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

Since 1976, methylcyclopentadienyl manganese tricarbonyl (MMT) has been used in Canada as an antiknock agent in gasoline and it completely replaced 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 two dispersion models (CALINE4, ISCLT). The results are compared to CO estimates since CO is often used as a surrogate for Mn human exposure assessment. The study area is located near a major highway (117,585 cars per day) in the city of Montreal. Model estimates were validated using results from two sampling stations located 25 and 250 m from the road centerline. Both models gave similar Mn estimates for distances over 250 m with values ranging from 1 to 3 ng m(-3). These predicted values 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 such 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 contribution to total atmospheric emissions is less than 20%, whereas for CO it may reach 75%. The total uncertainty in the model predictions was estimated at 50%.