exercise results in an increase in the slope when compared to Case I, but the correlation coefficient is unexpectedly lower. However, this manipulation does not represent true winter conditions because the temperature difference between the sea and the air would be different in the winter than it was for our February cruise. 

A simple physical relationship does not exist between synoptic measurements of wind speed and sea-salt aerosol concentrations in the marine atmosphere because of advection, hysteresis, condensation processes, and the varying stability of the marine boundary layer. In the region of the South Atlantic Ocean discussed in this chapter, the low correlation between the time series for sea-salt aerosol concentration and local wind speed is attributed to the high variability of the effects just mentioned. Removing the temporal constraint by ordering both data sets results in an extremely high (r = 0.99) correlation coefficient. This result provides promise for the... 

Figure 34 represents the average size distribution of sulfate and sea salt particles as observed by A. Meszaros and Vissy (1974) over different parts of the South Atlantic. One can see that on a number basis (solid lines) sulfate particles with much smaller mean radii predominate, while the major proportion of the particle volume (dotted lines) is occupied by the sea salt fraction in the coarse size range. On the basis of the data plotted, only 4 % of the total volume of the particulate matter is composed of sulfates (that total volume is 6.2 //m3 cm 3). This means that the mass concentration of aerosol particles in remote oceanic areas is also controlled by sea salt. 

Inorganic aerosols near the ocean-atmosphere interface are composed of both salt and dust. The source of the dust is the continents. This is indicated by analyses that show the elements present in proportions close to those found for average crustal rocks. Further evidence is obtained from mineralogical investigation. Dust over the equatorial Atlantic has been found to originate in Africa, and dust in the South Pacific was traced to Australia. Little is known about the chemistry of aerosol dusts over the oceans. Significant changes in the dust component may occur... 

---