Estimates of European uptake of CO2 inferred from GOSAT XCO2 retrievals: sensitivity to measurement bias inside and outside Europe
Estimates of European uptake of CO2 inferred from GOSAT XCO2 retrievals: sensitivity to measurement bias inside and outside Europe
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DOI：
10.5194/acp-16-1289-2016
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发表时间：
2016-01-01
影响因子：
6.3
通讯作者：
Sussmann, R.
中科院分区：
文献类型：
--
作者：
Feng, L.;Palmer, P. I.;Sussmann, R.
从大气CO2摩尔分数的现场测量推断的欧洲上空的自然CO2通量估计数以前曾被用来检查从空间载干空气CO2柱（X-CO2）检索推断的自上而下的通量估计数。最近的几项研究表明，从日本温室气体观测卫星和扫描成像吸收光谱仪的X-CO2数据推断的CO2通量比从现场数据推断的有更大的季节幅度和更负的年度净CO2平衡。欧洲吸收二氧化碳的原因尚不清楚，但最近的一些研究表明，这是一个真正的科学现象。在这里，我们提出了一个替代假设，并表明，现实水平的偏差GOSAT数据可能会导致错误的估计，在欧洲的吸收量增加。我们使用一个全球通量反演系统来研究测量偏差和欧洲CO2吸收估计之间的关系。我们建立了一个参考原位反演，使用Enhancement卡尔曼滤波（EnKF）同化传统的表面摩尔分数观测和X-CO2反演从基于表面的总碳柱观测网络（TCCON）。我们使用相同的EnKF系统同化两个独立版本的GOSAT X-CO2数据。我们发现GOSAT推断的欧洲陆地生物圈吸收在夏季达到峰值，与参考反演相似，但净年通量为1.40 +/- 0.19 GtC a（-1），而我们仅使用原位数据的对照反演值为0.58 +/- 0.14 GtC a（-1）。为了调和这两个估计，我们进行了一系列的数值实验，同化观测增加的偏见或同化合成观测的部分或全部的GOSAT X-CO2数据替换为模型数据。我们发现，对于我们的全球通量反演，2010年从GOSAT数据推断的欧洲吸收量增加的很大一部分（60- 90%）是由于直接欧洲地区以外的检索，而其余部分在很大程度上可以解释为亚ppm检索偏差超过欧洲。我们使用的数据同化的方法来估计每月GOSAT X-CO2偏差从联合同化的原位观测和GOSAT X-CO2检索。推断的偏差代表了GOSAT X-CO2反演和反演系统在区域或次区域尺度上的系统差异的估计。我们发现，每月变化的偏差高达0.5 ppm可以解释一个高估的年度汇高达0.20 GtC a（-1）。我们的研究结果强调了CO2通量估计对区域观测偏差的敏感性，目前的观测网络还没有完全表征。如果没有进一步的专门测量，我们无法证明或反驳欧洲生态系统吸收的二氧化碳量超过预期。还需要更强大的反演系统来从多种观测类型推断一致的通量。
Estimates of the natural CO2 flux over Europe inferred from in situ measurements of atmospheric CO2 mole fraction have been used previously to check top-down flux estimates inferred from space-borne dry-air CO2 column (X-CO2) retrievals. Several recent studies have shown that CO2 fluxes inferred from X-CO2 data from the Japanese Greenhouse gases Observing SATellite (GOSAT) and the Scanning Imaging Absorption Spectrometer for Atmospheric CHartographY (SCIAMACHY) have larger seasonal amplitudes and a more negative annual net CO2 balance than those inferred from the in situ data. The cause of this elevated European uptake of CO2 is still unclear, but some recent studies have suggested that this is a genuine scientific phenomenon. Here, we put forward an alternative hypothesis and show that realistic levels of bias in GOSAT data can result in an erroneous estimate of elevated uptake over Europe. We use a global flux inversion system to examine the relationship between measurement biases and estimates of CO2 uptake from Europe. We establish a reference in situ inversion that uses an Ensemble Kalman Filter (EnKF) to assimilate conventional surface mole fraction observations and X-CO2 retrievals from the surface-based Total Carbon Column Observing Network (TCCON). We use the same EnKF system to assimilate two independent versions of GOSAT X-CO2 data. We find that the GOSAT-inferred European terrestrial biosphere uptake peaks during the summer, similar to the reference inversion, but the net annual flux is 1.40 +/- 0.19 GtC a(-1) compared to a value of 0.58 +/- 0.14 GtC a(-1) for our control inversion that uses only in situ data. To reconcile these two estimates, we perform a series of numerical experiments that assimilate observations with added biases or assimilate synthetic observations for which part or all of the GOSAT X-CO2 data are replaced with model data. We find that for our global flux inversions, a large portion (60-90 %) of the elevated European uptake inferred from GOSAT data in 2010 is due to retrievals outside the immediate European region, while the remainder can largely be explained by a sub-ppm retrieval bias over Europe. We use a data assimilation approach to estimate monthly GOSAT X-CO2 biases from the joint assimilation of in situ observations and GOSAT X-CO2 retrievals. The inferred biases represent an estimate of systematic differences between GOSAT X-CO2 retrievals and the inversion system at regional or sub-regional scales. We find that a monthly varying bias of up to 0.5 ppm can explain an overestimate of the annual sink of up to 0.20 GtC a(-1). Our results highlight the sensitivity of CO2 flux estimates to regional observation biases, which have not been fully characterized by the current observation network. Without further dedicated measurements we cannot prove or disprove that European ecosystems are taking up a larger-than-expected amount of CO2. More robust inversion systems are also needed to infer consistent fluxes from multiple observation types.