2. Independent TD estimates are performed using methods that measure full-facility emissions (either directly or by summing emissions from major sources). Since these methods have limitations, including method detection limits (MDLs) that vary with environmental conditions, some of theses methods may not necessarily capture all emissions from a facility. Therefore, some of these methods are often supplemented with other information, such as per-source measurements and/or estimations based on BU emission factors, to produce a complete estimate of emissions, or an adjusted TD estimate.