Traceability of Long-Term Atmospheric Composition Observations across Global Monitoring Networks: Chemical Metrology Applied to the Measurements of Constituents in Air, Water, and Soil

High-quality and long-term comparable time series of the relevant atmospheric observations are the essential prerequisite to understand the dynamical, physical and chemical state of the atmosphere from seasonal to multi-decadal time scales. For relevant gaseous compounds such as ozone, methane (CH4) and carbon monoxide (CO), the requirements are secured by tracing back these observations to common primary standards. Periodical audits of the system in operation and the performance of measurement sites provide additional information about data quality and comparability. The results of 48 audits conducted by the World Calibration Centre for Surface Ozone, Carbon Monoxide and Methane (WCC-Empa) at global stations of the Global Atmosphere Watch programme (GAW) from 1996 to 2009 show that most of the audited sites meet the data quality objectives for ozone and methane whereas the situation is less uniform for carbon monoxide