Effective immediately, the use of NO2 cylinders to conduct NOx converter efficiency checks as specified in Method 7E, Section 8.2.4.1, is suspended. Gas cylinders of NO2 in either air or nitrogen have not been demonstrated to have long term stability and cannot be certified as an EPA Protocol Gas. (More background information can be found here: www.epa.gov/system/files/documents/2022-04/protocol-gas-memo_1.pdf NOx converter efficiency checks must be conducted in accordance with Method 7E, Section 16.2 (NOx generator or bag procedure). This applies to NOx converter efficiency checks for stack emissions compliance reference method testing as well as CEMS QA, per Technical Manual 1005.

Similarly, the use of SO2 in air gas cylinders, and blended SO2 and O2 with a balance of nitrogen gas cylinders, as calibration gases for Method 6C and CEMS calibrations is also suspended.