Abstract

Abstract. At the Izaña Atmospheric Research Center, high-resolution mid-infrared solar absorption spectra have been recorded for more than 12 yr using Fourier Transform InfraRed (FTIR) spectrometers. We use the spectral fitting algorithm PROFFIT to retrieve long-term time series of methane (CH4) from the measured spectra. We investigate the total column-averaged dry air mole fractions of methane (totXCH4) obtained from a profile scaling and a profile retrieval, and apply two approaches for deriving the tropospheric column-averaged dry air mole fractions: firstly, we use the FTIR hydrogen fluoride (HF) total column amounts as an estimator for the stratospheric CH4 contribution and a posteriori correct the totXCH4 data of a profile scaling retrieval accordingly (troXCH4post); secondly, we directly determine the tropospheric column-averaged dry air mole fractions of methane (troXCH4retr) from retrieved CH4 profiles. Our theoretical estimation indicates that the scaling retrieval leads to totXCH4 amounts that are subject to a large smoothing error, which can be widely avoided by applying a profile retrieval (for the latter we estimate an overall precision of 0.41%). We compare the different FTIR CH4 data to Izaña's Global Atmospheric Watch (GAW) surface in-situ CH4 data (CH4GAW), which in the case of the Izaña Atmospheric Research Center high mountain observatory are very representative for the free tropospheric CH4 amounts. Concerning totXCH4, the agreement between the FTIR data product and the in-situ measurement is rather poor documenting that totXCH4 is not a valid free tropospheric CH4 proxy, as it is significantly affected by the varying stratospheric CH4 contribution and it rather follows the variation in the tropopause altitude. The a posteriori correction method as applied here only removes a part of this stratospheric CH4 contribution. In contrast the profile retrieval allows for a direct estimation of the tropospheric column-averaged CH4 amounts. Results of the profile retrieval analysis correlate well with the CH4GAW data (correlation coefficient of 0.60, FTIR-GAW scatter of 0.97%), and both data sets show very similar annual cycles and trend behaviour for the 2001–2010 time period. Furthermore, we find a very good absolute agreement between the troXCH4retr and CH4GAW (mid-infrared FTIR/GAW scaling factor of 0.9987) suggesting that mid-infrared FTIR data can be well combined with the surface in-situ GAW data. Our study strongly supports the value of mid-infrared ground-based FTIR CH4 profile retrievals as well as the robustness of the approach for achieving total and tropospheric column-averaged XCH4 data of high quality.

Highlights

  • Methane (CH4) is the second most important anthropogenic greenhouse gas (GHG), after carbon dioxide (CO2)

  • In this work we present, discuss, and validate different ground-based Fourier Transform InfraRed (FTIR) CH4 products derived from mid-infrared spectral region: the total column-averaged volume mixing ratio, and two tropospheric column-averaged volume mixing ratios: a first derived by a posteriori correction method using hydrogen fluoride (HF) as indicator for the stratospheric contribution, and a second directly retrieved from the measured spectra

  • A scaling retrieval is indicated if there is no significant variation in the profile shape or if the variations in the profile shape are not reflected in the measured solar absorption spectra

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Summary

Introduction

Methane (CH4) is the second most important anthropogenic greenhouse gas (GHG), after carbon dioxide (CO2). Total column-averaged data are affected by the stratospheric contribution, the correct modelling of which is a significant error source when investigating the GHG cycling between the atmosphere, the biosphere, and the ocean. By means of the ground-based FTIR vertical profile data, one can calculate tropospheric column-averaged mixing ratios These ratios would neither be affected by small-scale near-surface processes nor by stratospheric contributions. In this work we present, discuss, and validate different ground-based FTIR CH4 products derived from mid-infrared spectral region: the total column-averaged volume mixing ratio (totXCH4), and two tropospheric column-averaged volume mixing ratios (troXCH4): a first derived by a posteriori correction method using HF as indicator for the stratospheric contribution (similar to Washenfelder et al, 2003), and a second directly retrieved from the measured spectra.

Site description
In-situ measurement program
FTIR measurement program
General setup of a ground-based FTIR analysis
The CH4 retrieval strategy
Error estimation
Characteristics of the retrieved CH4 profiles
Intercomparison between the Bruker spectrometers IFS 120M and IFS 125HR
FTIR versus surface in-situ GAW data
Interannual trend
De-trended CH4 annual cycle
Remark on non-transferability to the near-infrared spectra
Findings
Outlook and conclusions
Full Text
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