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. Atmospheric carbon monoxide (CO) and methane ( CH4 ) mole fractions are measured by ground-based insitu cavity ring-down spectroscopy (CRDS) analyzers and Fourier transforminfrared (FTIR) spectrometers at two sites (St Denis and Maido) onReunion Island (21 ∘  S, 55 ∘  E) in the Indian Ocean.Currently, the FTIR Bruker IFS 125HR at St Denis records the direct solarspectra in the near-infrared range, contributing to the Total Carbon ColumnObserving Network (TCCON). The FTIR Bruker IFS 125HR at Maido recordsthe direct solar spectra in the mid-infrared (MIR) range, contributing to the Network for the Detection of AtmosphericComposition Change (NDACC). In order to understand the atmospheric CO and CH4 variability on Reunion Island, the time series and seasonalcycles of CO and CH4 from in situ and FTIR (NDACC and TCCON)measurements are analyzed. Meanwhile, the difference between the in situ andFTIR measurements are discussed. The CO seasonal cycles observed from the in situ measurements at Maidoand FTIR retrievals at both St Denis and Maido are in good agreementwith a peak in September–November, primarily driven by the emissions frombiomass burning in Africa and South America. The dry-air column averaged molefraction of CO ( XCO ) derived from the FTIR MIR spectra (NDACC) isabout 15.7 ppb larger than the CO mole fraction near the surface atMaido, because the air in the lower troposphere mainly comes from theIndian Ocean while the air in the middle and upper troposphere mainly comesfrom Africa and South America. The trend for CO on Reunion Island is unclearduring the 2011–2017 period, and more data need to be collected to get arobust result. A very good agreement is observed in the tropospheric and stratospheric CH4 seasonal cycles between FTIR (NDACC and TCCON) measurements, andin 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 inDecember–January, which is due to the OH seasonal variation. In thestratosphere, the CH4 mole fraction has its maximum in March–Apriland 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 modelsimulation. The trend of X CH 4 is 7.6±0.4  ppb yr −1 fromthe TCCON measurements over the 2011 to 2017 time period, which is consistentwith the CH4 trend of 7.4±0.5  ppb yr −1 from the in situmeasurements for the same time period at St Denis.

atmospheric chemistry and physics

Atmospheric CO and CH&amp;lt;sub&amp;gt;4&amp;lt;/sub&amp;gt; time series and seasonal variations on Reunion Island from ground-based in situ and FTIR (NDACC and TCCON) measurements