An Autonomous Inexpensive Robust CO 2 Analyzer (AIRCOA) Britton Stephens, Andrew Watt, and Gordon Maclean National Center for Atmospheric Research, Boulder, Colorado, USA

Potential source of biasAIRCOA solution Relating to WMO CO 2 ScaleDedicated CO 2 and O 2 calibration transfer facility Short-term IRGA noiseAverage for 2 minutes to get better than 0.1 ppm precision Drift in IRGA sensitivity4-hourly 4-point calibrations and 30-minute 1-point calibrations IRGA pressure sensitivityAutomated 4-hourly pressure sensitivity measurements IRGA temperature sensitivity30-minute 1-point calibrations, temperature control at some sites Incomplete drying of airSlow enough flow (100 sccm), two 96” Nafion driers, downstream humidity sensor to verify performance Drying system altering CO 2 Continuous flows and pressures through Nafions and run surveillance gas through entire system Incomplete flushing of cell and dead volumes Fast enough flow (100 sccm), alternate calibration sequence low ‑ to- high / high-to-low to look for effects Leaks through fittings, solenoid valves, and pumps Automated 8-hourly positive pressure leak-down and 4-hourly ambient pressure leak-up checks Pressure broadening without ArUse calibration gases made with real air Fossil CO 2 in calibration gases and different field and lab 13 C sensitivities Laboratory tests limit current effect to 0.05 ppm, long-term plans to use cylinders with natural CO 2 Regulator temperature effectsTests suggest effect is negligible, but could be regulator dependent Regulator flushing effectsRepeat calibration tests suggest the effect is negligible Whole-system diagnostics and comparability verification Long-term surveillance tank analyzed every 8 hours, co-location with other programs, rotating cylinders, and laboratory comparisons Delay in diagnosis of errorsNear real-time data acquisition, processing, and dissemination

Potential source of biasAIRCOA solution Relating to WMO CO 2 ScaleDedicated CO 2 and O 2 calibration transfer facility Short-term IRGA noiseAverage for 2 minutes to get better than 0.1 ppm precision Drift in IRGA sensitivity4-hourly 4-point calibrations and 30-minute 1-point calibrations IRGA pressure sensitivityAutomated 4-hourly pressure sensitivity measurements IRGA temperature sensitivity30-minute 1-point calibrations, temperature control at some sites Incomplete drying of airSlow enough flow (100 sccm), two 96” Nafion driers, downstream humidity sensor to verify performance Drying system altering CO 2 Continuous flows and pressures through Nafions and run surveillance gas through entire system Incomplete flushing of cell and dead volumes Fast enough flow (100 sccm), alternate calibration sequence low ‑ to- high / high-to-low to look for effects Leaks through fittings, solenoid valves, and pumps Automated 8-hourly positive pressure leak-down and 4-hourly ambient pressure leak-up checks Pressure broadening without ArUse calibration gases made with real air Fossil CO 2 in calibration gases and different field and lab 13 C sensitivities Laboratory tests limit current effect to 0.05 ppm, long-term plans to use cylinders with natural CO 2 Regulator temperature effectsTests suggest effect is negligible, but could be regulator dependent Regulator flushing effectsRepeat calibration tests suggest the effect is negligible Whole-system diagnostics and comparability verification Long-term surveillance tank analyzed every 8 hours, co-location with other programs, rotating cylinders, and laboratory comparisons Delay in diagnosis of errorsNear real-time data acquisition, processing, and dissemination

