A comparison of atmospheric CO2 concentration GOSAT-based observations and model simulations

Satellite observations of atmospheric CO2 are able to truly capture the variation of global and regional CO2 concentration.The model simulations based on atmospheric transport models can also assess variations of atmospheric CO2 concentrations in a continuous space and time,which is one of approaches for qualitatively and quantitatively studying the atmospheric transport mechanism and spatio-temporal variation of atmospheric CO2 in a global scale.Satellite observations and model simulations of CO2 offer us two different approaches to understand the atmospheric CO2.However,the difference between them has not been comprehensively compared and assessed for revealing the global and regional features of atmospheric CO2.In this study,we compared and assessed the spatio-temporal variation of atmospheric CO2 using two datasets of the column-averaged dry air mole fractions of atmospheric CO2(XCO2)in a year from June 2009 to May 2010,respectively from GOSAT retrievals(V02.xx)and from Goddard Earth Observing System-Chemistry(GEOS-Chem),which is a global 3-D chemistry transport model.In addition to the global comparison,we further compared and analyzed the difference of CO2 between the China land region and the United States(US)land region from two datasets,and demonstrated the reasonability and uncertainty of satellite observations and model simulations.The results show that the XCO2 retrieved from GOSAT is globally lower than GEOS-Chem model simulation by 2 ppm on average,which is close to the validation conclusion for GOSAT by ground measures.This difference of XCO2 between the two datasets,however,changes with the different regions.In China land region,the difference is large,from 0.6 to 5.6 ppm,whereas it is 1.6 to 3.7 ppm in the global land region and 1.4 to 2.7 ppm in the US land region.The goodness of fit test between the two datasets is 0.81 in the US land region,which is higher than that in the global land region(0.67)and China land region(0.68).The analysis results further indicate that the inconsistency of CO2concentration between satellite observations and model simulations in China is larger than that in the US and the globe.This inconsistency is related to the GOSAT retrieval error of CO2 caused by the interference among input parameters of satellite retrieval algorithm,and the uncertainty of driving parameters in GEOS-Chem model.     

三峡澎溪河水-气界面温室气体模型估算及其敏感性分析

模型估算法是水-气界面温室气体通量监测的主要方法,所得成果也不胜枚举.然而监测过程中诸多环境因素会对最终结果产生不确定的影响.结合三峡库区澎溪河背景条件,利用模型估算法进行水-气界面温室气体通量(以CO_2为例)估算,并且采用修正Morris筛选法尝试分析模型估算法中各个参数对温室气体扩散通量(以CO_2为例)的局部敏感性.研究结果表明:利用模型估算法计算三峡澎溪河流域水-气界面温室气体通量具有较高的可行性和可靠性;风速、水温以及pH值会对监测结果产生影响,且风速越强、水温越高、pH值越小,CO_2扩散通量就越大;pH值是高灵敏参数,风速和水温是灵敏参数.在三峡库区澎溪河监测过程中更应注意pH值的精确性,每次采样前需校正仪器.     

Infrared Radiation in the Mesosphere and Lower Thermosphere: Energetic Effects and Remote Sensing

This paper discusses the formation mechanisms of infrared radiation in the mesosphere and lower thermosphere (MLT), the energetic effects of the radiative absorption/emission processes, and the retrieval of atmospheric parameters from infrared radiation measurements. In the MLT and above, the vibrational levels of the molecules involved in radiative transitions are not in local thermodynamic equilibrium (LTE) with the surrounding medium, and this then requires specific theoretical treatment. The non-LTE models for CO₂, O₃, and H₂O molecules are presented, and the radiative cooling/heating rates estimated for five typical atmospheric scenarios, from polar winter to polar summer, are shown. An optimization strategy for calculating the cooling/heating rates in general circulation models is proposed, and its accuracy is estimated for CO₂. The sensitivity of the atmospheric quantities retrieved from infrared observations made from satellites to the non-LTE model parameters is shown.     

