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As of NEON’s 2024 Data Release, there have been modifications in the eddy covariance storage exchange (ECSE) workflow and bundled HDF5 files (). NEON's atmospheric stable carbon isotope data () are now provided as 6- and 30-minute averages (previously 9- and 30-minute averages) utilizing values obtained from the Picarro G2131-i instrument. This adjustment removes additional time from the beginning of each measurement interval to increase flushing time to minimize memory effects resulting from residual gas of previous measurement levels or cylinders, which can introduce bias to the data. Previously, one minute of data was excluded from each averaging interval for the 9-minute product. This update increases the exclusion to four minutes and is based on an analysis performed by Dr. Rich Fiorella (Los Alamos National Lab) with support and concurrence from the rest of the NEON Atmospheric Stable Isotope Technical Working Group. Currently, this is only implemented for the atmospheric CO₂ isotopes data product, however in the future we plan to assess making similar updates to the atmospheric methane concentration () and atmospheric water isotopes () data products. Please note that there may be issues stacking released and provisional data in the neonUtilities R package while we finalize reprocessing provisional data (July 2023-present).

Additionally, we have adapted community code (NEONiso R-Package; Fiorella et al. 2021) to perform a daily field calibration in the eddy4R data processing software and reprocessed data in the 2024 data release to implement this calibration. While these instruments are calibrated annually, the lack of daily field calibration greatly limits the ability to perform cross-site analyses. Daily field calibrations are standard practice in the community, and NEON provides all the information necessary for a user to conduct them. However, this process requires a significant amount of work to be done by end users and has been identified as a barrier to data use by the Atmospheric Stable Isotope Technical Working Group.

The isotope ratios (δ¹³C) and CO₂ dry mole fraction data were calibrated using two different methods as outlined in Fiorella et al. (2021).

• The first method estimates calibration parameters for each isotopologue (¹²CO₂ and ¹³CO₂) independently. Then, the calibrated δ¹³C and CO₂ dry mole fractions are calculated directly from the calibrated ¹²CO₂ and ¹³CO₂ (Bowling et al. (2003)).
• The second method adopts a linear regression technique to directly correct δ¹³C and CO₂ dry mole fractions without converting to isotopologue mole fractions.

The calibrated δ¹³C and CO₂ dry mole fractions from both methods are available in the expanded HDF5 files, while only the calibrated δ¹³C and CO₂ dry mole fractions using the first method are provided in the basic HDF5 files.

References

Bowling, D. R., Sargent, S. D., Tanner, B. D., & Ehleringer, J. R. (2003). Tunable diode laser absorption spectroscopy for stable isotope studies of ecosystem-atmosphere CO₂ exchange. Agricultural and Forest Meteorology, 118(1â€�2), 1â€�19.

Fiorella, R. P., Good, S. P., Allen, S. T., Guo, J. S., Still, C. J., Noone, D. C., Anderegg, W. R. L., Florian, C. R., Luo, H., Pingintha-Durden, N., and Bowen, G. J. (2021). Calibration Strategies for Detecting Macroscale Patterns in NEON Atmospheric Carbon Isotope Observations. Journal of Geophysical Research: Biogeosciences, 126, e2020JG005862,