NCAR CO 2 and O 2 /N 2 Calibration Facility

Potential source of biasAIRCOA solution Relating to WMO CO 2 ScaleDedicated CO 2 and O 2 calibration transfer facility Short-term IRGA noiseAverage for 2 minutes to get better than 0.1 ppm precision Drift in IRGA sensitivity4-hourly 4-point calibrations and 30-minute 1-point calibrations IRGA pressure sensitivityAutomated 4-hourly pressure sensitivity measurements IRGA temperature sensitivity30-minute 1-point calibrations, temperature control at some sites Incomplete drying of airSlow enough flow (100 sccm), two 96” Nafion driers, downstream humidity sensor to verify performance Drying system altering CO 2 Continuous flows and pressures through Nafions and run surveillance gas through entire system Incomplete flushing of cell and dead volumes Fast enough flow (100 sccm), alternate calibration sequence low ‑ to- high / high-to-low to look for effects Leaks through fittings, solenoid valves, and pumps Automated 8-hourly positive pressure leak-down and 4-hourly ambient pressure leak-up checks Pressure broadening without ArUse calibration gases made with real air Fossil CO 2 in calibration gases and different field and lab 13 C sensitivities Laboratory tests limit current effect to 0.05 ppm, long-term plans to use cylinders with natural CO 2 Regulator temperature effectsTests suggest effect is negligible, but could be regulator dependent Regulator flushing effectsRepeat calibration tests suggest the effect is negligible Whole-system diagnostics and comparability verification Long-term surveillance tank analyzed every 8 hours, co-location with other programs, rotating cylinders, and laboratory comparisons Delay in diagnosis of errorsNear real-time data acquisition, processing, and dissemination

CO 2 signal averaged over 2.5 min. measurement cycle

Potential source of biasAIRCOA solution Relating to WMO CO 2 ScaleDedicated CO 2 and O 2 calibration transfer facility Short-term IRGA noiseAverage for 2 minutes to get better than 0.1 ppm precision Drift in IRGA sensitivity4-hourly 4-point calibrations and 30-minute 1-point calibrations IRGA pressure sensitivityAutomated 4-hourly pressure sensitivity measurements IRGA temperature sensitivity30-minute 1-point calibrations, temperature control at some sites Incomplete drying of airSlow enough flow (100 sccm), two 96” Nafion driers, downstream humidity sensor to verify performance Drying system altering CO 2 Continuous flows and pressures through Nafions and run surveillance gas through entire system Incomplete flushing of cell and dead volumes Fast enough flow (100 sccm), alternate calibration sequence low ‑ to- high / high-to-low to look for effects Leaks through fittings, solenoid valves, and pumps Automated 8-hourly positive pressure leak-down and 4-hourly ambient pressure leak-up checks Pressure broadening without ArUse calibration gases made with real air Fossil CO 2 in calibration gases and different field and lab 13 C sensitivities Laboratory tests limit current effect to 0.05 ppm, long-term plans to use cylinders with natural CO 2 Regulator temperature effectsTests suggest effect is negligible, but could be regulator dependent Regulator flushing effectsRepeat calibration tests suggest the effect is negligible Whole-system diagnostics and comparability verification Long-term surveillance tank analyzed every 8 hours, co-location with other programs, rotating cylinders, and laboratory comparisons Delay in diagnosis of errorsNear real-time data acquisition, processing, and dissemination

Calibration sequence

Potential source of biasAIRCOA solution Relating to WMO CO 2 ScaleDedicated CO 2 and O 2 calibration transfer facility Short-term IRGA noiseAverage for 2 minutes to get better than 0.1 ppm precision Drift in IRGA sensitivity4-hourly 4-point calibrations and 30-minute 1-point calibrations IRGA pressure sensitivityAutomated 4-hourly pressure sensitivity measurements IRGA temperature sensitivity30-minute 1-point calibrations, temperature control at some sites Incomplete drying of airSlow enough flow (100 sccm), two 96” Nafion driers, downstream humidity sensor to verify performance Drying system altering CO 2 Continuous flows and pressures through Nafions and run surveillance gas through entire system Incomplete flushing of cell and dead volumes Fast enough flow (100 sccm), alternate calibration sequence low ‑ to- high / high-to-low to look for effects Leaks through fittings, solenoid valves, and pumps Automated 8-hourly positive pressure leak-down and 4-hourly ambient pressure leak-up checks Pressure broadening without ArUse calibration gases made with real air Fossil CO 2 in calibration gases and different field and lab 13 C sensitivities Laboratory tests limit current effect to 0.05 ppm, long-term plans to use cylinders with natural CO 2 Regulator temperature effectsTests suggest effect is negligible, but could be regulator dependent Regulator flushing effectsRepeat calibration tests suggest the effect is negligible Whole-system diagnostics and comparability verification Long-term surveillance tank analyzed every 8 hours, co-location with other programs, rotating cylinders, and laboratory comparisons Delay in diagnosis of errorsNear real-time data acquisition, processing, and dissemination

