The international comparison of Dobson spectrophotometers of WMO Global Atmosphere Watch region III took place at the Villa Ortúzar Observatory, Buenos Aires, from 4 to 22 March 2019. The intercomparison was organized by the WMO secretariat, Argentina’s National Meteorological Service, the United States National Oceanic and Atmospheric Administration’s Global Monitoring Division, and the University of Colorado Cooperative Institute for Environmental Research. Participants included operators from: Argentina, Brazil (Dobson D 093 from Natal and Dobson D 114 from Cachoeira Paulista – São Paulo, both supported by the Brazilian Space Agency), Cuba, Mexico, Peru, Uruguay and the National Oceanic and Atmospheric Administration’s travelling standard instrument.

The primary purpose of the event was to check and harmonize calibrations and perform maintenance of Dobson spectrophotometers operated within WMO region III, as well as two instruments from region IV which also took part in the intercomparison. Overall, 12 Dobson instruments participated in the intercomparison campaign: D 065 (the World Secondary Standard instrument), D 067, D 070, D 087, D 093, D 097, D 098, D 099, D 114, D 131, D 133, D 134.

All 11 Dobson spectrophotometers from 6 different monitoring and research institutions in Latin America and the Caribbean were inspected, serviced and calibrated, as necessary, so that data obtained at the measurement sites could be standardized and comparable locally and globally. The Regional Dobson Standard for South America was calibrated against the Word Secondary Standard stationed in Boulder, Colorado, United States.

Instructions on the operation and routine maintenance of the instruments were provided to participants along with the final intercomparison to ensure that calibrations and repairs are correct and sufficient to bring instruments within an error of 1 per cent. The relative uncertainty of each Dobson was also estimated.

This event is part of the WMO Global Atmosphere Watch quality control requirements for monitoring atmospheric total ozone and ensures the quality of total ozone data sets. In addition to the instruments participating in the Dobson intercomparison, the regional Brewer Ozone spectrophotometer and Pandora spectrometers, owned by NASA, conducted measurements at Villa Ortúzar during that period. Special Umkehr observations on the zenith sky were made by all participating instruments in the morning of 21 March 2019 to create a reliable data set for verification of different technologies used for processing these observations.