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Atmos. Meas. Tech., 5, 2969–2980, 2012 www.atmos-meas-tech.net/5/2969/2012/ doi:10.5194/amt-5-2969-2012 © Author(s) 2012. CC Attribution 3.0 License.

Atmospheric Measurement Techniques

XCO2-measurements with a tabletop FTS using solar absorption spectroscopy M. Gisi1 , F. Hase1 , S. Dohe1 , T. Blumenstock1 , A. Simon2 , and A. Keens2 1 Karlsruhe 2 Bruker

Institute of Technology (KIT), Institute for Meteorology and Climate Research (IMK-ASF), Karlsruhe, Germany Optics GmbH, Ettlingen, Germany

Correspondence to: M. Gisi ([email protected]) Received: 24 July 2012 – Published in Atmos. Meas. Tech. Discuss.: 17 August 2012 Revised: 24 October 2012 – Accepted: 3 November 2012 – Published: 29 November 2012 Abstract. A commercial low-resolution (0.5 cm−1 ) Fourier Transform Spectrometer (FTS) has been modified and is used for determining the total column XCO2 of the atmosphere by analysing direct solar radiation. The spectrometer has a small home-built solar tracker attached, so that it is a ready-touse instrument. The results are validated with temporally coinciding on-site measurements taken with a high-resolution Total Carbon Column Observing Network (TCCON) FTIR spectrometer. For the whole comparison period of 5 months (26 measurement days) an agreement with TCCON results of (0.12 ± 0.08) % is achieved. This makes the spectrometer a promising candidate for a low-cost addition to the TCCON core FTIR sites, especially suitable for locations with limited infrastructure. An impressive mechanical and thermal stability is proved, enabling the spectrometer for use in field campaigns and for the monitoring of local sources.

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Introduction

Within the last several years, it has been recognised that a precise knowledge of the global abundances of greenhouse gases such as CO2 , N2 O and CH4 is required for understanding the mechanisms of the global carbon cycle and to determine it’s sources and sinks (Olsen and Randerson, 2004). To measure column-averaged mole fractions of these gases, several satellite-borne instruments were developed such as SCIAMACHY (http://www.sciamachy.de/), GOSAT (http://www.gosat.nies.go.jp/index e.html) and OCO/OCO-2 (http://oco.jpl.nasa.gov/news/). The latter two missions are exclusively dedicated on measuring greenhouse gases and OCO-2 is scheduled for launch in 2014.

To support and validate these measurements great efforts have been undertaken to set up the Total Carbon Column Observation Network (TCCON). It consists of 23 globally distributed, ground-based solar absorption Fourier transform spectrometers operating in the Near Infrared (NIR) spectral region to measure the CO2 total column in the atmosphere. These high-resolution TCCON spectrometers achieve an unprecedented accuracy of 0.25 % for the atmospheric XCO2 content (Wunch et al., 2011). The XCO2 , which is the column-averaged dry-air mole fraction, is derived by the ratio of the measured CO2 and O2 total columns by XCO2 =

CO2,column · 0.2095 O2,column

(1)

where 0.2095 is the dry air O2 mole fraction. This approach compensates for numerous systematic errors, such as a pointing offset or an erroneous surface pressure. To make the network results comparable to WMO (World Meteorological Organisation) standards, the retrieved XCO2 value is divided by a calibration factor of 0.989. This value was determined by comparisons with in situ profiles measured in the framework of aircraft campaigns (Wunch et al., 2010; Messerschmidt et al., 2011). Despite its outstanding capabilities such as the precision and stability during normal operations, the IFS125HR spectrometers from Bruker™ used by the TCCON also have their limitations. The IFS125HR is an expensive high-precision instrument, its large dimensions on the order of 1 × 1 × 3 m and the mass well beyond 100 kg makes it difficult to transport, and its operation requires a significant amount of infrastructure, such as a stable support platform, air conditioning and su