Results of surface and total column CO measurements at Obninsk in 2015-2024

The direct greenhouse effect of CO due to absorption of solar radiation in the atmospheric transparency window of 3-5 microns is negligible, however, due to reactions with the hydroxyl radical, carbon monoxide indirectly affects the content of the second most efficient greenhouse gas, methane. This, as well as toxicity, determine the urgency of monitoring the CO content in the atmosphere.

The report considers the methodology and results of measurements of total column CO and the surface layer concentrations at Obninsk station  in 2015-2024 with the medium resolution MR-32 spectroscopic instrument. In the surface layer CO concentration measurements are carried out in air samples in a multi-pass cell with an optical path length of about 30 m. The main interfering gases are H₂O and N₂O. The CO concentration was determined during a sequential iteration – comparison of the spectrum measured with a resolution of 0.12 cm^-1 in the spectral ranges 2164-2181 and 2144.5-2149 cm^-1, and calculations based on the HITRAN spectral lines.

The CO total colum (CO TC) was determined in the spectral ranges 2155-2160 and 2126-2132 cm^-1, where the main interfering gases are N₂O, H₂O, and O₃. The inverse problem of determining CO in the atmospheric column was solved using the SFIT4 software package. 

The maximum values of the surface concentration observed at Obninsk station during the winter period gradually decreased during 2015-2024. At the same time, the minimum summer values increased by about 20 ppb in the period 2021-2023, which led to a generally positive but insignificant trend over the entire observation period.

The measurement results of the MR-32 instrument are compared with satellite measurements of the total column and vertical profiles  of carbon monoxide by the MOPITT and AIRS orbiting complexes. Satellite data were averaged relative to  Obninsk in the area of 34×38° E and 53×57° N.

The average surface concentrations of CO for the period from 2015 to 2024 according to MP-32 and AIRS (layer 925 – 1000 hPa) are 158 ± 47 ppb and 136 ± 22 ppb.

The total column CO_TC measurements by the MR-32 instrument are in good agreement with the satellite data from MOPITT with R²=0.8. The average values of СО_ТС within the margin of error are the same and equal to (2.1 ± 0.2) 10¹⁸ molec/cm². Small linear trends are insignificant.

According to AIRS and MOPITT data the spectral composition of oscillations and linear trends оf xCO volume mixing ratios vary radically with altitude. At altitudes of 200-300 hPa, the maximum of the annual harmonic shifts from February-March to August. It is noteworthy to mark a significant increase in хСО at the levels of 50-300 hPa in the summer period in 2021-2024. If the trends are negative for surfaces of 925, 1000 hPa (-1.3 and -1.8 ppb/year), then at levels 50-200 hPa the xCO trends are positive (from 0.08 to 0.35 ppb/year),. A change in the trend sign occurs near 200-300 hPa.

Keywords: carbon monoxide CO, surface concentrations, total content, altitude profiles, spectral and wavelet analysis, satellite data