*Update 2* EPA has modified the document "Concern with Certain Calibration Gas Mixtures Sold as Protocol Gases Not Meeting Long-Term Stability Requirements (pdf) (February 2022)". This update reflects that cylinders of "oxides of nitrogen" (NO2 in Air) which are listed in Table 2-3 with a footnote of "f" are acceptable for performing NOx converter efficiency checks as specified in Method 7E, Section 8.2.4.1. The use of NO2 in N2 cylinders to conduct NOx converter efficiency checks remains suspended until such time that their long-term stability has been demonstrated to EPA and Table 2-3 has been revised to include them. We apologize for any confusion our prior Listserv may have caused.