Q77. For a unit that has an O2 monitor and doesn’t meet the conditions required to use Tier 4, but elects to use Tier 4, what are the monitoring requirements?

A77. If a facility elects to use Tier 4, then all of the Tier 4 requirements must be met. In this case, a stack flow monitor must be installed and certified, and the O2 monitor may be used in lieu of installing and certifying a CO2 monitor, provided that: (1) only fuels listed in Table 1 in section 3.3.5 of Part 75, Appendix F are combusted in the unit; and (2) the effluent stream does not include any CO2 emissions from non-combustion processes or emission control systems. (See §98.33(a)(4)(iv)).  However, the relative accuracy test audit (RATA) of the O2 monitor must be conducted on a %CO2 basis comparing the calculated CO2 concentrations to the concentrations measured by the CO2 reference method. (See §98.34(c)(2).)