IRIS analyser assessment reveals sub-hourly variability of isotope ratios in carbon dioxide at Baring Head, New Zealand's atmospheric observatory in the Southern Ocean

Abstract. We assess the performance of an isotope ratio infrared spectrometer (IRIS) to measure carbon (δ13C) and oxygen (δ18O) ratios in atmospheric dioxide (CO2) report observations from a 26 d field deployment trial at Baring Head, New Zealand, NIWA's observatory for Southern Ocean baseline air. Our study describes operational method improve comparison previous publications on this analytical instrument. By using calibration technique that reflected principle identical treatment sample reference gases, we achieved reproducibility 0.07 ‰ δ13C-CO2 0.06 δ18O-CO2 over multiple days. This is within extended compatibility goal 0.1 both δ18O-CO2, which was recommended by World Meteorological Organization (WMO). Further improvement measurement desirable also meet WMO network goals 0.01 0.05 needed resolve small variability typical background air observatories such as Head. One objective capabilities limitations IRIS analyser variations under conditions. Therefore, selected events record Keeling plot analysis. resolved isotopic composition endmembers with uncertainty ≤ 1 when magnitude CO2 signals larger than 10 ppm. The analysis strongly increased smaller (2–7 ppm), where instrument limiting factor may only allow distinction between very different endmembers, role terrestrial versus oceanic cycle processes.