Empirical pressure correction

Potential source of biasAIRCOA solution Relating to WMO CO 2 ScaleDedicated CO 2 and O 2 calibration transfer facility Short-term IRGA noiseAverage for 2 minutes to get better than 0.1 ppm precision Drift in IRGA sensitivity4-hourly 4-point calibrations and 30-minute 1-point calibrations IRGA pressure sensitivityAutomated 4-hourly pressure sensitivity measurements IRGA temperature sensitivity30-minute 1-point calibrations, temperature control at some sites Incomplete drying of airSlow enough flow (100 sccm), two 96” Nafion driers, downstream humidity sensor to verify performance Drying system altering CO 2 Continuous flows and pressures through Nafions and run surveillance gas through entire system Incomplete flushing of cell and dead volumes Fast enough flow (100 sccm), alternate calibration sequence low ‑ to- high / high-to-low to look for effects Leaks through fittings, solenoid valves, and pumps Automated 8-hourly positive pressure leak-down and 4-hourly ambient pressure leak-up checks Pressure broadening without ArUse calibration gases made with real air Fossil CO 2 in calibration gases and different field and lab 13 C sensitivities Laboratory tests limit current effect to 0.05 ppm, long-term plans to use cylinders with natural CO 2 Regulator temperature effectsTests suggest effect is negligible, but could be regulator dependent Regulator flushing effectsRepeat calibration tests suggest the effect is negligible Whole-system diagnostics and comparability verification Long-term surveillance tank analyzed every 8 hours, co-location with other programs, rotating cylinders, and laboratory comparisons Delay in diagnosis of errorsNear real-time data acquisition, processing, and dissemination

SPL 9/4-9/18 NWR 9/18 Empirical temperature correction

Potential source of biasAIRCOA solution Relating to WMO CO 2 ScaleDedicated CO 2 and O 2 calibration transfer facility Short-term IRGA noiseAverage for 2 minutes to get better than 0.1 ppm precision Drift in IRGA sensitivity4-hourly 4-point calibrations and 30-minute 1-point calibrations IRGA pressure sensitivityAutomated 4-hourly pressure sensitivity measurements IRGA temperature sensitivity30-minute 1-point calibrations, temperature control at some sites Incomplete drying of airSlow enough flow (100 sccm), two 96” Nafion driers, downstream humidity sensor to verify performance Drying system altering CO 2 Continuous flows and pressures through Nafions and run surveillance gas through entire system Incomplete flushing of cell and dead volumes Fast enough flow (100 sccm), alternate calibration sequence low ‑ to- high / high-to-low to look for effects Leaks through fittings, solenoid valves, and pumps Automated 8-hourly positive pressure leak-down and 4-hourly ambient pressure leak-up checks Pressure broadening without ArUse calibration gases made with real air Fossil CO 2 in calibration gases and different field and lab 13 C sensitivities Laboratory tests limit current effect to 0.05 ppm, long-term plans to use cylinders with natural CO 2 Regulator temperature effectsTests suggest effect is negligible, but could be regulator dependent Regulator flushing effectsRepeat calibration tests suggest the effect is negligible Whole-system diagnostics and comparability verification Long-term surveillance tank analyzed every 8 hours, co-location with other programs, rotating cylinders, and laboratory comparisons Delay in diagnosis of errorsNear real-time data acquisition, processing, and dissemination

