Intercomparison of total ozone measured by the Brewer and Dobson spectrophotometers at Toronto

Both Brewer and Dobson Spectrophotometers have been taking routine simultaneous direct-Sun total ozone measurements from the five stations in the Canadian ozone monitoring network. The longest record of intercomparison data is at Toronto, where the well-maintained Canadian regional standard Dobson instrument 77 is stationed and where routine ozone measurements made by Brewer instrument 8 commenced in February 1983. Results of this intercomparison are presented, and the causes for minor differences are discussed. A suitable transfer function is developed to calculate simulated Dobson ozone data from the Brewer data in order to ensure that the ozone record remains continuous and uninterrupted by the transition from the Dobson to the Brewer instrument. With appropriate corrections applied, the Dobson ozone values are 3.8% greater than those of the Brewer, and the standard deviation between the daily mean values is 0.5%. The relative drift between the two instruments over the 5-year period was found to be 0.05% per year, smaller than the 95% confidence estimate of 0.27% per year.