The objective of this project was to investigate the degree to which simple addition of maximum concentrations overestimates the actual maximum concentrations and to assess the accuracy of other formulations. This has been done by taking a year of consecutive hourly measurements of SO2, NOx and NO2 from monitoring stations in Liverpool and Manchester. Using hourly local meteorological data for the same period, consecutive hourly ground level concentrations have been modelled for emissions from representative hypothetical new industrial sources. Modelled source and measured background concentrations have been added hour by hour to provide a data set of consecutive hourly combined concentrations for the year. This has been used as a baseline for the determination of the highest combined concentrations occurring in the year against which the addition of maximum percentiles and estimates from other formulations can be compared.