The Total Carbon Column Observing Network (TCCON) is the baseline ground-based network of instruments that record solar absorption spectra from which accurate and precise column-averaged dry-air mole fractions of CO₂ (XCO₂), CH₄ (XCH₄), CO (XCO), and other gases are retrieved. The TCCON data have been widely used for carbon cycle science and validation of satellites measuring greenhouse gas concentrations globally. The number of stations in the network (currently about 25) is limited and has a very uneven geographical coverage: the stations in the Northern Hemisphere are distributed mostly in North America, Europe, and Japan, and only 20 % of the stations are located in the Southern Hemisphere, leaving gaps in the global coverage. A denser distribution of ground-based solar absorption measurements is needed to improve the representativeness of the measurement data for various atmospheric conditions (humid, dry, polluted, presence of aerosol), various surface conditions such as high albedo (>0.4) and very low albedo, and a larger latitudinal distribution. More stations in the Southern Hemisphere are also needed, but a further expansion of the network is limited by its costs and logistical requirements. For this reason, several groups are investigating supplemental portable low-cost instruments. The European Space Agency (ESA) funded campaign Fiducial Reference Measurements for Ground-Based Infrared Greenhouse Gas Observations (FRM4GHG) at the Sodankylä TCCON site in northern Finland aims to characterise the assessment of several low-cost portable instruments for precise solar absorption measurements of XCO₂, XCH₄, and XCO. The test instruments under investigation are three Fourier transform spectrometers (FTSs): a Bruker EM27/SUN, a Bruker IRcube, and a Bruker Vertex70, as well as a laser heterodyne spectroradiometer (LHR) developed by the UK Rutherford Appleton Laboratory. All four remote sensing instruments performed measurements simultaneously next to the reference TCCON instrument, a Bruker IFS 125HR, for a full year in 2017. The TCCON FTS was operated in its normal high-resolution mode (TCCON data set) and in a special low-resolution mode (HR125LR data set), similar to the portable spectrometers. The remote sensing measurements are complemented by regular AirCore launches performed from the same site. They provide in situ vertical profiles of the target gas concentrations as auxiliary reference data for the column retrievals, which are traceable to the WMO SI standards. The reference measurements performed with the Bruker IFS 125HR were found to be affected by non-linearity of the indium gallium arsenide (InGaAs) detector. Therefore, a non-linearity correction of the 125HR data was performed for the whole campaign period and compared with the test instruments and AirCore. The non-linearity-corrected data (TCCONmod data set) show a better match with the test instruments and AirCore data compared to the non-corrected reference data. The time series, the bias relative to the reference instrument and its scatter, and the seasonal and the day-to-day variations of the target gases are shown and discussed. The comparisons with the HR125LR data set gave a useful analysis of the resolution-dependent effects on the target gas retrieval. The solar zenith angle dependence of the retrievals is shown and discussed. The intercomparison results show that the LHR data have a large scatter and biases with a strong diurnal variation relative to the TCCON and other FTS instruments. The LHR is a new instrument under development, and these biases are currently being investigated and addressed. The campaign helped to characterise and identify instrumental biases and possibly retrieval biases, which are currently under investigation. Further improvements of the instrument are ongoing. The EM27/SUN, the IRcube, the modified Vertex70, and the HR125LR provided stable and precise measurements of the target gases during the campaign with quantified small biases. The bias dependence on the humidity along the measurement line of sight has been investigated and no dependence was found. These three portable low-resolution FTS instruments are suitable to be used for campaign deployment or long-term measurements from any site and offer the ability to complement the TCCON and expand the global coverage of ground-based reference measurements of the target gases.

中文翻译：

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低分辨率和高分辨率红外光谱仪的比较，用于地面太阳遥感测量CO ₂，CH ₄和CO的总列浓度

总碳柱观测网络（TCCON）是基于地面的基线仪器网络，该仪器记录太阳吸收光谱，从中可以准确，精确地计算出CO ₂（XCO ₂），CH ₄（XCH ₄）的柱平均干空气摩尔分数。），CO（XCO）和其他气体。TCCON数据已广泛用于碳循环科学和全球范围内用于测量温室气体浓度的卫星的验证。网络中的站点数量有限（目前约25个），并且地理覆盖范围非常不均匀：北半球的站点主要分布在北美，欧洲和日本，而只有20％的站点位于南半球，在全球覆盖范围上尚存空白。需要更密集的地面太阳能吸收测量分布，以提高各种大气条件（潮湿，干燥，污染，存在气溶胶），各种表面条件（例如高反照率（> 0.4））的测量数据的代表性。）和非常低的反照率，以及较大的纬度分布。在南半球也需要更多台站，但是网络的进一步扩展受到其成本和后勤要求的限制。因此，几个小组正在研究补充便携式低成本仪器。欧洲航天局（ESA）在芬兰北部的SodankyläTCCON站点资助了“地面红外温室气体观测基准基准测量（FRM4GHG）”运动，旨在表征对几种低成本便携式仪器的评估，以对XCO进行精确的太阳吸收测量₂，XCH ₄和XCO。研究中的测试仪器是三种傅立叶变换光谱仪（FTS）：Bruker EM27 / SUN，Bruker IRcube和Bruker Vertex70，以及英国卢瑟福·阿普尔顿实验室开发的激光外差光谱仪（LHR）。在2017年全年，这四台遥感仪器都与参考TCCON仪器（布鲁克IFS 125HR）相邻，同时进行了测量。TCCONFTS以其常规高分辨率模式（TCCON数据集）和特殊的低分辨率操作。分辨率模式（HR125LR数据集），类似于便携式光谱仪。在同一地点执行的常规AirCore发射补充了遥感测量。它们提供目标气体浓度的原位垂直剖面作为柱检索的辅助参考数据，标准。发现使用Bruker IFS 125HR进行的参考测量受砷化铟镓（InGaAs）检测器非线性影响。因此，在整个战役期间对125HR数据进行了非线性校正，并与测试仪器和AirCore进行了比较。与未经校正的参考数据相比，经过非线性校正的数据（TCCONmod数据集）显示出与测试仪器和AirCore数据的更好匹配。显示并讨论了时间序列，相对于参考仪器的偏差及其散布，以及目标气体的季节性和日常变化。与HR125LR数据集的比较提供了有用的分析方法，该方法分析了分辨率对目标气体采收率的影响。显示并讨论了太阳天顶角与取回的关系。相互比较的结果表明，相对于TCCON和其他FTS仪器，LHR数据具有较大的散度和偏差，并且昼夜变化很大。LHR是一种正在开发的新工具，目前正在研究和解决这些偏见。该运动有助于表征和确定仪器偏差，可能还有检索偏差，这些偏差目前正在调查中。仪器的进一步改进正在进行中。EM27 / SUN，IRcube，改良的Vertex70和HR125LR在运动期间以定量的小偏差对目标气体进行了稳定而精确的测量。已经研究了沿测量视线对湿度的偏差依赖性，但未发现依赖性。