Drying system monitoring A change of 0.5% RH is approximately 300 ppm H 2 O, which would cause a dilution error of 0.1 ppm in CO 2

Potential source of biasAIRCOA solution Relating to WMO CO 2 ScaleDedicated CO 2 and O 2 calibration transfer facility Short-term IRGA noiseAverage for 2 minutes to get better than 0.1 ppm precision Drift in IRGA sensitivity4-hourly 4-point calibrations and 30-minute 1-point calibrations IRGA pressure sensitivityAutomated 4-hourly pressure sensitivity measurements IRGA temperature sensitivity30-minute 1-point calibrations, temperature control at some sites Incomplete drying of airSlow enough flow (100 sccm), two 96” Nafion driers, downstream humidity sensor to verify performance Drying system altering CO 2 Continuous flows and pressures through Nafions and run surveillance gas through entire system Incomplete flushing of cell and dead volumes Fast enough flow (100 sccm), alternate calibration sequence low ‑ to- high / high-to-low to look for effects Leaks through fittings, solenoid valves, and pumps Automated 8-hourly positive pressure leak-down and 4-hourly ambient pressure leak-up checks Pressure broadening without ArUse calibration gases made with real air Fossil CO 2 in calibration gases and different field and lab 13 C sensitivities Laboratory tests limit current effect to 0.05 ppm, long-term plans to use cylinders with natural CO 2 Regulator temperature effectsTests suggest effect is negligible, but could be regulator dependent Regulator flushing effectsRepeat calibration tests suggest the effect is negligible Whole-system diagnostics and comparability verification Long-term surveillance tank analyzed every 8 hours, co-location with other programs, rotating cylinders, and laboratory comparisons Delay in diagnosis of errorsNear real-time data acquisition, processing, and dissemination

Nafion absorption effect Flow pulled through Nafion went from 300 to 50 sccm at t = 30 sec

Potential source of biasAIRCOA solution Relating to WMO CO 2 ScaleDedicated CO 2 and O 2 calibration transfer facility Short-term IRGA noiseAverage for 2 minutes to get better than 0.1 ppm precision Drift in IRGA sensitivity4-hourly 4-point calibrations and 30-minute 1-point calibrations IRGA pressure sensitivityAutomated 4-hourly pressure sensitivity measurements IRGA temperature sensitivity30-minute 1-point calibrations, temperature control at some sites Incomplete drying of airSlow enough flow (100 sccm), two 96” Nafion driers, downstream humidity sensor to verify performance Drying system altering CO 2 Continuous flows and pressures through Nafions and run surveillance gas through entire system Incomplete flushing of cell and dead volumes Fast enough flow (100 sccm), alternate calibration sequence low ‑ to- high / high-to-low to look for effects Leaks through fittings, solenoid valves, and pumps Automated 8-hourly positive pressure leak-down and 4-hourly ambient pressure leak-up checks Pressure broadening without ArUse calibration gases made with real air Fossil CO 2 in calibration gases and different field and lab 13 C sensitivities Laboratory tests limit current effect to 0.05 ppm, long-term plans to use cylinders with natural CO 2 Regulator temperature effectsTests suggest effect is negligible, but could be regulator dependent Regulator flushing effectsRepeat calibration tests suggest the effect is negligible Whole-system diagnostics and comparability verification Long-term surveillance tank analyzed every 8 hours, co-location with other programs, rotating cylinders, and laboratory comparisons Delay in diagnosis of errorsNear real-time data acquisition, processing, and dissemination

