6.6.1. Sources and Sinks of Aerosols

Atmospheric aerosol particles may be emitted as particles (primary sources) or formed in the atmosphere from gaseous precursors (secondary sources). Table 6.6 summarises the estimated recent annual emissions into the troposphere or stratosphere from the major sources of atmospheric aerosol, both natural and anthropogenic, primary and secondary. These include sulphates from the oxidation of sulphur-containing gases, nitrates from gaseous nitrogen species, organic materials from biomass combustion and oxidation of VOCs, soot from combustion, and mineral dust from aeolian (wind-blown) processes (Andreae, 1994). It should be noted that each aerosol flux estimate is a best guess, and uncertainty ranges of 100% are not untypical.

Removal of aerosol mass is mainly achieved by transfer to the Earth’s surface or by volatilisation (Jonas et al., 1995). Such transfer is brought about by precipitation (wet deposition) and by direct uptake at the surface (dry deposition). The efficiency of both these deposition processes, and hence the time spent in the atmosphere by an aerosol particle, is a complex function of the aerosol’s physical and chemical characteristics (e.g. particle size), and the time and location of its release.

For fine sulphate aerosols (0.01 to 0.1m) released into or formed near the Earth’s surface, an average lifetime is typically of the order of several days (Chamberlain, 1991). This time scale is dominated mainly by the frequency of recurrence or precipitation. Conversely, particles transported into or formed in the upper troposphere are likely to remain there for weeks or months because of the less efficient precipitation scavenging (Balkanski et al., 1993). Stratospheric aerosols, formed as a consequence of large volcanic eruptions (section 2.6.3) can remain there for up to one or two years.

Table 6.6. Recent global annual emissions estimates from major aerosols, Mt (after Andreae, 1994)

� fine: <1m diameter; coarse: >1m diameter

Owing to the short lifetime of aerosol particles in the atmosphere, particularly in the troposphere, and the non-uniform distribution of sources, their geographical distribution is highly non-uniform. As a consequence, the relative importance of the numerous sources shown in Table 6.6 varies considerably over the globe. In certain areas of the Northern Hemisphere, for example Europe and North America, industrial sources are relatively much more important.