The Aburra Valley (Colombia) comprises an urban territory between a mountainous and complex orography. Motorized transport emits polluting particulate matter that is not easily evacuated outside the valley due to the obstacle that the mountains imply to the advective wind. Also the top of the atmospheric boundary layer retains contaminants that rise convectively. Searching to investigate the amount of emissions that should be reduced to improve air quality in the Aburra Valley, the diurnal behavior of the concentration of particulate matter smaller than 2.5 microns was simulated from a box model.