Empirical flushing correction

Potential source of biasAIRCOA solution Relating to WMO CO 2 ScaleDedicated CO 2 and O 2 calibration transfer facility Short-term IRGA noiseAverage for 2 minutes to get better than 0.1 ppm precision Drift in IRGA sensitivity4-hourly 4-point calibrations and 30-minute 1-point calibrations IRGA pressure sensitivityAutomated 4-hourly pressure sensitivity measurements IRGA temperature sensitivity30-minute 1-point calibrations, temperature control at some sites Incomplete drying of airSlow enough flow (100 sccm), two 96” Nafion driers, downstream humidity sensor to verify performance Drying system altering CO 2 Continuous flows and pressures through Nafions and run surveillance gas through entire system Incomplete flushing of cell and dead volumes Fast enough flow (100 sccm), alternate calibration sequence low ‑ to- high / high-to-low to look for effects Leaks through fittings, solenoid valves, and pumps Automated 8-hourly positive pressure leak-down and 4-hourly ambient pressure leak-up checks Pressure broadening without ArUse calibration gases made with real air Fossil CO 2 in calibration gases and different field and lab 13 C sensitivities Laboratory tests limit current effect to 0.05 ppm, long-term plans to use cylinders with natural CO 2 Regulator temperature effectsTests suggest effect is negligible, but could be regulator dependent Regulator flushing effectsRepeat calibration tests suggest the effect is negligible Whole-system diagnostics and comparability verification Long-term surveillance tank analyzed every 8 hours, co-location with other programs, rotating cylinders, and laboratory comparisons Delay in diagnosis of errorsNear real-time data acquisition, processing, and dissemination

Automated (4- or 8-hourly) leak checks A positive trend of 0.3 kPa/min would be a leak rate of 0.1 sccm which if 100 ppm different would cause a 0.1 ppm bias

Potential source of biasAIRCOA solution Relating to WMO CO 2 ScaleDedicated CO 2 and O 2 calibration transfer facility Short-term IRGA noiseAverage for 2 minutes to get better than 0.1 ppm precision Drift in IRGA sensitivity4-hourly 4-point calibrations and 30-minute 1-point calibrations IRGA pressure sensitivityAutomated 4-hourly pressure sensitivity measurements IRGA temperature sensitivity30-minute 1-point calibrations, temperature control at some sites Incomplete drying of airSlow enough flow (100 sccm), two 96” Nafion driers, downstream humidity sensor to verify performance Drying system altering CO 2 Continuous flows and pressures through Nafions and run surveillance gas through entire system Incomplete flushing of cell and dead volumes Fast enough flow (100 sccm), alternate calibration sequence low ‑ to- high / high-to-low to look for effects Leaks through fittings, solenoid valves, and pumps Automated 8-hourly positive pressure leak-down and 4-hourly ambient pressure leak-up checks Pressure broadening without ArUse calibration gases made with real air Fossil CO 2 in calibration gases and different field and lab 13 C sensitivities Laboratory tests limit current effect to 0.05 ppm, long-term plans to use cylinders with natural CO 2 Regulator temperature effectsTests suggest effect is negligible, but could be regulator dependent Regulator flushing effectsRepeat calibration tests suggest the effect is negligible Whole-system diagnostics and comparability verification Long-term surveillance tank analyzed every 8 hours, co-location with other programs, rotating cylinders, and laboratory comparisons Delay in diagnosis of errorsNear real-time data acquisition, processing, and dissemination

