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

Yu Hoshina,Shin-Ichiro Nakaoka,Keiichi Katsumata,Toshinobu Machida,Yasunori Tohjima

doi:10.5194/acp-18-9283-2018

Yu Hoshina, Shin-Ichiro Nakaoka + Show 3 more

Open Access

https://doi.org/10.5194/acp-18-9283-2018

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Abstract

Abstract. Atmospheric oxygen (O2) and carbon dioxide (CO2) variations in the North Pacific were measured aboard a cargo ship, the New Century 2 (NC2), while it cruised between Japan and the United States between December 2015 and November 2016. A fuel cell analyzer and a nondispersive infrared analyzer were used for the measurement of O2 and CO2, respectively. To achieve parts-per-million precision for the O2 measurements, we precisely controlled the flow rates of the sample and reference air introduced into the analyzers and the outlet pressure. A relatively low airflow rate (10 cm3 min−1) was adopted to reduce the consumption rate of the reference gases. In the laboratory, the system achieved measurement precisions of 3.8 per meg for δ(O2 ∕ N2), which is commonly used to express atmospheric O2 variation, and 0.1 ppm for the CO2 mole fraction. After the in situ observation started aboard NC2, we found that the ship's motion caused false wavy variations in the O2 signal with an amplitude of more than several tens of ppm and a period of about 20 s. Although we have not resolved the problem at this stage, hourly averaging considerably suppressed the variation associated with ship motion. Comparison between the in situ observation and flask sampling of air samples aboard NC2 showed that the averaged differences (in situ–flask) and the standard deviations (±1σ) are −2.8 ± 9.4 per meg for δ(O2 ∕ N2) and −0.02 ± 0.33 ppm for the CO2 mole fraction. We compared 1 year of in situ data for atmospheric potential oxygen (APO; O2 +1.1×CO2) obtained from the broad middle-latitude region (140∘ E–130∘ W, 29∘ N–45∘ N) with previous flask sampling data from the North Pacific. This comparison showed that longitudinal differences in the seasonal amplitude of APO, ranging from 51 to 73 per meg, were smaller than the latitudinal differences.

Highlights

The balance of CO2 emissions from fossil fuel combustion, land biotic CO2 uptake, and ocean CO2 uptake determines long-term change in the atmospheric CO2 burden
The role of atmospheric O2 as a tracer of air–sea gas exchange is emphasized by the introduction of the tracer atmospheric potential oxygen (APO), which is defined as APO = O2 +1.1 × CO2 (Stephens et al, 1998), where 1.1 represents the −O2 / CO2 exchange ratio associated with land biotic activity (Severinghaus, 1995)
After beginning the in situ measurements aboard New Century 2 (NC2), we found that the ship motions did not affect the response of the CO2 analyzer but did seriously affect the response of the O2 analyzer

Summary

Introduction

The balance of CO2 emissions from fossil fuel combustion, land biotic CO2 uptake, and ocean CO2 uptake determines long-term change in the atmospheric CO2 burden. Fossil fuel combustion consumes atmospheric O2, while the land biotic CO2 uptake is accompanied by the emission of O2 into the atmosphere. Atmospheric O2 measurements are useful for understanding air–sea gas exchange. This is based on the fact that the air–sea exchange of O2 is more than 1 order of magnitude faster than that of CO2; the chemical equilibrium of dissolved inorganic carbon (dissolved CO2, bicarbonate, and carbonate ions) in seawater suppresses the air–sea exchange of CO2 (e.g., Keeling et al, 1993).

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