There are important fluctuations in instantaneous pollutant concentration at small distances from the roadway due to variability in vehicle spacings and emission rates. The pollutant concentration from a single-lane highway is represented as a two-sided filtered Markov renewal process. Explicit formulae for the filter or impact function and the average concentration are derived for isotropic Gaussian diffusion model with variances proportional to time or to time-squared. A numerical example for a single lane of rush-hour traffic yields coefficients of variation of about 20% at 20 meters from the roadway, for reasonable parameters.