EPA has approved the use of direct-reading analyzers to measure nitrogen dioxide (NO2) in ambient air. In the past, this pollutant has been measured by chemiluminescence analyzers as the difference between measurements of total nitrogen oxides (NOx) and nitric oxides (NO). The new NO2 analyzers need to use National Institute of Standards and Technology (NIST)-traceable NO2 calibration gases (i.e., EPA Protocol Gases) to make accurate direct-reading measurements. Although NIST is not currently selling NO2 reference standards, it does recognize as equivalent the NO2 reference standards from the Von Swinden Laboratory (VSL) in the Netherlands. VSL uses specially prepared aluminum cylinders in which NO2 gas mixtures are stable.
The two EPA memoranda alert specialty gas producers, EPA regional offices, and state/local agencies that nitrogen dioxide (NO2) EPA Protocol Gases can be assayed and certified using VSL NO2 reference standards. It also notes that the EPA's Office of Air Quality Planning and Standards (OAQPS) will be assessing the long-term stability of these NO2 EPA Protocol Gases.