The Global Atmosphere Watch (GAW) program currently coordinates 22 ground-based atmospheric background monitoring stations of global scope. The GAW World Calibration Centre for Surface Ozone, Carbon Monoxide and Methane (WCC-EMPA) is responsible for tracing surface ozone measurements at these stations to the designated reference within the GAW program, the Standard Reference Photometer SRP 2 maintained at the National Institute of Standards and Technology (NIST). The recommended method for surface ozone measurements is based on UV absorption at 254 nm (Hg line). Repeated and regular intercomparisons of station instruments are necessary to achieve and maintain high and known data quality. In this paper, the traceability chain is explained, and standard uncertainties for each element are evaluated. Data of 26 intercomparisons performed at 14 stations between 1996 and 2002 are analyzed. On 23 occasions, the instruments passed the audit with good agreement, in one case with sufficient agreement. On 2 occasions, both first audits at the site, the audited instrument did not comply with the minimal data quality requirements. The best instruments in use exhibit a median absolute bias of approximately 0.32 ppbv and a standard uncertainty of approximately 0.8 ppbv (0--100 ppbv). The quantitative improvement of data quality as a result of repeated audits can be demonstrated with several stations.