Comparison of SO<Subscript>2</Subscript> column retrievals from BRD and DOAS algorithms

Atmospheric SO₂ has a significant impact on the urban environment and global climate. Band Residual Difference Algorithm (BRD) and Differential Optical Absorption Spectroscopy (DOAS) were used respectively by NASA and ESA science team to derive SO₂ columns from satellite observations, but there are few studies on the comparison and validation of BRD and DOAS SO₂ retrievals under the same observation conditions. In this study, the radiative transfer model SCIATRAN was firstly used to validate the accuracies of BRD and DOAS SO₂ retrievals, and analyse the uncertainty of SO₂ retrieval caused by band selection, O3 absorption, aerosol, surface reflectance, solar and viewing zenith angle. Finally, BRD and DOAS algorithms were applied to the same radiances from satellite observations, and comparisons of BRD and DOAS SO₂ retrievals were conducted over volcanic eruption and North China. Results show that, for the case with low SO₂ columns, BRD SO₂ retrievals have higher retrieval accuracy than DOAS, but typical seasonal variation with high SO₂ column in winter and low in summer can be more clearly discernible in DOAS SO₂ retrievals than BRD from satellite observations. For the case with high SO₂ columns, the differences between BRD (310.8–314.4 nm) and DOAS (315–327 nm) retrievals are large, and the value and accuracy of BRD (310.8–314.4 nm) SO₂ retrievals are lower than those of DOAS (315–327 nm) retrievals. Compared with the SO₂ inputs in forward model, both BRD (310.8–314.4 nm) and DOAS (315–327 nm) SO₂ retrievals are underestimated for the case with high SO₂ columns. The selection of wavelength range can significantly affect the accuracy of SO₂ retrieval. The error of BRD SO₂ retrieval from 310.8–314.4 nm is lower than other bands in the ultraviolet spectral region (306–327 nm). The increase of wavelength in the ultraviolet spectral region 306–330 nm can reduce the underestimation of DOAS SO₂ retrievals in the case of high SO₂ column, but slight overestimation of SO₂ retrieval is found from the 315–327 nm range in the case of low SO₂ column. The values of BRD and DOAS SO₂ retrieval decrease with atmospheric O₃ column and aerosol optical depth increasing, but increase with surface reflectance increasing. Large solar zenith angle and viewing zenith angle can introduce more errors to the BRD and DOAS SO₂ retrievals. This study is important for the improvement of retrieval algorithm and the application of SO₂ products from satellite observations.     

The importance of ocean temperature to global biogeochemistry

Variations in the mean temperature of the ocean, on time scales from millennial to millions of years, in the past and projected for the future, are large enough to impact the geochemistry of the carbon, oxygen, and methane geochemical systems. In each system, the time scale of the temperature perturbation is key. On time frames of 1-100 ky, atmospheric CO₂ is controlled by the ocean. CO₂ temperature-dependent solubility and greenhouse forcing combine to create an amplifying feedback with ocean temperature; the CaCO₃ cycle increases this effect somewhat on time scales longer than ∼5-10 ky. The CO₂/T feedback can be seen in the climate record from Vostok, and a model including the temperature feedback predicts that 10% of the fossil fuel CO₂ will reside in the atmosphere for longer than 100 ky. Timing is important for oxygen, as well; the atmosphere controls the ocean on short time scales, but ocean anoxia controls atmospheric pO₂ on million-year time scales and longer. Warming the ocean to Cretaceous temperatures might eventually increase pO₂ by approximately 25%, in the absence of other perturbations. The response of methane clathrate to climate change in the coming century will probably be small, but on longer time scales of 1-10 ky, there may be a positive feedback with ocean temperature, amplifying the long-term climate impact of anthropogenic CO₂ release.     

The greenhouse effect

The greenhouse effect on the Earth is identified by the difference between the effective radiating temperature of the planet and its surface temperature. The difference between the energy emitted by the surface and that emitted upward to space by the upper atmosphere quantifies it; it can therefore be defined as the long wave energy trapped in the atmosphere. Climate forcing and the response of the climate system within which climate feedback mechanisms are contained, will be defined in this review. Quantitative examples will illustrate what could happen if the greenhouse effect is perturbed by the human activities, in particular if atmospheric CO₂ concentrations would double in the future. Recent measurements by satellites of the greenhouse effect will be given. The net cooling effect of clouds on the Earth and whether or not there will be less cooling by clouds as the planet warms, are discussed following a series of papers recently published by Ramanathan and his collaborators.     

