(a) Application. You may use a chemiluminescent detector (CLD) to measure NOX concentration in raw or diluted exhaust for batch or continuous sampling. We generally accept a CLD for NOX measurement, even though it measures only NO and NO2, when coupled with an NO2-to-NO converter, since conventional engines and aftertreatment systems do not emit significant amounts of NOX species other than NO and NO2. Measure other NOX species if required by the standard- setting part. While you may also use other instruments to measure NOX, as described in § 1065.272, use a reference procedure based on a chemiluminescent detector for comparison with any proposed alternate measurement procedure under § 1065.10.