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March 31, 2022
Fukuoka University

“Iodine Fountain”: An Area with Significantly High Atmospheric Iodine Concentrations in the Tropical Western Pacific
―Implications for climate change predictions―

1. Key Points

Based on wide-area observations using the oceanographic research vessel Mirai, an area with significantly high concentrations of atmospheric iodine (iodine monoxide) has been discovered in the tropical Western Pacific.
In this area, the concentrations of iodine monoxide and the greenhouse gas ozone in the atmosphere are negatively correlated.
Atmospheric iodine is released from the ocean. Previous studies have indicated that the higher the ozone concentration in the atmosphere, the higher is the amount of iodine supplied to the atmosphere from the ocean, i.e., a positive correlation. Therefore the release process needs reevaluation to explain the negative correlation newly found.
Researchers estimate that the effects of the iodine supply and ozone depletion on climate change, not considered by the Sixth Intergovernmental Panel on Climate Change (IPCC) Assessment Report, could be stronger than presumed. Efforts will be made to improve the assessment of these effects and include them in the Seventh IPCC Assessment Report.

2. Overview

Visiting Senior Researcher Hisahiro Takashima, Center Director Yugo Kanaya, and other members of the Japan Agency for Marine–Earth Science and Technology (JAMSTEC) Earth Surface System Research Center (ESS) conducted wide-area observations of atmospheric composition globally using the oceanographic research vessel Mirai. They observed high concentrations of atmospheric iodine (iodine monoxide) in an area of the tropical Western Pacific, where the sea surface temperature reaches up to 30 °C (warm pool region).

Iodine from the ocean is released into the atmosphere. It is known that atmospheric iodine causes depletion of tropospheric ozone (*1), being a greenhouse gas, and hence, must be a part of climate change predictions. However, surveys are limited, and the dynamics and effects of atmospheric iodine remain unclear because the concentrations are small and difficult to measure.

Therefore, the research group developed a shipborne multi-axis differential optical absorption spectroscopy (MAX–DOAS) apparatus (*2, Figure 1) capable of observing an iodine compound in the atmosphere to ultra-low levels. From 2014 to 2018, the oceanographic research vessel Mirai was used to conduct wide-area surveys of iodine monoxide concentrations over the open ocean, from the Southern Hemisphere to the Arctic Ocean.

The results showed that the concentration of atmospheric iodine monoxide increased with the increase in sea surface temperature; the highest concentrations of iodine monoxide were detected over the tropical Western Pacific warm pool, where the sea surface temperatures exceed 30 °C (Figure 2). The results also show that the atmospheric iodine monoxide and ozone concentrations in the warm pool region are negatively correlated (Figure 3). Previous studies have reported that iodine is released into the atmosphere from the ocean upon reaction of atmospheric ozone on the water surface; this mechanism should have resulted in increases in the iodine emissions with increasing ozone concentration, i.e., a positive correlation. Therefore, previous findings about the releasing process and photochemical reactions in the atmosphere need reevaluation.

As future studies, researchers will conduct more observations to clarify the mechanisms of iodine release from the ocean and ozone depletion, to evaluate its climate change impact and integrate the results in the Seventh IPCC Assessment Report.

The findings of this study were published in Atmospheric Chemistry and Physics journal on March 31, 2022 (Japan Standard Time). The paper was selected as an Atmospheric Chemistry and Physics highlight article. This research was conducted as part of a Grant-in-Aid for Scientific Research (A; No. 21H04933) of the Ministry of Education, Culture, Sports, Science, and Technology of Japan.

Full latitudinal marine atmospheric measurements of iodine monoxide
Hisahiro Takashima1,2, Yugo Kanaya1, Saki Kato2, Martina M. Friedrich3, Michel Van Roozendael3, Fumikazu Taketani1, Takuma Miyakawa1, Yuichi Komazaki1, Carlos A. Cuevas4, Alfonso Saiz-Lopez4, and Takashi Sekiya1
1. Japan Agency for Marine–Earth Science and Technology (JAMSTEC), Yokohama, Japan
2. Faculty of Science, Fukuoka University, Fukuoka, Japan
3. Belgian Institute for Space Aeronomy (BIRA-IASB), Brussels, Belgium
4. Department of Atmospheric Chemistry and Climate, Institute of Physical Chemistry Rocasolano (CSIC), Madrid, Spain
Tropospheric ozone
Approximately 90% of the ozone in the Earth’s atmosphere is in the stratosphere (altitude ~15–30 km). The remaining 10% of ozone is in the troposphere (altitude of up to 10 km). The ozone in the stratosphere (ozone layer) protects life from ultraviolet solar radiation. However, tropospheric ozone is a major cause of photochemical smog (atmospheric pollution), which affects health and agricultural crops, and acts as a greenhouse gas, contributing to global warming. Stratospheric and tropospheric ozone are sometimes referred to as “good” and “bad” ozone, respectively.
Multi-axis differential optical absorption spectroscopy (MAX-DOAS)
MAX-DOAS is a remote sensing method used to analyze trace gases and aerosols in the troposphere by measuring scattered sunlight in the ultraviolet to visible range at multiple low elevation angles. In addition to land-based measurements, JAMSTEC also conducts observations over the ocean by mounting the equipment on a ship.

Fig.1 Outdoor telescope (left) and indoor spectrometer (right) portions of the MAX-DOAS equipment installed on the oceanographic research vessel Mirai.


Fig.2 Concentrations of iodine monoxide over the ocean measured using the MAX-DOAS method between 2014 and 2018 from the oceanographic research vessel Mirai (differential slant column densities for an elevation angle of 3° in the unit of molecules/cm2;the maximum level found in the Western Pacific warm pool region is equivalent to 0.8 pptv). The contours indicate average sea surface temperatures (°C; using the optimum interpolated sea surface temperature averaged for 2014–2018). High concentrations of iodine were detected in the tropical Western Pacific warm pool region, where the sea surface temperatures are high.


Fig.3 Time series of atmospheric iodine monoxide (IO) and ozone (O3) concentrations measured in the warm pool region showing a negative correlation between the two parameters.


(For this study)
Hisahiro Takashima, Visiting Senior Researcher, Research Institute for Global Change (RIGC), Earth Surface System Research Center (ESS), Environmental Geochemical Cycle Research Group, JAMSTEC / Associate professor, Faculty of Science, Fukuoka University
Yugo Kanaya, Principal Researcher, Research Institute for Global Change (RIGC), Earth Surface System Research Center (ESS), JAMSTEC
(For press release)
Press Office, Marine Science and Technology Strategy Department, JAMSTEC
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