亚热带城市湖泊与河流CO2气体通量特征及其影响因素

湖泊、河流等内陆水体是连接陆地生态系统和海洋的“长程碳环路”的重要节点,也是温室气体二氧化碳(CO₂)排放源,在调节陆地、海洋间的碳迁移转换中发挥着重要作用。相对于自然水体,城市水体因面积小、水深浅且受监测方法限制,水-气界面碳通量经常被忽略。为探讨我国亚热带城市水体温室气体排放特征,本研究以湖南省长沙市典型城市水体,包括洋湖、西湖、松雅湖、月湖4个湖泊和湘江长沙段为研究对象,分别于2022年4和10月采用光化学反馈-腔增强吸收光谱法(OF-CEAS)和扩散模型法对水-气界面CO₂通量进行对比测定。结果表明,长沙城市湖泊与河流春季为CO₂排放源,秋季为吸收汇,河流水-气界面CO₂通量呈显著季节差异。河湖之间CO₂通量在春季表现为显著差异,秋季差异不显著。CO₂通量与水体溶解氧、水体总氮浓度等呈显著正相关。2种方法的CO₂通量对比测定在湖泊上显著相关,但对河流而言相关性不显著。研究揭示的城市湖泊与河流CO₂气体的排放特征有利于深入探究城市水体碳的迁移转化,可对全面了解全球气候变化过程和河湖湿地温室气体减排和调控提供科学支撑。     

城市湖泊不同水生植被区水体温室气体溶存浓度及其影响因素

为了解城市湖泊不同水生植被区水体温室气体的溶存浓度及其影响因素,于2015年4-11月按每月2次的频率采用顶空平衡法对聊城市铃铛湖典型植被区——菹草区、莲藕区和睡莲区表层水中CO₂、CH₄和N₂O的溶存浓度进行监测,计算水中温室气体的饱和度和排放通量,并测定水温（T）、pH、溶解氧（DO）、叶绿素a及营养盐浓度等理化指标,以探究水体环境因子对温室气体溶存浓度的影响.结果表明,铃铛湖各植被区水体温室气体均处于过饱和状态,是大气温室气体的"源";莲藕区CH₄浓度、饱和度和排放通量均显著高于菹草区,而各植被区N₂O和CO₂均无显著性差异;不同植被区湖水中DO、总氮（TN）、总磷（TP）和硝态氮（NO₃^--N）浓度具有显著差异,其中DO、TN和NO₃^--N浓度均表现为菹草区最高,莲藕区最低,而TP浓度则正好相反;各植被区温室气体浓度和水环境参数间的相关分析和多元回归分析的结果表明,水生植物可通过影响水体的理化性质对温室气体的产生和排放产生显著差异影响,在菹草区亚硝态氮（NO₂^--N）、NO₃^--N、T和DO是控制水体温室气体浓度的主要因子;睡莲区为TP和pH;莲藕区则为pH、NO₂^--N和DO.     

Green's functions and response functions in geochemical modelling

The use of Green's functions and response functions in modelling atmospheric and oceanic tracers is discussed. It is shown that low-resolution Green's function representations can describe tracer distributions more accurately than low-resolution models. The Green's function formalism is also used to discuss the problem of determining surface CO₂ sources from observations of surface CO₂ concentrations.     

Greenhouse gas emissions (CO2, CH4, and N2O) from several perialpine and alpine hydropower reservoirs by diffusion and loss in turbines

We investigated greenhouse gas emissions (CO₂, CH₄, and N₂O) from reservoirs located across an altitude gradient in Switzerland. These are the first results of greenhouse gas emissions from reservoirs at high elevations in the Alps. Depth profiles were taken in 11 reservoirs located at different altitudes between the years 2003 and 2006. Diffusive trace gas emissions were calculated using surface gas concentrations, wind speeds and transfer velocities. Additionally, methane entering with the inflowing water and methane loss at the turbine was assessed for a subset of the reservoirs. All reservoirs were emitters of carbon dioxide and methane with an average of 970?±?340?mg?m^?2?day^?1 (results only from four lowland and one subalpine reservoir) and 0.20?±?0.15?mg?m^?2?day^?1, respectively. One reservoir (Lake Wohlen) emitted methane at a much higher rate (1.8?±?0.9?mg?m^?2?day^?1) than the other investigated reservoirs. There was no significant difference in methane emissions across the altitude gradient, but average dissolved methane concentrations decreased with increasing elevation. Only lowland reservoirs were sources for N₂O (72?±?22???g?m^?2?day^?1), while the subalpine and alpine reservoirs were in equilibrium with atmospheric concentrations. These results indicate reservoirs from subalpine/alpine regions to be only minor contributors of greenhouse gases to the atmosphere compared to other reservoirs.     

