Measurements of delta^<13>C of atmospheric CO_2 and its application to the carbon cycle

在过去的200年里，由于化石燃料燃烧和森林砍伐等人类活动的影响，大气中的二氧化碳浓度一直在稳步上升。为了定量地解释这种浓度增加的原因，有必要弄清楚地球表面的碳循环。然而，生物圈和海洋在碳循环中的作用还没有得到很好的理解。为了解决这一困难，需要测量大气中CO_2的碳同位素比值及其浓度。本研究对日本对流层、西太平洋和南极地区采集的CO_2样品进行了分析，其精度优于0.02*。由于生物圈活动的季节性变化，在北纬和赤道存在明显的季节变化，而在南半球则很难观察到。研究发现，由于化石燃料的燃烧作用，Delta^&lt；13&gt；C值长期下降的速度约为0.03*。长期趋势的年际变化也与ENSO事件等因素有关，与CO_2浓度的变化趋势基本相反。用箱扩散模式和二维大气输送模式分析了大气和生物圈之间的CO2浓度变化，结果表明，大气和生物圈之间的CO2交换的不平衡可能是造成这种年际变化的原因，生物圈并不是大气CO2的强大净源。

The concentration of atmospheric CO_2 has been increasing steadily for the last 200 years, due to human activities such as fossil fuel combustion and deforestation. In order to elucidate the causes of such an increase of the concentration quantitatively, it is necessary to figure out the carbon cycle on the earth's surface. However, the roles of the biosphere and the oceans in the carbon cycle are not yet well understoond. To solve this difficulty, measurements are requireed for the carbon isotopic ratio delta^<13>C of atmospheric CO_2 as well as for its concentration.In this study, CO_2 samples collected in the troposphere over Japan and in the western Pacific and the Antarctic regions were analyzed for delta^<13>C with a precision of better than 0.02*. The seasonal cycle of delta^<13>C was clearly found at northern latitudes and the equator, due to the seasonal change in the biospheric activities, but it was hardly observable in the southern hemisphere. It was found that values of delta^<13>C decreased seculary at a rate of about 0.03*, due mainly to fossil fuel combustion. Interannual variations in the long-term trend of delta^<13>C were also found in association with ENSO event and other factors, which were almost opposite in phase with those of the CO_2 concentration. The analyzes of delta^<13>C and the CO_2 concentration with a box diffusion model and a 2-D atmospheric transport model showed that an imbalance of the CO_2 exchange between the atmosphere and the biosphere could be responsible for such interannual variations and that the biosphere does not act as a strong net source for atmospheric CO_2.