The Evolution of Surface Ozone Data Quality over the Past 3 Decades

For nearly 30 years, Empa has operated the World Calibration Centre for Carbon Monoxide, Methane, Carbon Dioxide and Surface Ozone (WCC-Empa) as a Swiss contribution to the Global Atmosphere Watch (GAW) programme. In this capacity, WCC-Empa has played a central role in sustaining and improving the data quality and availability required for climate and environmental research.

A core activity of WCC-Empa is the quality control of GAW stations through on-site system- and performance audits. We have conducted more than 120 audits as an independent verification of the traceability of measurements to the accepted standards of the WMO/GAW programme, which are hosted and distributed by Central Calibration Laboratories (CCLs). These audits also include operator training and capacity building to improve data availability and quality, especially in less developed regions.

Our presentation will focus on the evolution of surface ozone data quality over the past three decades. We have assessed the stability of the traceability chain from the primary reference standard (the NIST Standard Reference Photometer family) to the transfer standards and the analysers used in the field. Unlike other parameters, the technique for measuring ozone has not changed in the last 30 years, and most surface ozone measurements are made using the UV absorption technique. About two-thirds of the comparisons met the data quality objectives (maximum bias of 1 nmol mol^-1) of the GAW programme.

Looking ahead, a new ozone absorption cross-section will be implemented in 2025 (bipm.org/en/ozone). This will improve the accuracy of ozone measurements but will have an impact on exceedances of air quality standards by increasing ozone values by 1.29%. We will discuss the implications of the new cross-section value and provide guidance on how to make the transition.