The oxygen minimum zone (OMZ) off Chile as intense source of CO2 and N2O

The oxygen minimum zones (OMZs) are recognized as intense sources of N₂O greenhouse gas (GHG) and could also be potential sources of CO₂, the most important GHG for the present climate change. This study evaluates, for one of the most intense and shallow OMZ, the Chilean East South Pacific OMZ, the simultaneous N₂O and CO₂ fluxes at the air–sea interface. Four cruises (2000–2002) and 1 year of monitoring (21°–30°–36°S) off Chile allowed the determination of the CO₂ and N₂O concentrations at the sea surface and the analysis of fluxes variations associated with different OMZ configurations. The Chilean OMZ area can be an intense GHG oceanic local source of both N₂O and CO₂. The mean N₂O fluxes are 5–10 times higher than the maximal previous historical source in an OMZ open area as in the Pacific and Indian Oceans. For CO₂, the mean fluxes are also positive and correspond to very high oceanic sources. Even if different coupling and decoupling between N₂O and CO₂ are observed along the Chilean OMZ, 65% of the situations represent high CO₂ and/or N₂O sources. The high GHG sources are associated with coastal upwelling transport of OMZ waters rich in N₂O and probably also in CO₂, located at a shallow depth. The integrated OMZ role on GHG should be better considered to improve our understanding of the past and future atmospheric CO₂ and N₂O evolutions.     

The characteristics of atmospheric CO<Subscript>2</Subscript> concentration variation of four national background stations in China

Atmospheric carbon dioxide is an important kind of greenhouse gas which influences global temperature. Its concentration variation could indicate the distribution of human and natural activities in various regions. Through the non-dispersive infrared method, flask sampling of atmospheric CO₂ concentration was measured weekly at four national background stations including Waliguan, Shangdianzi, Lin’an, and Longfengshan. Based on the data collected from September 2006 to August 2007, along with the Waliguan station’s experience on in situ observational data processing, the selection methods for sampling data through the atmospheric background CO₂ concentration analysis were preliminarily discussed. On the basis of this result, the variation features of the four typical regions’ atmospheric background CO₂ concentration was analyzed for the first time. The results show that the atmospheric CO₂ concentration at Waliguan, Shangdianzi, Lin’an, and Longfengshan is 383.5, 385.9, 387.8, and 384.3 ppm, respectively. During the research period, CO₂ concentration at the Waliguan station changed slightly. However, the CO₂ concentration changed sharply at the Shangdianzi and the Lin’an stations due to the great influence of human activities in the Jingjinji and the Changjiang Delta economic zones, and changed regularly with seasons at Longfengshan station under dual influences of human activities and plant photosynthesis. The results from this study can lay the foundation for more profound studies on atmospheric CO₂ concentration level of different areas in China, and could be used to improve the understanding of carbon source and sink distribution.     

Monitoring air pollution in China from geostationary satellite: A synthetic study using simulated observations

We simulated geostationary satellite observations to assess the potential for high spatial-and temporal-resolution monitoring of air pollution in China with a focus on tropospheric ozone(O_3), nitrogen dioxide(NO_2), sulfur dioxide(SO_2), and formaldehyde(HCHO). Based on the capabilities and parameters of the payloads onboard sun-synchronous satellites, we simulated the observed spectrum based on a radiative transfer model using a geostationary satellite model. According to optimal estimation theory, we analyzed the sensitivities and retrieval uncertainties of the main parameters of the instrument for the target trace gases. Considering the retrieval error requirements of each trace gas, we determined the major instrument parameter values(e.g., observation channel, spectral resolution, and signal-to-noise ratio). To evaluate these values, retrieval simulation was performed based on the three-dimensional distribution of the atmospheric components over China using an atmospheric chemical transportation model. As many as 90% of the experiments met the retrieval requirements for all target gases. The retrieval precision of total-column and stratospheric O_3 was 2%. In addition, effective retrieval of all trace gases could be achieved at solar zenith angles larger than 70°. Therefore, the geostationary satellite observation and instrument parameters provided herein can be used in air pollution monitoring in China. This study offers a theoretical basis and simulation tool for improving the design of instruments onboard geostationary satellites.     

