Numerical Modeling of Carbon Monoxide Concentrations Near Highways

Abstract

Numerical models have been designed to compute carbon monoxide concentrations in the vicinity of a plane highway and a depressed highway in Toronto. The non-steady two-dimensional diffusion equation is integrated in time until steady conditions are attained. Diffusivities vary both horizontally and vertically. Near the highway, diffusivities are higher over the highway (20 m2 sec?1) than to the sides. At higher levels, diffusivities are greater for cold air advection (9 m2 sec?1) than for warm (3 m2 sec?1). Winds vary logarithmically with height in the lowest 10 m and linearly above that level. The highway is treated as an area source. Computed concentrations at a fixed measuring site (15 m north of the plane highway and 2 m above the ground) have simple correlation coefficients of 0.82?0.94 with observed concentrations. Numerical simulations show that the order of importance of various factors is, with the most significant one first, low-level diffusivities wind component perpendicular to the highway, mixing height, and type of advection. Particular attention is paid to the decrease of concentration with distance from the highway. For both models, the reduction is relatively small with light winds or low mixing heights. As expected, the decrease with distance is larger for the depressed highway than for the plane highway.