Wintertime measurements of atmospheric aerosol over the size range 3-500 nm made approximately 30 in above street level in central Manchester are reported. These are compared with measurements of trace gases. It is found that secondary accumulation mode particles follow closely the values of NO(x) in the atmosphere and are closely related to vehicle activity, reaching a maximum at about 9 am. New particle formation close to the observation point is closely related to SO2 concentrations, predominately during daylight hours, and is a maximum around mid-day but persists after dark. The concentrations of fine and Aitken mode particles were much higher than in nearby rural areas, but the concentration of particles in the size range 200-500 nm were similar at the urban and rural sites. In addition, measurements of ammonia, a gas often important in nucleation kinetics, revealed concentrations up to 17 ppb in Manchester.