On the use of data from commercial NOx analyzers for air pollution studies

Reactive oxidized nitrogen species play a central role in environmental pollution, and long term monitoring is widespread. But conventional NOx (defined as NO + NO2) analyzers employing heated converters respond to many species in addition to the compounds NO and NO2. The response of these instruments to nitric acid, peroxyacetyl nitrate, alkyl nitrates and other oxidized nitrogen species (the sum of these plus NOx is defined as NOy) is well established, but the ratio of NOx to NOy varies widely in time and space making the accuracy of commercial NOx monitors uncertain. Care must be taken when comparing spectroscopic measurements of NO2 or numerical models to output from commercial NOx monitors. Correction factors can be developed for specific conditions, and long term trends can be meaningful. Recent studies comparing modeled NOx to measurements with large interferences can involve errors of a factor of two or more and produce misleading guidance on science and policy; the need for rigorous model evaluation adds urgency to the deployment of true NOx monitors.