Our research found that most CEMS are operated and maintained by third-parties with support provided by the plants while large power plants often operated CEMS in-house. Nearly all CEMS operators have established developed written CEMS operation and maintenance routines. However, in the absence of systematic onsite inspection of CEMS operation by ESCs, it is unclear how those written rules have been followed in practice. Many EPB officials believed that few CEMS operators regularly calibrate CEMS and perform reference tests, and inconsistent calibration gas casts doubts about the validity of the measurements.