On the accumulative contribution of CO2 emission from China to global climate change

A study of the contribution to global climate change from China’s CO₂ emission is conducted using the FIO-ESM v1.0 climate model. A series of sensitivity experiments are performed to identify two kinds of contributions to global climate change of China’s CO₂ emission due to fossil fuel combustion: one is the pure contribution which is the historical climate response from the sensitivity experiment forced only by China’s CO₂ emission, the other is the accumulative contribution which is proposed in this research and defined as the difference of historical climate responses between the experiments forced by all countries’ CO₂ emission and other countries’ CO₂ emission excluding China. The pure contribution approach considers the total CO₂ discharged by China, while the accumulative contribution approach considers not only the discharge amount of China but also the discharge order of China and other countries. The latter is a more realistic approach to quantify the contribution of CO₂ emission to the historical change of atmospheric CO₂ concentration, surface air temperature (SAT), sea surface temperature (SST) and sea ice coverage in the Arctic. Model results show that from the accumulative perspective, the ratio of the contribution of CO₂ emission from China for the increase of atmospheric CO₂ concentration, SAT and SST, and the decrease of the sea ice coverage in the Arctic to that from all other countries excluding China varies from 8% to 92%, 5% to 95%, 9% to 91% and 18% to 82%, respectively. Here we take the contribution of China’s CO₂ emission as an example, the contribution of CO₂ emission from any other country or area can be evaluated by the same approach.     

Erta'ale lava lake: heat and gas transfer to the atmosphere

Data on uncontaminated samples of volcanic gases can be counted on the fingers of one hand, yet estimation of total volcanic gas flow cannot be made without such data. In this paper the flux of gas from the lava lake to the atmosphere is calculated by a heat budget based on the excess heat loss caused by combustion of H₂ and CO and by the mass rate of loss of other gases on the basis of their ratios to H₂ and CO in the unoxidized gas samples. The estimated rates of loss of H₂O, CO₂, SO₂ and HCl are consistent with the rate of loss of heat if this heat is generated by crystallization and if the initial magma contains concentrations of gas appropriate for submarine basalt from oceanic ridges. The moderate activity of permanent degassing from the two active lava ponds studied gives a lower flux than that of other volcanoes.     

The snowball Earth aftermath: Exploring the limits of continental weathering processes

Carbonates capping Neoproterozoic glacial deposits contain peculiar sedimentological features and geochemical anomalies ascribed to extraordinary environmental conditions in the snowball Earth aftermath. It is commonly assumed that post-snowball climate dominated by CO₂ partial pressures several hundred times greater than modern levels, would be characterized by extreme temperatures, a vigorous hydrological cycle, and associated high continental weathering rates. However, the climate in the aftermath of a global glaciation has never been rigorously modelled. Here, we use a hierarchy of numerical models, from an atmospheric general circulation model to a mechanistic model describing continental weathering processes, to explore characteristics of the Earth system during the supergreenhouse climate following a snowball glaciation. These models suggest that the hydrological cycle intensifies only moderately in response to the elevated greenhouse. Indeed, constraints imposed by the surface energy budget sharply limit global mean evaporation once the temperature has warmed sufficiently that the evaporation approaches the total absorbed solar radiation. Even at 400 times the present day pressure of atmospheric CO₂, continental runoff is only 1.2 times the modern runoff. Under these conditions and accounting for the grinding of the continental surface by the ice sheet during the snowball event, the simulated maximum discharge of dissolved elements from continental weathering into the ocean is approximately 10 times greater than the modern flux. Consequently, it takes millions of years for the silicate weathering cycle to reduce post-snowball CO₂ levels to background Neoproterozoic levels. Regarding the origin of the cap dolostones, we show that continental weathering alone does not supply enough cations during the snowball melting phase to account for their observed volume.     

