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 (XCO) 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 XCH4 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.