[A-AS11] [JJ] 大気化学

[AAS11-26] 酸素・二酸化炭素濃度比による大気CO₂濃度変動成分の起源推定法の開発

*保科 優¹、遠嶋 康徳¹、寺尾 有希夫¹、勝又 啓一¹、向井 人史¹、町田 敏暢¹、遅野井 祐美¹ (1.国立環境研究所)

Atmospheric carbon dioxide (CO₂) concentrations observed in urban areas and the surroundings often show short-term elevations on a time scale ranging from several hours to a few days. These variations are considered to be attributed to the CO₂ emissions from biotic activities and burning of fossil fuels. Partitioning the contribution rate between the individual CO₂ sources would be useful to reduce the uncertainty of the CO₂ emissions estimated from atmospheric inversion calculations. The ratio of oxygen (O₂) consumption to CO₂ emission for fossil fuel burning (−O₂/CO₂) ranges from 1 to 2, depending on the elemental composition of the fossil fuel. The average −O₂/CO₂ exchange ratio for the fossil fuel consumed in Japan is more than 1.4 while that for land biotic process is 1.1. Therefore, the difference in the exchange ratios could allow us to determine the contributions from the individual sources. In this study, we started a continuous observation of the atmospheric CO₂ and O₂ concentration at Tsukuba in February 2015, and evaluated the individual contributions of the biotic and fossil fuel-derived emissions to the short-term CO₂ variations based on the observed −ΔO₂/ΔCO₂ changing ratios. The observed −ΔO₂/ΔCO₂ changing ratios show clear seasonal variation with minimum in summer and maximum in winter, ranging from 1.0 to 1.6. Taking into account of the average −O₂/CO₂ exchange ratio of 1.58 for the fossil fuel consumption in Tsukuba, we obtained the contribution ratios of fossil fuel burning to the CO₂ variations were more than 70% in winter and 0–30% in summer. To validate the above estimations based on the −O₂/CO₂ exchange ratio, we conducted flask samplings of air 6 times at the different months during the observation period and measure ¹⁴CO₂, which is known as good indicator of the burning of fossil fuels. Note that 5 flask samples were usually collected for each sampling at an interval of 3 hours to detect the diurnal variations. The contribution rates of the fossil fuel CO₂ estimated from −O₂/CO₂ ratios and ¹⁴CO₂ agree each other within the uncertainties.