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

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. [ABSTRACT FROM AUTHOR]