Potential source of biasAIRCOA solution Relating to WMO CO 2 ScaleDedicated CO 2 and O 2 calibration transfer facility Short-term IRGA noiseAverage for 2 minutes to get better than 0.1 ppm precision Drift in IRGA sensitivity4-hourly 4-point calibrations and 30-minute 1-point calibrations IRGA pressure sensitivityAutomated 4-hourly pressure sensitivity measurements IRGA temperature sensitivity30-minute 1-point calibrations, temperature control at some sites Incomplete drying of airSlow enough flow (100 sccm), two 96” Nafion driers, downstream humidity sensor to verify performance Drying system altering CO 2 Continuous flows and pressures through Nafions and run surveillance gas through entire system Incomplete flushing of cell and dead volumes Fast enough flow (100 sccm), alternate calibration sequence low ‑ to- high / high-to-low to look for effects Leaks through fittings, solenoid valves, and pumps Automated 8-hourly positive pressure leak-down and 4-hourly ambient pressure leak-up checks Pressure broadening without ArUse calibration gases made with real air Fossil CO 2 in calibration gases and different field and lab 13 C sensitivities Laboratory tests limit current effect to 0.05 ppm, long-term plans to use cylinders with natural CO 2 Regulator temperature effectsTests suggest effect is negligible, but could be regulator dependent Regulator flushing effectsRepeat calibration tests suggest the effect is negligible Whole-system diagnostics and comparability verification Long-term surveillance tank analyzed every 8 hours, co-location with other programs, rotating cylinders, and laboratory comparisons Delay in diagnosis of errorsNear real-time data acquisition, processing, and dissemination

13 C bias test

Potential source of biasAIRCOA solution Relating to WMO CO 2 ScaleDedicated CO 2 and O 2 calibration transfer facility Short-term IRGA noiseAverage for 2 minutes to get better than 0.1 ppm precision Drift in IRGA sensitivity4-hourly 4-point calibrations and 30-minute 1-point calibrations IRGA pressure sensitivityAutomated 4-hourly pressure sensitivity measurements IRGA temperature sensitivity30-minute 1-point calibrations, temperature control at some sites Incomplete drying of airSlow enough flow (100 sccm), two 96” Nafion driers, downstream humidity sensor to verify performance Drying system altering CO 2 Continuous flows and pressures through Nafions and run surveillance gas through entire system Incomplete flushing of cell and dead volumes Fast enough flow (100 sccm), alternate calibration sequence low ‑ to- high / high-to-low to look for effects Leaks through fittings, solenoid valves, and pumps Automated 8-hourly positive pressure leak-down and 4-hourly ambient pressure leak-up checks Pressure broadening without ArUse calibration gases made with real air Fossil CO 2 in calibration gases and different field and lab 13 C sensitivities Laboratory tests limit current effect to 0.05 ppm, long-term plans to use cylinders with natural CO 2 Regulator temperature effectsTests suggest effect is negligible, but could be regulator dependent Regulator flushing effectsRepeat calibration tests suggest the effect is negligible Whole-system diagnostics and comparability verification Long-term surveillance tank analyzed every 8 hours, co-location with other programs, rotating cylinders, and laboratory comparisons Delay in diagnosis of errorsNear real-time data acquisition, processing, and dissemination

Three cylinders were in the oven and one (green dots) was not Regulator oven tests

Potential source of biasAIRCOA solution Relating to WMO CO 2 ScaleDedicated CO 2 and O 2 calibration transfer facility Short-term IRGA noiseAverage for 2 minutes to get better than 0.1 ppm precision Drift in IRGA sensitivity4-hourly 4-point calibrations and 30-minute 1-point calibrations IRGA pressure sensitivityAutomated 4-hourly pressure sensitivity measurements IRGA temperature sensitivity30-minute 1-point calibrations, temperature control at some sites Incomplete drying of airSlow enough flow (100 sccm), two 96” Nafion driers, downstream humidity sensor to verify performance Drying system altering CO 2 Continuous flows and pressures through Nafions and run surveillance gas through entire system Incomplete flushing of cell and dead volumes Fast enough flow (100 sccm), alternate calibration sequence low ‑ to- high / high-to-low to look for effects Leaks through fittings, solenoid valves, and pumps Automated 8-hourly positive pressure leak-down and 4-hourly ambient pressure leak-up checks Pressure broadening without ArUse calibration gases made with real air Fossil CO 2 in calibration gases and different field and lab 13 C sensitivities Laboratory tests limit current effect to 0.05 ppm, long-term plans to use cylinders with natural CO 2 Regulator temperature effectsTests suggest effect is negligible, but could be regulator dependent Regulator flushing effectsRepeat calibration tests suggest the effect is negligible Whole-system diagnostics and comparability verification Long-term surveillance tank analyzed every 8 hours, co-location with other programs, rotating cylinders, and laboratory comparisons Delay in diagnosis of errorsNear real-time data acquisition, processing, and dissemination

