There exists strong evidence for long-range transport of pollutant aerosols to areas downwind of large anthropogenic sources. The impact of anthropogenic particulate matter can easily be observed, especially in organic-rich top soils. The elements Cd, Se, Pb, Sb, Hg, Ag, Zn, As, Cu, Sn, and Mo belong to the most enriched elements in both, urban particulate matter and organic top soils, which suggests industrialized and densely populated regions as source areas. Urban particulate matter comprises motor vehicle emissions, abrasion of tires and brake linings, paved road dust, abrasion of buildings, fly ashes from coal, lignite and oil combustion, refuse incineration, cement and steel production, dust from crustal weathering, sea spray, plant detritus, and additional sulfur and chlorine compounds of gas-phase reactions. To obtain a more complete impression of the sources and sinks of these materials, the distribution of urban particulates, the mechanisms of their removal from the atmosphere, and first of all the magnitude of anthropogenic sources must be known.
From 1988 until 1991 atmospheric particulate matter was collected in 11 urban areas. In this study spider webs as natural particle traps and air-filter samples have been analyzed for up to 40 elements. Chemical mass balances require a detailed knowledge of the composition of particulates from all major sources. The atmospheric particulate matter collected at monitoring sites can be divided into components, when the contributing sources are characterized by specific marker elements. Particulate matter in urban areas usually originates from direct emissions. If additional sulfur and chlorine compounds of gas-phase reactions are considered, the composition of particulates at the receptor may be regarded as an almost linear combination of concentration patterns of particles from the contributing sources. Since absolute trace-element concentrations only have little influence on the calculation, values have to be normalized. Otherwise, trace elements with low abundance, which might serve as nearly unique tracers for some source types, will be ignored. In this study element concentrations were normalized to the natural abundance in the upper continental earth's crust.
The different receptor models provided the following results: tire dust was responsible for 25% (range: 22-30), soot from
diesel combustion for 24% (11-30) and tar for 14% (1-27) of the anthropogenic particulate load. The wide range of the last two components can be attributed to the small number of available marker elements. The total motor vehicle emissions account for 28%. Soot from gasoline combustion contributed about 4 % to the particle mass. Leaded automobile exhausts can be traced by the receptor model. But the calculation of motor vehicle exhaust suffers from changes in Pb concentration with time as the number of new, catalyst-equipped and older vehicles using leaded fuel is changing. The motor vehicle emissions are accompanied by other vehicular emissions including tire wear, break-lining dust, and paved road dust, especially tar. Our preliminary assessment indicates that components from traffic sources comprise more than 60 % of the particulate matter in urban air. These results are generally valid for a time period before 1992, because the increasing number of catalyst-equipped cars and changing patterns of fuel usage significantly affects the emission characteristics. The major natural source contributing to the total atmospheric particulate concentrations is the earths upper continental crust. The soil dust component (upper continental crust) may represent both, soil particles as well as
materials originating from the abrasion of bricks, which are very similar in their elemental composition. The soil dust source and the abrasion of bricks were responsible for 12% (9-13) of the particulate load, whereas abrasion of cement contributed 6%. About 10% of the mass was attributed to large industrial and municipal combustion processes with the following approximate contribution: 3% stack fly ashes from cement production, 2% from coal and lignite combustion, 1% from steel production, 0.8% from municipal refuse incineration and 0.5% from oil combustion. Sea salt
represents the remaining 0.8%. In densely populated and highly industrialized areas man-made emissions make up more than 85% of the total particulate matter.
Only Cu, Sb, Pb, Zn, and Mo are definitely associated with vehicular emissions. These elements are apparently derived from three major sources: automobile exhausts, tire dust, and brake lining particles. Up to 90% of urban Cu may be explained by
vehicular emissions, especially diesel exhausts. Abrasion of motor-vehicle related surfaces, such as tires and brake linings contribute large amounts of Zn, Sb, Mo, and also Cu. The major fraction of ambient Zn originates from tire dust. The contribution of a brake lining component is based primarily on the presence of Sb, Cu, Sn, and Mo. Motor vehicle emissions account for substantial fractions of As and Cr. By contrast, metals like Cd, Ag, and Co seem to originate from small contributions from a large number of industrial sources and vehicular traffic. The atmospheric concentrations of V and Ni are strongly dominated by oil combustion. The major
fraction of Tl is due to cement production, whereas Sn and Bi are likely associated with refuse incineration. The emission of Pb decreased drastically. The contribution of leaded gasoline to total gasoline burning has dropped from 50% in 1988 to 37% in 1990 and is still declining.
More detailed information on urban particulate matter
composition and an improved data base for rural and forested areas will provide a more complete view of sources and sinks of
anthropogenic particulate matter.