Molecular spectroscopy provides the primary link between radiance and atmospheric gas composition, and is a primary component of the theory of radiative transfer through the atmosphere. The spectroscopic properties of a gas are constant and therefore, if they are robustly characterised and all of the external and instrumental influence factors on a spectroscopic measurement method are assessed, then formal traceability could, in theory, be realised for any measurement using that method.

In addition to the spectroscopic issues relating to those techniques that directly use spectroscopic measurement methods to derive information on ECVs, spectroscopic parameters are also an integral part of radiative transfer (RT) codes. RT codes constitute the core of radiometric physical retrievals, such as optimal estimation methods. In addition, any data intercomparison/validation method that includes the use of RT codes will also be influenced by spectroscopic uncertainties. Such uncertainties will contribute to the overall uncertainty of the data intercomparison, and could be the source of, potentially unexpected, correlation between the different data sources.

The exact nature of the influence of spectroscopic uncertainties on the derived ECV products will vary according to the spectral region being measured and the specific details of the measurement technique being employed – and a series of related gaps have been identified that give examples of this. However, there would be a clear benefit in a top-level spectroscopic coordination activity that took an overview of the more detailed technical developments; identified and disseminated common issues and solutions; and potentially developed a harmonised process for dealing with spectroscopic uncertainties and establishing spectroscopic traceability. This final goal of formal traceability based purely on the spectroscopic assessment of the measurement is a very challenging one that is unlikely to be resolved in the short term. However intermediate steps to improve the knowledge of spectroscopic uncertainties and their impact on measurement methods and intercomparison results, will have immediate impact which will be enhanced through an overall spectroscopic coordination activity.      

Historically, other sources of uncertainty have tended to be much larger than spectroscopic uncertainties such that spectroscopic uncertainty has tended to be seen as an ignorable effect. As satellite and non-satellite instrumentation become more stable and better characterised and understanding of collocation effects improves it is increasingly the case that spectroscopic uncertainties become important or even the limiting factor in the comparison, particularly as they are a potential source of long term correlation within individual measurement methods but also in comparisons between methods. It is thus increasingly important that spectroscopic uncertainties be considered afresh and better quantified.