A foreign substance mixed with or attached to a given substance to enable the distribution or location of the latter to be determined subsequently. There are several types of tracers which are used: (i) A physical tracer is one that is attached by physical means to the object being traced; (ii) A chemical tracer is a chemical with properties similar to those of the substance being traced with which it is mixed homogeneously; (iii) An @[email protected] is a unique isotope, either @[email protected] or an enriched, uncommon @[email protected] isotope, of the element to be traced; (iv) A @[email protected] is a physical or chemical tracer having @[email protected] as its distinctive property which allows detection at small concentrations and hence after large transport distances. The composition of aerosols in the @[email protected] has been used as a qualitative tracer of air masses. The elemental analyses (determined by @[email protected] @[email protected], @[email protected], etc.) of the aerosols transported from various sources or source regions sometimes have characteristic patterns which are used to define qualitatively the origin of tropospheric aerosols collected in other geographical regions.