Natural Background Aerosol

The atmospheric aerosol—that is, the particles that are normally found in oiu atmosphere—is a complex and dynamic mixture of solid and liquid particles from natural and anthropogenic sources. We consider first the natural background aerosol—the aerosol that would be present in the absence of human activity. At the other extreme the urban aerosol is dominated by anthropogenic sources. In both cases, primary particles are continuously emitted into, and secondary particles are formed in, the atmosphere. Both kinds of particles may undergo growth, evaporation, or chemical reactions and are subject to various removal mechanisms. The size distributions one finds in the atmosphere reflect the complex interaction of all these processes. We omit from this discussion the largest and most important atmospheric aerosol natural clouds of water droplets. 

Simoneit, B.R.T., and Mazurek, M.A. (1982) Organic matter of the troposphere—II. Natural background of biogenic lipid matter in aerosols over the rural western United States. Atmos. Environ. 16, 2139-2159. 

Due to the considerably different nature of aerosols and their living conditions in different layers of the atmosphere (which are nearly isolated in the sense of energy- and mass-exchange), the atmospheric aerosol is subdivided into tropospheric, stratospheric, mesospheric, etc., aerosols. Its background and disturbing components are, accordingly, differentiated. 

The problem of the global background aerosol has been briefly discussed above. The sophisticated nature and substantial uncertainty of this notion have been mentioned. Apparently, of all the global regions most favourable for studies of GBA the Antarctic continent is the best. 

Away from cities the natural background level of chemical species in Amazon Basin atmosphere is extremely low. Thus, Artaxo Netto et al. (1982a,b) used proton induced X-ray fluorescence analysis (PIXE) to determine that the total concentration of the natural Amazon aerosol was under 10 Pg/m , one of the lowest natural backgrounds ever recorded. This measurement was taken about 60 km north of the busy city of Manaus, confirming incidentally the prevailing east-west direction of the wind. 

Early estimates of source contributions to particulate pollution were based on emission inventories—that is, compilations of mass rates of discharge of particulate matter from vtuious sources. Such inventories by themselves are of limited value in determining quantitatively contributions to the aerosol concentration at a given point, such as an air inonitoring station. Emission inventories make no provision for natural background, or particle deposition between the source and the point of measurement. They also do not account for products of gas-to-panicle conversion, which contribute significantly to the... 

The stratospheric aerosol is composed of an aqueous sulfuric acid solution of 60-80% sulfuric acid for temperatures from — 80 to — 45°C, respectively (Shen et al. 1995). The source of the globally distributed, unperturbed background stratospheric aerosol is oxidation of carbonyl sulfide (OCS), which has its sources at the Earth s surface. OCS is chemically inert and water insoluble and has a long tropospheric lifetime. It diffuses into the stratosphere where it dissociates by solar ultraviolet radiation to eventually form sulfuric acid, the primary component of the natural stratospheric aerosol. Other surface-emitted sulfur-containing species, for example, S02, DMS, and CS2, do not persist long enough in the troposphere to be transported to the stratosphere. 

The submicron aerosol populations in the European background air are variable from location to location. The concentrations and variability of aerosol distributions do, however, show similarities over large geographical areas (Fig. 11). The particle number concentrations are generally lower in more northern and higher mountain locations, naturally as they are generally located farther from the emission areas. 

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