Author: Mahesh Kumar Sha

Measurement of greenhouse gases from novel ground-based remote sensing instruments the FRM4GHG campaign at the Sodankylä TCCON site, N. Finland
Mahesh Kumar Sha1*, Martine De Mazière1, Christian Hermans1, Francis Scolas1, Justus Notholt2, Christof Petri2, Huilin Chen3, Marcel de Vries3, David Griffith4, Nicholas Jones4, Nicholas Deutscher4, Frank Hase5, Thomas Blumenstock5, Qiansi Tu5, Rigel Kivi6, Pauli Heikkinen6, Damien Weidmann7, Alex Hoffmann7, Marko Huebner7, Angelika Dehn8 (*)mahesh.sha@aeronomie.be

Koninklijk Belgisch Instituut voor Ruimte-Aeronomie (BIRA) Institut royal d’Aéronomie Spatiale de Belgique (IASB) Royal Belgian Institute for Space Aeronomy (BIRA-IASB) Koninklijk Belgisch Instituut voor Ruimte-Aeronomie (BIRA) Institut royal d’Aéronomie

Motivation for the project — Fiducial Reference Measurements for Greenhouse Gases (FRM4GHG)
Greenhouse gas (GHG) measuring satellites require ground-based measurements for validation of temporal and spatial trends and/or dependencies on satellite observational characteristics.
Total Carbon Column Observing Network (TCCON) provides accurate and precise measurements of GHGs and is the current standard for validation of GHG measuring satellites (e.g. SCIAMACHY, GOSAT, OCO-2, and upcoming S5P, GOSAT-2).
TCCON network has limited geographic coverage, especially in remote locations and locations with high/low albedo.
Limitations for TCCON — very expensive 400 - 500 k€, requires good infrastructure (container, Bruker IFS 125HR as spectrometer, solar tracker) - not easy to relocate, requires special operating conditions, requires trained personnel for both operation and maintenance
— expensive if further expansion of the network is desired.
Development of portable, easy to maintain and low cost systems for remote sensing GHG observations by several research institutes.
These spectrometers have the potential to complement TCCON, however their performances have not yet been fully characterized.
Aim of the FRM4GHG project
Perform an intercomparison of simultaneously measured total column amounts of CO2, CH4, CO using several different portable low-cost spectrometric instruments under different atmospheric conditions in comparison to the TCCON instrument.

Implementation of the FRM4GHG campaign
Sodankylä TCCON site (67.37 N, 26.63 E, 188 m.a.s.l) has been selected for performing approximately one year of measurement campaign starting as of March this year.
Four portable spectrometers together with the TCCON are taking part in the campaign. In addition, regular Aircore launches are performed from the site, the data of which will be used to verify the calibration of the remote sensing instruments.

Fig. 1: (left) TCCON sites, (center) FRM4GHG container in front and TCCON building behind, (right) Aircore launch at Sodankylä

Aircore — University of Groningen / FMI
In-situ sampling of CO2, CH4, CO to get vertical profiles of these gases calibrated to WMO standards. Resolution: 13.4 mbar (Amb. P. > 232 mbar), 3.9 mbar (Amb, P. < 232 mbar).

Fig. 2: (left—right) Aircore coil, vertical profiles of CO2, CH4 and CO from four different Aircore launch at Sodankylä TCCON site.

