XCO2 and XCH4 total column measurement during POLARSTERN cruise PS83 (ANT-XXIX/10), north-south gradient

A portable Fourier transform spectrometer (FTS), model EM27/SUN, was deployed onboard the research vessel Polarstern to measure the column-average dry air mole fractions of carbon dioxide (XCO2) and methane (XCH4) by means of direct sunlight absorption spectrometry. We report on technical developments as well as data calibration and reduction measures required to achieve the targeted accuracy of fractions of a percent in retrieved XCO2 and XCH4 while operating the instrument under field conditions onboard the moving platform during a 6-week cruise on the Atlantic from Cape Town (South Africa, 34° S, 18° E; 5 March 2014) to Bremerhaven (Germany, 54° N, 19° E; 14 April 2014). We demonstrate that our solar tracker typically achieved a tracking precision of better than 0.05° toward the center of the sun throughout the ship cruise which facilitates accurate XCO2 and XCH4 retrievals even under harsh ambient wind conditions. We define several quality filters that screen spectra, e.g., when the field of view was partially obstructed by ship structures or when the lines-of-sight crossed the ship exhaust plume. The measurements in clean oceanic air, can be used to characterize a spurious air-mass dependency. After the campaign, deployment of the spectrometer alongside the TCCON (Total Carbon Column Observing Network) instrument at Karlsruhe, Germany, allowed for determining a calibration factor that makes the entire campaign record traceable to World Meteorological Organization (WMO) standards. Comparisons to observations of the GOSAT satellite and concentration fields modeled by the European Centre for Medium-Range Weather Forecasts (ECMWF) Copernicus Atmosphere Monitoring Service (CAMS) demonstrate that the observational setup is well suited to provide validation opportunities above the ocean and along interhemispheric transects.

本研究将型号为EM27/SUN的便携式傅里叶变换光谱仪（Fourier Transform Spectrometer，FTS）搭载于极星号（Polarstern）科考船，采用直接太阳吸收光谱法测量二氧化碳柱平均干空气摩尔分数（XCO2）与甲烷柱平均干空气摩尔分数（XCH4）。本研究报道了为实现反演得到的XCO2与XCH4达到百分之零点几的目标精度所需的技术改进、数据校准与数据约简手段，实验期间仪器搭载于移动科考平台，于2014年3月5日至4月14日开展为期6周的大西洋航次，航线从南非开普敦（南纬34°，东经18°）延伸至德国不来梅哈芬（北纬54°，东经19°）。研究表明，在整个科考航次中，本系统搭载的太阳跟踪器对太阳中心的跟踪精度通常优于0.05°，即便在恶劣的野外风力条件下，也可保障XCO2与XCH4的精准反演。本研究设定了多组光谱数据质量过滤规则，用于筛除无效光谱：例如当观测视场被船体结构部分遮挡，或观测视线穿过船舶排气羽流时的光谱数据。清洁海洋大气中的观测数据可用于表征虚假的气团相关性。航次结束后，将该光谱仪与总碳柱观测网络（Total Carbon Column Observing Network，TCCON）仪器一同部署于德国卡尔斯鲁厄，由此确定了校准因子，使得本次航次的全部观测数据可追溯至世界气象组织（World Meteorological Organization，WMO）标准。通过与全球温室气体观测卫星（GOSAT）的观测数据，以及欧洲中期天气预报中心（European Centre for Medium-Range Weather Forecasts，ECMWF）哥白尼大气监测服务（Copernicus Atmosphere Monitoring Service，CAMS）模拟的大气浓度场进行对比，结果表明本观测装置能够很好地适配海洋上空以及跨半球断面上的大气成分验证工作需求。