Towards a traceable global solar UV monitoring network.

In view of improving the status of solar UV measurements within the UV monitoring community, the World Calibration Center for UV (WCC-UV) organized in 2022 the 3^rd solar ultraviolet broadband radiometer campaign UVC-III under the auspices of the WMO. 73 instruments were characterized in the laboratory of the WCC-UV and calibrated outdoors relative to the QASUME reference spectroradiometer. After an up to three-month calibration period all devices were returned to their owners including a calibration certificate demonstrating traceability to the international system of units (SI). The expanded uncertainty of the responsivity was less than 5 % for the majority of the radiometers. The deviation to the original responsivity factors was analyzed. From this data one can extract three components affecting the overall measurement uncertainty of solar UV measurements using broadband radiometers on different time scales: Usage of additional correction factors next to the responsivity, control of the humidity inside the device and recalibration frequency. However, it must be noted that the responsivity and measurement equations used by the participating institutes not always follow the recommendations of the WMO as well the sometimes scarce to non-existent recalibration frequency of the solar UV radiometers. [ABSTRACT FROM AUTHOR]