水库温室气体净通量评估模型(G-res Tool)及在长江上游典型水库初步应用

目前准确量化温室气体排放量已成为气候变化研究和政策制定的关键.在IPCC水库温室气体净通量的概念性框架下,国际水电协会汇总分析了全球223座水库的CO₂和CH₄研究成果,构建了G-res Tool,其可以用于评估已建或待建水库在长时间尺度下的温室气体净通量.本文介绍了G-res Tool模型的基本原理与模型框架,利用模型内置数据库中所涉及的中国长江上游12座典型水库数据进行初步应用分析,12座水库温室气体净通量平均值为88.17 g CO₂e/（m²·a）,在全球约7000座水库中所处水平为11.67%,处于低阈值范围.在水库温室气体净通量分析结果中,其他非相关人类活动产生的水库温室气体通量（UAS）在蓄水后总通量（Post）中所占比重远高于蓄水前温室气体通量（Pre）.长江上游水库蓄水后的CH₄和CO₂通量对于温室效应的贡献量相当.通过将G-res Tool模型蓄水后的温室气体通量评估结果和所涉及到的12座水库中已发表的数据对比分析发现,G-res Tool具有简便、适用面广等特点.但G-res Tool毕竟仍为经验性模型,其基本原理和模块设计上的内在缺陷在很大程度上限制了其应用范围并造成了一定的不确定性.对个案水库而言,长期跟踪观测与机理研究仍是未来减少水库温室气体净通量不确定性的关键.     

Impacts of Methane on Carbon Dioxide Storage in Brine Formations

In the context of geological carbon sequestration (GCS), carbon dioxide (CO₂) is often injected into deep formations saturated with a brine that may contain dissolved light hydrocarbons, such as methane (CH₄). In this multicomponent multiphase displacement process, CO₂ competes with CH₄ in terms of dissolution, and CH₄ tends to exsolve from the aqueous into a gaseous phase. Because CH₄ has a lower viscosity than injected CO₂, CH₄ is swept up into a ‘bank’ of CH₄‐rich gas ahead of the CO₂ displacement front. On the one hand, this may provide a useful tracer signal of an approaching CO₂ front. On the other hand, the emergence of gaseous CH₄ is undesirable because it poses a leakage risk of a far more potent greenhouse gas than CO₂ if the cap rock is compromised. Open fractures or faults and wells could result in CH₄ contamination of overlying groundwater aquifers as well as surface emissions. We investigate this process through detailed numerical simulations for a large‐scale GCS pilot project (near Cranfield, Mississippi) for which a rich set of field data is available. An accurate cubic‐plus‐association equation‐of‐state is used to describe the non‐linear phase behavior of multiphase brine‐CH₄‐CO₂ mixtures, and breakthrough curves in two observation wells are used to constrain transport processes. Both field data and simulations indeed show the development of an extensive plume of CH₄‐rich (up to 90 mol%) gas as a consequence of CO₂ injection, with important implications for the risk assessment of future GCS projects.     

Variations of near-surface atmospheric CO2 and H2O concentrations during summer on Muztagata

Expeditions during the summers of 2002 and 2003 implemented continuous monitoring of near-surface (2 m height) atmospheric CO₂ and H₂O concentrations at the 4500 m elevation on Muztagata. The resultant data sets reveal a slight decrease of CO₂ concentrations (of about 5 μmol·mol^-1) and changes in the diurnal variations from the end of June to the middle August. The daily maximum CO₂ concentrations occur between 02:30-05:30 AM (local time) and the minimum levels occur between 12:00-15:30 PM. The atmospheric CO₂ concentrations in the summer of 2002 were around 5 μmol·mol^-1 lower than those during the same period of 2003, whereas the diurnal amplitude was higher. In contrast, we found that the daily mean atmospheric H₂O content in 2003 was much lower than that in 2002 and there exists a striking negative correlation between CO₂ and H₂O concentrations. We therefore suggest that the near-surface atmospheric CO₂ concentration is affected not only by photosynthesis and respiration, but also by the air H₂O content in the glaciated region around Muztagata.