Estimation of the tropospheric and stratospheric CO2 content by ground-based IR technique

Published: 05 July 2021;

Measured solar spectra of high spectral resolution possess specific information about the vertical distribution of some atmospheric gases. The assessment of the spectra informational value is provided by determining a degree of freedom for signal (dofs). This can be obtained from the solar spectra. Such information can be used to retrieve CO2 content in the troposphere and stratosphere. The aim of this study was to analyze the retrievals of average CO2 content (XCO2) in both the troposphere and stratosphere obtained from the measurements of solar spectra in the Peterhof suburban area (St. Petersburg, Russia) conducted during 2018-2019. 

To retrieve XCO2 the PROFIT (PROfile FITting) software and 6 different spectral schemes were used. Fig.1 depicts annual variation of retrieved daily average XCO2 in total atmospheric column by ground-based Fourier-spectrometer Bruker 125HR (located in Peterhof) and satellite measurements (OCO-2). Large systematic discrepancies between XCO2 retrievals reaching 20 ppm were identified. However, all XCO2 retrievals represented a seasonal cycle relatively well. Satellite retrievals of XCO2 fitted well the ground-based observations and in particular with schemes 2 and 3. 

Fig.1 Annual variation of daily average XCO2 in total atmospheric column according  to ground-based (Bruker 125 HR) and satellite (OCO-2) measurements for the Peterhof suburban area of St. Petersburg in 2018-2019 

The XCO2 was also retrieved in two layers – below and above of 12 km which approximately corresponded to the troposphere and stratosphere in this geographical region. The optimal combinations of mean values, standard deviation, random and systematic errors in both layers were found for schemes 2 and 3. It was found that tropospheric and stratospheric XCO2 were quite close to each other (Fig. 2). The tropospheric XCO2 was higher than stratospheric from November to May for the scheme 2 (~5 ppm) and from November to July for the scheme 3 (~10-15 ppm). These enhancements were caused by CO2 anthropogenic emissions from the Saint-Petersburg metropolitan area with a large number of industrial sources of emissions. By contrast, the stratospheric XCO2 was higher than the tropospheric at the beginning of autumn and during summer (~20-30 ppm for the scheme 2 and ~10 ppm for the scheme 3) which could be caused by the photosynthesis processes. Both schemes presented a distinguishable seasonal cycle of XCO2 in the troposphere with a minimum in summer. The XCO2 in the stratosphere did not show a notable seasonal variation. However, in autumn the maximum in XCO2 according to the scheme 3 was found which coincided with the satellite observations. 

Fig. 2 Annual variation of XCO2 in the troposphere and stratosphere according to ground-based measurements (Bruker 125 HR, schemes 2 and 3) for the Peterhof suburban area of St. Petersburg in 2018-2019 

This study also demonstrated that the best agreement between XCO2 according to the observations and CAMS reanalysis data was registered for the total column and the troposphere in particular for the scheme 2. In general, the ground-based and satellite retrievals of XCO2 in the stratosphere (ACE-FTS) fitted relatively well. 

See more details in Timofeyev Yu., Nerobelov G., Poberovskii A., Filippov N. (2021) Evaluation of CO2 content in troposphere and stratosphere by ground-based IR method. Izvestiya. Atmospheric and Oceanic Physics, 57(3), pp 322-333

Text by Yury Timofeyev, Georgy Nerobelov, Anatoly Poberovskii and Nikolay Filippov, Saint-Petersburg State University, St. Petersburg, Russia. 

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