Abstract: The reactions of phenanthrene coated on silica gel or on fly ash with NO₂ are studied. GC and GC/MS analyses show that the main products on siqca gel in dark arc nitro-phenanthrencs, as a result of a dominate substitution reaction, while under UV irradiation 2,2′-diformyl dibenzene, nitroso-2-formyl dibenzene, etc. are formed due to photodegradation. There is a remarkable difference between the reactions on silica gel and on fly ash in dark. It may be the results of competitive reaction of water molecule and phenanthrene with NO₂ on silica gel. Ames test of the reaction Products shows that the non-mutagenic phenanthrene undergoes transformation with NO₂ into direct mutagenic secondary pollutant which is more harmful to the health of human than phenanthrene itself.

Key words: Phcnanthrene, Air pollutant, Photochemical reaction, NO₂