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Abstract. Atmospheric oxygen (O2) and carbon dioxide (CO2) variations in theNorth Pacific were measured aboard a cargo ship, the New Century 2(NC2), while it cruised between Japan and the United States betweenDecember 2015 and November 2016. A fuel cell analyzer and a nondispersiveinfrared analyzer were used for the measurement of O2 andCO2, respectively. To achieve parts-per-million precision for theO2 measurements, we precisely controlled the flow rates of thesample and reference air introduced into the analyzers and the outletpressure. A relatively low airflow rate (10 cm3 min−1) wasadopted to reduce the consumption rate of the reference gases. In thelaboratory, the system achieved measurement precisions of 3.8 per meg forδ(O2 ∕ N2), which is commonly used to expressatmospheric O2 variation, and 0.1 ppm for the CO2 molefraction. After the in situ observation started aboard NC2, we found thatthe ship's motion caused false wavy variations in the O2 signalwith an amplitude of more than several tens of ppm and a period of about20 s. Although we have not resolved the problem at this stage, hourlyaveraging considerably suppressed the variation associated with ship motion.Comparison between the in situ observation and flask sampling of airsamples aboard NC2 showed that the averaged differences (in situ–flask)and the standard deviations (±1σ) are −2.8 ± 9.4 per megfor δ(O2 ∕ N2) and −0.02 ± 0.33 ppmfor the CO2 mole fraction. We compared 1 year of in situ data foratmospheric potential oxygen (APO; O2 +1.1×CO2)obtained from the broad middle-latitude region(140∘ E–130∘ W, 29∘ N–45∘ N) withprevious flask sampling data from the North Pacific. This comparison showedthat longitudinal differences in the seasonal amplitude of APO, ranging from51 to 73 per meg, were smaller than the latitudinal differences.

In situ observation of atmospheric oxygen and carbon dioxide in the North Pacific using a cargo ship