Concentration measurements of selective CH4, CO, CO2, NOx, and O2 are combined with exhaust flow information to determine mass emissions. Methane slip is the release of unburned methane from natural gaspowered engines. It has become a critical environmental issue due to methane's high global warming potential. Compressor stations, which predominantly use large reciprocating or turbine engines, are major sources of methane slip, with combustion slip contributing approximately 75% of total emissions.