Guidance on NO2 and SO2 EPA Protocol Gases for State-Local Agencies and Specialty Gas Producers
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 oxide (NO). The EPA reference method for NO2 (see 40 CFR Part 50, Appendix F) specifies that these analyzers be calibrated using NO-in-N2 EPA Protocol Gases and gas-phase titration. While it would be desirable to calibrate the direct-reading NO2 analyzers using NO2 calibration gases, there currently are no National Institute of Standards and Technology (NIST)-certified NO2 reference standards that can be used to establish the traceability of NO2 EPA Protocol Gases. Such NO2 reference standards are available from the Von Swinden Laboratory (VSL) in the Netherlands and from the National Physical Laboratory (NPL) in the United Kingdom and NIST has Declarations of Equivalence (DoE) with these national metrology institutes (NMIs). However, a recent international comparison among NMIs performed by the European Association of National Metrology Institutes (EURAMET) Metrology for Nitrogen Dioxide, showed that 10-ppm NO2-in-N2 standards in passivated cylinders appear to experience a NO2 concentration decay of from 1 to 4 percent relative over 15 to 26 months’ time. Given that EPA has received only limited data from specialty gas producers concerning the long-term stability of their NO2 calibration gases, such calibration gases cannot be certified as EPA Protocol Gases at this time. EPA seeks long-term NO2 stability data from the producers to justify certifying NO2 EPA Protocol Gases.
Similar long-term stability problems exist for SO2-in-air calibration gases although SO2-in-N2 calibration gases have been long recognized as being stable. EURAMET also conducted a 2014 comparison of 100-ppb SO2-in-air standards in passivated cylinders among six NMIs, which found a SO2 concentration change of +0.6 to-2.2 percent (mean -0.6 percent) over 6 months’ time. Earlier, a 2004 international comparison of 280-ppb SO2-in-air standards in passivated cylinders among thirteen NMIs found a median change of -2.2 percent over 6 months’ time. EPA also seeks long-term SO2-in-air stability data from the producers to justify certifying SO2-in-air EPA Protocol Gases. Such calibration gases cannot be certified as EPA Protocol Gases at this time.
The EPA memorandum alerts specialty gas producers, EPA regional offices, and state/local agencies that that gaseous calibration standards used for the calibration and the QA/QC of monitors intended to demonstrate compliance with the National Ambient Air Quality Standards (NAAQS) must be EPA Protocol Gases.