

of a maritime environment in which 

 aerosol distribution was made up of 

 sea-salt particles which provide the 

 clouds with giant hygroscopic nuclei 

 for condensation and with possible 

 sites for freezing. The Barbados 

 measurements, however, show that 

 the bulk dust density in the air is 

 greater than the expected concen- 

 tration of sea-salt particles, even near 

 the surface. Additional measure- 

 ments made recently from aircraft 

 near Barbados show that the ice 

 nuclei were as high as 10 ; to 10 4 

 per cubic meter in visibly dusty areas, 

 values that are comparable to those 

 found over the continents. At other 

 times, the ice-nuclei concentrations 

 were found to be negligible in areas 

 of dense haze. These measurements 

 suggest that the ice nuclei are deacti- 

 vated under certain conditions, pos- 

 sibly by surface contamination with 

 Aitken nuclei, water droplets, or 

 some form of pollution. 



Such ambiguities in the physics of 

 ice nuclei and the lack of aerosol 

 measurements in the tropics preclude 

 even an educated guess as to the 

 effect of African dust on the growth 

 of disturbances. At present, argu- 

 ments can be made for either sup- 

 pression or enhancement of cloud 

 growth given an abundant supply of 

 aerosols. 



Much more evidence is required to 

 form a quantitative picture of how 

 much dust is entering the convective 

 clouds associated with the disturb- 

 ances and what the distributions are 

 of ice and condensation nuclei in the 

 cloud environment and the popula- 

 tion of ice crystals and water drops 

 in the clouds. Additional aerosol and 

 dust measurements need to be made 

 along the African coast and by air- 

 craft flying in the vicinity of African 

 disturbances. A more detailed knowl- 

 edge of the vertical distribution of 

 dust and other aerosols should be 

 sought in these flights. If efforts are 

 going to be made to seed disturb- 

 ances, it would be important to know 

 exactly what the background seeding 

 capacity of the environment is during 



a period of exceptionally high dust 

 content in order to estimate the seed- 

 ability of the clouds in these hazy 

 areas. Aerosol measurements of any 

 sort made over Africa itself would 

 be most useful. 



Dust as a Tracer of Air Motion — 

 Besides being an active participant in 

 the condensation and energetics of 

 cumulus clouds, the dust is useful as 

 a tracer of air motions in the trade 

 winds, thereby leading to an under- 

 standing- of the dynamics of air mo- 

 tion at low altitudes. Some tentative 

 evidence exists showing that the 

 dust transport off the African coast 

 is much enhanced by the passage of 

 an African disturbance south of the 

 dust-producing area. Intensely hazy 

 areas, visible on satellite photos, were 

 concentrated immediately to the rear 

 (east) of an African disturbance on 

 two or three occasions in the sum- 

 mer of 1969. In these particular dust 

 outbreaks, the leading edge of the 

 dust mass remained close to the axis 

 of the easterly wave disturbance as 

 it crossed the ocean and passed the 

 island of Barbados. Statistics for the 

 past three years show that the pas- 

 sage of African disturbances by 

 Barbados is accompanied by a sig- 

 nificant diminution in dust loading 

 just prior to its arrival and a marked 

 increase, leading to maximum dust 

 loading, immediately after passage of 

 the wave axis by Barbados. It is not 

 clear whether the disturbance actu- 

 ally prevents the dust from passing 

 the wave. 



Examination of radiosonde data 

 shows that the temperature, stability, 

 and water-vapor content of the air is 

 singularly different in the dusty area. 

 In general, air of high dust content 

 is accompanied by a minimum of 

 cloudiness. This is probably due to a 

 more rapid subsidence of the strong 

 northeasterly trades that are espe- 

 cially susceptible to the raising of 

 dust over the continent and to the 

 increased stability at low levels found 

 in the dusty air, rather than to an in- 

 teraction of the dust with the clouds. 



Chemical, mineralogical, and 

 analysis of the dust is another pos- 

 sible method for determining the 

 origin, composition, and seeding pos- 

 sibilities of the dust. This has been 

 done on a number of selected occa- 

 sions using the Barbados dust sam- 

 ples. The results so far are inconclu- 

 sive, but they do show significant 

 variations in quartz, calcite, iron, and 

 other substances between winter and 

 summer dust. In addition, the lead 

 and zinc content of the summer dust 

 is anomalously high, especially in 

 comparison to the very low amounts 

 of these elements in the winter dust. 

 These two elements owe their abun- 

 dance to industrial contamination, 

 notably fossil fuels. Therefore, the 

 air that carried the dust from the 

 northwestern corner of the Sahara 

 was likely to have been over indus- 

 trial Europe immediately before its 

 arrival over Africa; conversely, the 

 winter dust is carried in an airstream 

 of long-standing duration in the 

 tropics. 



Measurement Techniques and 

 Their Implications 



Radon-222 — Some indirect meas- 

 urements of dust content can be made 

 using radon-222 as a tracer of Sa- 

 haran dust. Radon-222 is liberated 

 from soils in large quantities and is 

 mixed throughout the lower layers 

 of the atmosphere in much the same 

 way as water vapor and dust are 

 mixed from their sources at the 

 earth's surface. Unlike dust, how- 

 ever, radon gas is not washed out by 

 rain. This property (insolubility) can 

 provide a means of studying the 

 washout of dust and the later move- 

 ment of Saharan air after it has 

 passed through a cycle of cumulus 

 convection. 



Thus, radon-222 measured in the 

 high troposphere may be useful in 

 tracing the outflow of dusty air from 

 the tops of cumulonimbus and can 

 lead to a substantiation of the theory 

 that the high concentrations of ice 

 nuclei and dust particles sometimes 



193 



