Aerosol Sources and Global Production Rates

Source mechanisms are the main subject of this section. We describe the production of mineral dust, sea salt, and condensation nuclei, then present estimates of global production rates. 

Soils are formed by the weathering of crustal material of the earth. Rocks, stones, and pebbles slowly disintegrate through the action of water, chemically by the leaching of soluble components, and mechanically by the 

Natural soils are polydisperse systems of particles, which are rarely present in the form of loose beds. Nevertheless, if loose particles are available, they will be preferentially mobilized. Other factors that are important are the coverage of the soil surface with roughness elements like pebbles, stubbles, bushes, etc., which partially absorb momentum coherence forces between soil particles due to clay aggregation, organic material, or moisture content and soil texture, that is, the composition of the soil in terms of particle size classes (see Table 7-7). 

Gillette (1980) and Gillette et al. (1980) have presented measured threshold friction velocities for a number of dry soils of different types. The lowest velocities, 0.2-0.4m/s, were found for disturbed soils having less than 50% clay content and less than 20% pebble cover, or for tilled bare soils. At the upper end of the range (s 1.5m/s) were soils with more than 50% clay content and surface crusts or a cover of coarse ( 5cm) pebbles. The corresponding wind speeds when measured about 2 m above ground were 4-8 and 33 m/s, respectively. 

Once the threshold velocity is surpassed, the flux of saltating particles increases rapidly with wind speed. Measurements (Gillette, 1974, 1978) show that the horizontal flux of particles through a plane perpendicular to both ground surface and wind direction increases with u (u - u 0) where u is the friction velocity and u 0 is the threshold value. For large velocities 

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The total rate of aerosol pnoduction is of the order of 2000 Tg/yr. Some comparisons are of interest. Anthropogenic sources, both direct and indirect, contribute about 15% to the global aerosol production rate. The percentage of direct emissions from anthropogenic sources is only about 20% of the total contribution, so that most of it derives from gas-to-particle conversion. With regard to natural aerosol formation, the percentage contribution of direct emissions is 60%, so that direct and secondary aerosol production are approximately equivalent. 

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