Motor vehicle emissions are the most significant source of particulate matter (PM) in urban environments. This study was undertaken to observe the evolution of aerosols downwind from a busy road, concentrating specifically on the aerosol total number maximum and number size distribution. A Grimm Aerosol Technik (5.400) CPC and DMA 5.5-900 classifier were used to measure ultra-fine particles from 9.8 nm to 1.1 μm at varying distances up to 100 m from the road side. Contrary to current accepted aerosol theory particle number concentration was seen to rise with increasing horizontal distance from the road side up to a maximum distance of about 100m. As this occurred the number of fine particle was seen to increase as numbers of larger particles declined. These findings are discussed in light of existing aerosol literature on aerosol formation and dispersion.