Atmospheric CO and CH4 time series and seasonal variations on Reunion Island from ground-based in situ and FTIR (NDACC and TCCON) measurements

Abstract

Atmospheric carbon monoxide (CO) and methane (CH₄) mole fractions are measured by ground-based in situ cavity ring-down spectroscopy (CRDS) analyzers and Fourier transform infrared (FTIR) spectrometers at two sites (St Denis and Maïdo) on Reunion Island (21^∘ S, 55^∘ E) in the Indian Ocean. Currently, the FTIR Bruker IFS 125HR at St Denis records the direct solar spectra in the near-infrared range, contributing to the Total Carbon Column Observing Network (TCCON). The FTIR Bruker IFS 125HR at Maïdo records the direct solar spectra in the mid-infrared (MIR) range, contributing to the Network for the Detection of Atmospheric Composition Change (NDACC). In order to understand the atmospheric CO and CH₄ variability on Reunion Island, the time series and seasonal cycles of CO and CH₄ from in situ and FTIR (NDACC and TCCON) measurements are analyzed. Meanwhile, the difference between the in situ and FTIR measurements are discussed.

The CO seasonal cycles observed from the in situ measurements at Maïdo and FTIR retrievals at both St Denis and Maïdo are in good agreement with a peak in September–November, primarily driven by the emissions from biomass burning in Africa and South America. The dry-air column averaged mole fraction of CO (X_CO) derived from the FTIR MIR spectra (NDACC) is about 15.7 ppb larger than the CO mole fraction near the surface at Maïdo, because the air in the lower troposphere mainly comes from the Indian Ocean while the air in the middle and upper troposphere mainly comes from Africa and South America. The trend for CO on Reunion Island is unclear during the 2011–2017 period, and more data need to be collected to get a robust result.

A very good agreement is observed in the tropospheric and stratospheric CH₄ seasonal cycles between FTIR (NDACC and TCCON) measurements, and in situ and the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) satellite measurements, respectively. In the troposphere, the CH₄ mole fraction is high in August–September and low in December–January, which is due to the OH seasonal variation. In the stratosphere, the CH₄ mole fraction has its maximum in March–April and its minimum in August–October, which is dominated by vertical transport. In addition, the different CH₄ mole fractions between the in situ, NDACC and TCCON CH₄ measurements in the troposphere are discussed, and all measurements are in good agreement with the GEOS-Chem model simulation. The trend of $X_{{\mathrm{CH}}_{\mathrm{4}}}$ is 7.6±0.4 ppb yr⁻¹ from the TCCON measurements over the 2011 to 2017 time period, which is consistent with the CH₄ trend of 7.4±0.5 ppb yr⁻¹ from the in situ measurements for the same time period at St Denis.