Abstract:In this study, the above-road PM concentrations, together with in-use vehicle activity (the number of heavy-duty and light-duty vehicles) were collected at the Caldecott Tunnel in Orinda, California, on November 18, 1997. The data were analyzed using multivariate-lagged regression, which incorporates the auto-correlation of the dependent variable across time and the cross-correlation between the dependent variable and the explanatory variables across time. The results provide insights on the relative contributions of heavy-duty vehicles and light-duty vehicles to the above-road PM concentrations.