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 @I03350@ is a unique isotope, either @R05077@ or an enriched, uncommon @S05900@ isotope, of the element to be traced; (iv) A @R05089@ is a physical or chemical tracer having @R05092@ as its distinctive property which allows detection at small concentrations and hence after large transport distances. The composition of aerosols in the @T06522@ has been used as a qualitative tracer of air masses. The elemental analyses (determined by @N04116@ @A00093@, @X06710@, 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.