Preprints
https://doi.org/10.5194/egusphere-2024-291
https://doi.org/10.5194/egusphere-2024-291
17 May 2024
 | 17 May 2024
Status: this preprint is open for discussion and under review for Atmospheric Chemistry and Physics (ACP).

To what extent does CO2 diurnal cycle impact carbon flux estimates in CarboScope?

Saqr Munassar, Christian Roedenbeck, Michał Gałkowski, Frank-Thomas Koch, Kai U. Totsche, Santiago Botía, and Christoph Gerbig

Abstract. Ignoring the diurnal cycle in surface-to-atmosphere CO2 fluxes leads to a systematic bias in CO2 mole fraction simulations sampled at daytime, because the daily mean flux systematically misses the CO2 uptake during the daytime hours. In an atmospheric inversion using daytime-selected CO2 measurements at most continental sites and not resolving diurnal cycles in the flux, this leads to systematic biases in the estimates of the annual sources and sinks of atmospheric CO2. This study focuses on quantifying the impact of this diurnal cycle effect on the annual carbon fluxes estimated with the CarboScope (CS) atmospheric inversion at regional, continental, and global scales for the period of time 2010–2020. Biogenic fluxes of hourly Net Ecosystem Exchange (NEE) obtained from the data-driven FLUXCOM estimates are used in the inversion together with global and regional atmospheric transport models. Differences between CO2 mixing ratios simulated with daily averaged and hourly NEE range between around -2.5 and 7 ppm averaged annually throughout a site network across the world. As a consequence, these differences lead to systematic biases in CO2 flux estimates when ignoring the diurnal variations of the CO2 flux in the atmospheric inversions. Although the impact on the global average of estimated annual flux is negligible (around 2 % of the overall land flux of -1.79 Pg C yr-1), we find significant biases in the annual flux budgets at continental and regional scales. For Europe, the annual mean difference in the fluxes arising from the diurnal cycle of CO2 represents around 48 % of the annual posterior fluxes (0.31 Pg C yr-1) estimated with CarboScope-Regional (CSR). Furthermore, the differences in NEE estimates calculated with CS increase the magnitude of the flux budgets for some regions such as northern American temperate and northern Africa by a factor of about 1.5. To the extent that FLUXOM diurnal cycles are realistic at all latitudes and for the station set used in our inversions here, we conclude that ignoring the diurnal variations in the land CO2 flux leads to overestimation of both CO2 sources in the tropical lands and CO2 sinks in the temperate zones.

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Saqr Munassar, Christian Roedenbeck, Michał Gałkowski, Frank-Thomas Koch, Kai U. Totsche, Santiago Botía, and Christoph Gerbig

Status: open (until 28 Jun 2024)

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  • RC1: 'Comment on egusphere-2024-291', Anonymous Referee #1, 07 Jun 2024 reply
Saqr Munassar, Christian Roedenbeck, Michał Gałkowski, Frank-Thomas Koch, Kai U. Totsche, Santiago Botía, and Christoph Gerbig
Saqr Munassar, Christian Roedenbeck, Michał Gałkowski, Frank-Thomas Koch, Kai U. Totsche, Santiago Botía, and Christoph Gerbig

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Short summary
CO2 mole fractions simulated over a global stations showed an overestimation of CO2 if the diurnal cycle is missing NEE. This led to biases in the estimated fluxes derived from the inversions at continental and regional scales. IAVof estimated NEE was affected by the diurnal effect. The findings point to the importance of including the diurnal variations of CO2 in the biosphere priors used in inversions to better converge flux estimates among inversions, in particular those contributing to GCB.