CONTRIBUTION OF METHYLCYCLOPENTADIENYL MANGANESE TRICARBONYL (MMT) TOATMOSPHERIC MN CONCENTRATION NEAR EXPRESSWAY - DISPERSION MODELING ESTIMATIONS

Since 1976, methylcyclopentadienyl manganese tricarbonyl (MMT) has bee n used in Canada as an antiknock agent in gasoline and it completely r eplaced lead in 1990. The combustion of MMT leads to the formation of Mn oxides, especially Mn3O4. This paper calculates the contribution of Mn from MMT source to the total atmospheric Mn concentration using tw o dispersion models (CALINE4, ISCLT). The results are compared to CO e stimates since CO is often used as a surrogate for Mn human exposure a ssessment. The study area is located near a major highway (117,585 car s per day) in the city of Montreal. Model estimates were validated usi ng results from two sampling stations located 25 and 250 m from the ro ad centerline. Both models gave similar Mn estimates for distances ove r 250 m with values ranging from 1 to 3 ng m(-3). These predicted valu es underestimate by a factor of ten the measured values 250 m from the road. The Mn contribution from MMT may be masked by other sources suc h as Mn enriched road dust or naturally occurring crustal material. On the other hand, CO estimates and measured values are almost identical . This may be explained by the fact that the mobile source Mn contribu tion to total atmospheric emissions is less than 20%, whereas for CO i t may reach 75%. The total uncertainty in the model predictions was es timated at 50%.