Regulator flushing tests

Potential source of biasAIRCOA solution Relating to WMO CO 2 ScaleDedicated CO 2 and O 2 calibration transfer facility Short-term IRGA noiseAverage for 2 minutes to get better than 0.1 ppm precision Drift in IRGA sensitivity4-hourly 4-point calibrations and 30-minute 1-point calibrations IRGA pressure sensitivityAutomated 4-hourly pressure sensitivity measurements IRGA temperature sensitivity30-minute 1-point calibrations, temperature control at some sites Incomplete drying of airSlow enough flow (100 sccm), two 96” Nafion driers, downstream humidity sensor to verify performance Drying system altering CO 2 Continuous flows and pressures through Nafions and run surveillance gas through entire system Incomplete flushing of cell and dead volumes Fast enough flow (100 sccm), alternate calibration sequence low ‑ to- high / high-to-low to look for effects Leaks through fittings, solenoid valves, and pumps Automated 8-hourly positive pressure leak-down and 4-hourly ambient pressure leak-up checks Pressure broadening without ArUse calibration gases made with real air Fossil CO 2 in calibration gases and different field and lab 13 C sensitivities Laboratory tests limit current effect to 0.05 ppm, long-term plans to use cylinders with natural CO 2 Regulator temperature effectsTests suggest effect is negligible, but could be regulator dependent Regulator flushing effectsRepeat calibration tests suggest the effect is negligible Whole-system diagnostics and comparability verification Long-term surveillance tank analyzed every 8 hours, co-location with other programs, rotating cylinders, and laboratory comparisons Delay in diagnosis of errorsNear real-time data acquisition, processing, and dissemination

Laboratory intercomparisons Field surveillance tanks Laboratory offsets less than 0.06 ppm (1-sigma = 0.12 ppm) Field differences from assigned values less than 0.15 ppm (1-sigma = 0.15 ppm)

Potential source of biasAIRCOA solution Relating to WMO CO 2 ScaleDedicated CO 2 and O 2 calibration transfer facility Short-term IRGA noiseAverage for 2 minutes to get better than 0.1 ppm precision Drift in IRGA sensitivity4-hourly 4-point calibrations and 30-minute 1-point calibrations IRGA pressure sensitivityAutomated 4-hourly pressure sensitivity measurements IRGA temperature sensitivity30-minute 1-point calibrations, temperature control at some sites Incomplete drying of airSlow enough flow (100 sccm), two 96” Nafion driers, downstream humidity sensor to verify performance Drying system altering CO 2 Continuous flows and pressures through Nafions and run surveillance gas through entire system Incomplete flushing of cell and dead volumes Fast enough flow (100 sccm), alternate calibration sequence low ‑ to- high / high-to-low to look for effects Leaks through fittings, solenoid valves, and pumps Automated 8-hourly positive pressure leak-down and 4-hourly ambient pressure leak-up checks Pressure broadening without ArUse calibration gases made with real air Fossil CO 2 in calibration gases and different field and lab 13 C sensitivities Laboratory tests limit current effect to 0.05 ppm, long-term plans to use cylinders with natural CO 2 Regulator temperature effectsTests suggest effect is negligible, but could be regulator dependent Regulator flushing effectsRepeat calibration tests suggest the effect is negligible Whole-system diagnostics and comparability verification Long-term surveillance tank analyzed every 8 hours, co-location with other programs, rotating cylinders, and laboratory comparisons Delay in diagnosis of errorsNear real-time data acquisition, processing, and dissemination

Automated web-based output