[A-AS09] Atmospheric Chemistry

[AAS09-06] Characteristics and its origin of variations of CO₂ concentration at Minamitorishima

*Asaka Hasegawa¹, Ryoichi Imasu¹, Masafumi Ohashi², Yosuke Niwa³ (1.Atmosphere and Ocean Research Institute, The University of Tokyo , 2.Kagoshima University, 3.National Institute for Environmental Studies)

Keywords:CO2, Minamitorishima, NDIR, GOSAT, Ground-based remote sensing instruments

Minamitorishima is expected to be suitable for verifying the satellite observation data of CO₂ over the ocean. Also, this station’s observation data is utilized as the background in atmospheric transport models of CO₂ such as NICAM-TM. However, the previous studies showed the episodic events at Minamitorishima due to the continental outflow. Also, the impacts of plants, a power station, and an incinerator on the island are a matter of concern. In this research, to evaluate whether Minamitorishima is suitable station for verification of satellite and models’ background, we attempted to elucidate the frequency, timing, and scale of unique concentration variations.
Hourly data of CO₂ from NDIR analyzer by JMA which are posted on WDCGG were used (28 years). Extremely high and low concentration (EHC, ELC) events were distinguished from seasonal cycles and trends. The backward trajectory analysis, the time, duration, and winds of the events are acquired, and footprint predictions were conducted, so as to consider the effects of atmospheric transport mentioned in previous studies and emission and absorption in the island. Carbon monoxide (CO) was also used as a tracer for anthropogenic combustions and/or biomass burning.
There were 128 EHC events with concentrations higher than the set reference. While some short period events were occurring, both CO and CO₂ were high, and the winds were from the island. These events suggest the influence of island’s incinerators and power plants. Whereas, during some long period events, both CO and CO₂ were high. Backward trajectory analysis showed that these EHC events were affected by outflow from the Asian combustion emission. In one event of them, both the column-averaged CO₂ (XCO₂) from Fiber Etalon spectrometer (FES-C) and the ground concentration may have increased. The outflow from the continent also affects XCO₂. During some short EHC events, CO concentrations were low, the winds were from the island. As the result of footprint calculation, they were likely caused by the respiration of the plants on the island.
The ELC events occurred 87 times. During long ELC events, winds were mainly from outside the island. Also, like previous studies, they occurred in summer. Backward trajectory analysis showed there were outflows from the Russian Far East vegetation sinks during the events. Focusing on the ELC events with winds from the island, they were short and tended to occur during daytime. They suggested the influence of photosynthesis in the island, and the footprint calculation also showed it.
Based on these characteristics of the variations, we considered whether these variations are reproduced by NICAM-TM. As a result, trend, seasonal cycles, and synoptic time scale variations were well reproduced. Also, the events caused by outflows were also reproduced. However, the global offset value used in the model deviates by 2 ppm because the seasonal cycle of NICAM-TM was smaller than the observational data. Additionally, this study showed that it is difficult to reproduce short timescale events which are shorter than one day corresponded to the model’s horizontal resolution about 240km.
Therefore, it can be said that it is necessary to treat EHC and ELC events’ data carefully in case of using CO₂ observation data at Minamitorishima as background.