Discussions
Progress report on the early part of the project is presented here with a principal aim to identify and correct biases and errors between the participating remote sensing instruments.
XCO2, XCH4, XCO total columns retrieved from different instruments (as applicable) are compared with respect to the TCCON retrievals. XAir which is a measure of the instrument performance is calculated from the O2 total column and ground pressure measured independently.
Most instruments have been operating well since their deployment. Five Aircore launches made.
Vertex70: significant bias in ΔXgas values w.r.t TCCON till beginning of July, after instrument modification it has better ILS, resulting in comparable Xgas values relative to the TCCON. Bias in XAir is the smallest for Vertex70 compared to IRCube.
EM27/SUN: stable ILS, the XCO2 and XCH4 values are scaled with pre-determined values derived from comparison measurements performed w.r.t another TCCON at Karlsruhe, bias in the scaled Xgas values are very small relative to the TCCON, however a small seasonality is observed in the ΔXgas plots.
IRCube: stable ILS, optical fiber cable broke shortly after deployment which was replaced in April, XCO2 values show a significant jump after instrument intervention, this is not so obvious for other species and also not seen in XAir, pointing towards a retrieval error for CO2. XCH4 bias is the smallest for IRCube compared to all other instruments.
LHR: is a homemade relatively new instrument, XCO2 shows large discrepancies (very high bias) with diurnal variability relative to the TCCON. Few days of data processed to check the newly developed retrieval algorithm, once the problem is cornered all data will be processed.
Next steps and outlook:
— Comparison of retrieval results for the whole campaign duration and determination of the calibration factors w.r.t the TCCON. Verify calibration of remote sensing instruments w.r.t the Aircore.
— Provide a significant dataset of GHG measurements which can be used for satellite validation purposes.
— Provide a guideline for further development of new observation sites to complement the TCCON network and provide better support for the validation of existing and future satellite missions.

Bruker IFS 125HR spectrometer (TCCON) — FMI
Spectral range: 1800—15000 cm-1, max resolution: 0.004 cm-1, main species: XCO2, XCH4, XCO measured at 0.02 cm-1. This is our reference instrument for the campaign.

Fig. 3: (left) Bruker IFS 125HR at Sodankylä TCCON site, ILS parameters — modulation efficiency (center) and phase error (right) for the Bruker IFS 125HR.

Vertex70 — University of Bremen / BIRA
Spectral range: 2500—15000 cm-1, max resolution: 0.16 cm-1, main species: XCO2, XCH4, XCO.

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Fig. 4: (a) Vertex70, (b) ILS parameters plot for Vertex70, timeseries of XCO2, XCH4, XCO and XAir for the Vertex70 (green) and TCCON (red) (top panel of c-f) and the difference of Vertex70 relative to the TCCON (bottom panel of c-f).

EM27SUN — KIT
Spectral range: 4000—9000 cm-1, max resolution: 0.5 cm-1, main species: XCO2, XCH4, XCO.

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Fig. 5: (a) EM27/SUN, (b) ILS parameters plot for EM27/SUN, timeseries of XCO2, XCH4 and XCO for the EM27/SUN (magenta) and TCCON (red) (top panel of c-e) and the difference of EM27/SUN relative to the TCCON (bottom panel of c-e).

IRCube — University of Wollongong
Spectral range: 4500—15000 cm-1, max resolution: 0.5 cm-1, main species: XCO2, XCH4.

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Fig. 6: (a) IRCube, (b) ILS parameters plot for IRCube, timeseries of XCO2, XCH4 and XAir for the IRCube (cyan) and TCCON (red) (top panel of c-e) and the difference of IRCube relative to the TCCON (bottom panel of c-e).

LHR (Laser Heterodyne Radiometer) — Rutherford Appleton Laboratory
Spectral range: 954—960 cm-1, max resolution: 0.002 and 0.02 cm-1, main species: CO2, H2O.

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Fig. 7: (a) LHR, (b) ILS plot for LHR, timeseries of XCO2 for the LHR (blue) and TCCON (red) (plotted in top panel of c) and the difference of IRCube relative to the TCCON (plotted in the bottom panel of c).

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Affiliations
1Royal Belgian Institute for Space Aeronomy, Brussels, Belgium, 2University of Bremen, Bremen, Germany, 3University of Groningen, Groningen, Netherlands, 4University of Wollongong, Wollongong, Australia, 5Karlsruhe Institute of Technology, Karlsruhe, Germany, 6Finnish Meteorological Institute, Sodankylä, Finland, 7Rutherford Appleton Laboratory, United Kingdom 8European Space Agency (ESA)/ ESRIN
Acknowledgements — The FRM4GHG campaign is funded by ESA under contract number ESA-IPL-POE-LG-cl-LE-2015-1129.
The author thanks other FRM4GHG team members for their contribution and Paulo Castracane from ESA.