Atmospheric CO and CH<sub>4</sub> time series and seasonal variations on Reunion Island from ground-based in situ and FTIR (NDACC and TCCON) measurements

Abstract

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.

An intercomparison of total column-averaged nitrous oxide between ground-based FTIR TCCON and NDACC measurements at seven sites and comparisons with the GEOS-Chem model

“…where TC H 2 O is total column of H 2 O, P s is the surface pressure, g is the column-averaged gravitational acceleration, and m H 2 O and m dry air are molecular masses of H 2 O and dry air, respectively (Deutscher et al, 2010;Zhou et al, 2018). The total column of N 2 O is calculated by integrating the partial column of each layer.…”

Retrieval of atmospheric CH<sub>4</sub> vertical information from TCCON FTIR spectra

“…The relative standard deviation of the CH 4 monthly means from the WACCM model in 1980-2020 is calculated as the random uncertainty of the CH 4 profile. For the systematic uncertainty, we have chosen a value of 5% (about 90 ppb in the troposphere), based on the difference between the a priori CH 4 mole fraction near the surface and the local in situ measurements (Zhou et al, 2018). As CH 4 is relatively stable in the atmosphere with a life time of ∼ 9 years, it is assumed that 5% systematic uncertainty is acceptable for all altitudes.…”

“…The measurements at 180 m is used here as it is less affected by the boundary layer (Schmidt et al, 2014). In situ measurements for the St Denis site is taken from the measurements performed at Maïdo (2155 m) located at about 20 km away from St Denis (Zhou et al, 2018). Both ground-based in situ measurements at Orléans and Maïdo are well calibrated frequently at the Laboratoire des Sciences du Climat et de l'Environnement (LSCE).…”

“…The in situ measurements at St Denis are found to be about 24 ppb lower than the SFIT4TCCON tropospheric X CH4 . Zhou et al (2018) pointed out that the air near the surface above St Denis (0-2 km) is mainly coming from the Indian Ocean and partly from Southern African region, whereas the air mass in the middle and upper troposphere (4-12 km) is mainly coming from Africa and South America.…”

An inter-comparison of total column-averaged nitrous oxide between ground-based FTIR TCCON and NDACC measurements at seven sites and comparisons with the GEOS-Chem model

“…10 where T C H2O is total column of H 2 O; P s is the surface pressure; g is the column-averaged gravitational acceleration; m H2O and m dry air are molecular masses of H 2 O and dry air, respectively (Deutscher et al, 2010;Zhou et al, 2018 of N 2 O is calculated by integrating the partial column of each layer. For each site, the mean of the monthly means during 1980-2020 from the Whole Atmosphere Community Climate Model (WACCM) version 4 is applied to be the a priori profile for the NDACC retrievals (constant in time).…”