Reaction mechanisms and products of benzo[a]pyrene and benzo[e]pyrene with oxygen and nitrogen oxides

B21

Thermal and photochemical reactions of benzo[e]pyrene (BeP) and benzo[a]pyrene (BaP) with nitrogen oxides and oxygen that take place in polluted atmospheres could produce direct mutagenic compounds. We have determined that, under laboratory conditions, these reactions generate diones, diols, hydroxy- and nitro- derivatives of BeP and BaP. The reaction mechanism of both isomers are different, therefore the microenvironment affects BeP and BaP in different ways. BaP react photochemically with oxygen through a singlet oxygen mediated mechanism whereas BeP reacts via a radical cation intermediate. We have determined that BeP also forms a radical cation intermediate when exposed to NO₂ in the dark, in solution or adsorbed on surfaces. The radical cation then reacts with water, traces of oxygen or with nitrogen dioxide to yield stable products. Some of the products observed are the same as those formed by photoreaction of BeP in the presence of oxygen; the other products were identified as mononitro-derivatives. In the case of BaP, the products formed after reaction with NO₂ are different to those observed by irradiation under aerobic conditions. Nitro derivatives are the main products of BaP exposed to NO₂; the nitration of BaP is thought to occur through an electrophilic substitution. We have identified thirteen products of BeP and BaP, and at least three of them presented direct mutagenic properties. BeP is considered non mutagenic and BaP is mutagenic only after metabolic activation; therefore, reactions with oxygen and NO₂ are processes that increase the toxicity of these pollutants in the atmosphere.