Koren, Gerbrand: Resolving the diurnal cycle of Δ¹⁷O in CO₂ at the ecosystem level: Simulations and observations at the mid-latitude pine forest Loobos
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1) This study presents the first measurements of the diurnal cycle of Δ17O in CO2 at a pine forest field site in the Netherlands. Flask samples were collected over 24 hours in summer 2019. 2) Modeling with an MXL model showed reasonable agreement with observations and allowed exploration of sensitivities driving Δ17O variations. A TM5 model simulation indicated biospheric exchange and fossil fuel combustion were the main processes influencing the Δ17O signal. 3) Observations showed the Δ17O signal exceeded measurement uncertainty, though interpretation was difficult. The timing of different processes in the TM5 model reduced the amplitude of the diurnal Δ17O cycle in CO2. The largest diurnal cycles were
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Koren, Gerbrand: Resolving the diurnal cycle of Δ¹⁷O in CO₂ at the ecosystem level: Simulations and observations at the mid-latitude pine forest Loobos
- 1. Resolving the diurnal cycle of Δ17O in CO2 at the ecosystem level: Simulations and observations at the mid-latitude pine forest Loobos Tea Talk, 20 Feb 2020 Gerbrand Koren, Getachew A. Adnew, Jordi Vilà-Guerau de Arellano, Michiel van der Molen, Bart Kruijt, Thomas Röckmann, Wouter Peters
- 2. Definition of Δ17O Gerbrand Koren Wageningen University 1 Thiemens (2006)
- 3. Definition of Δ17O Gerbrand Koren Wageningen University 2 Thiemens (2006)
- 4. Definition of Δ17O Gerbrand Koren Wageningen University 3 Δ17O ≈ δ17O - λ × δ18O Thiemens (2006)
- 5. Δ17O in CO2 as tracer of GPP • Stratospheric CO2 is enriched in Δ17O Δ17Oleaf Δ17Ostrat ≈ 0.66 ‰ Δ17Otrop Gerbrand Koren Wageningen University 4
- 6. Δ17O in CO2 as tracer of GPP • Stratospheric CO2 is enriched in Δ17O • CO2 can exchange oxygen isotopes after dissolving into water • Most exchange of isotopes between CO2 and water occurs in vegetation (carbonic anhydrase) Δ17Oleaf ≈ -0.07 ‰ Δ17Ostrat ≈ 0.66 ‰ Δ17Otrop Gerbrand Koren Wageningen University 5
- 7. Δ17O in CO2 as tracer of GPP • Stratospheric CO2 is enriched in Δ17O • CO2 can exchange oxygen isotopes after dissolving into water • Most exchange of isotopes between CO2 and water occurs in vegetation (carbonic anhydrase) • Δ17O in tropospheric CO2 represents a dynamic balance between stratosphere and biosphere exchange Δ17Oleaf ≈ -0.07 ‰ Δ17Ostrat ≈ 0.66 ‰ Δ17Otrop = ?? Gerbrand Koren Wageningen University 6
- 8. • Δ17O in CO2 was first proposed as tracer for GPP by Hoag et al. (2005) • Less sensitive to hydrological cycle than δ18O in CO2 Δ17O as tracer of GPP Gerbrand Koren Wageningen University 7
- 9. Global budget of Δ17O in CO2 stratosphere troposphere FAS = −64 Δ17Ostrat = 0.66 FAL = −514 FSA = 64 FLA = 400 Fresp = 111 Δ17Oleaf = −0.07 Δ17O isotope signatures in ‰ CO2 fluxes in PgC/yr Gerbrand Koren Wageningen University 8
- 10. Global budget of Δ17O in CO2 Δ17Oocean = −0.01 stratosphere troposphere FAS = −64 Δ17Ostrat = 0.66 FAL = −514 FOA = 87 FAO = −90 FSA = 64 FLA = 400 Fresp = 111 Δ17Osoil = −0.01 Δ17Oleaf = −0.07 FSIA = 30 FASI = −30 Δ17O isotope signatures in ‰ CO2 fluxes in PgC/yr Gerbrand Koren Wageningen University 9
- 11. Global budget of Δ17O in CO2 Δ17Oocean = −0.01 stratosphere troposphere FAS = −64 Δ17Ostrat = 0.66 FAL = −514 Fff = 9 FOA = 87 FAO = −90 FSA = 64 FCO+OH = 1 Δ17Off = −0.39 FLA = 400 Fresp = 111 Fbb = 2 Δ17Obb = −0.23 Δ17Osoil = −0.01 Δ17Oleaf = −0.07 Δ17OCO = 5.0 FSIA = 30 FASI = −30 Δ17O isotope signatures in ‰ CO2 fluxes in PgC/yr Gerbrand Koren Wageningen University 10
- 12. Global budget of Δ17O in CO2 Δ17Oocean = −0.01 stratosphere troposphere FAS = −64 Δ17Ostrat = 0.66 FAL = −514 Fff = 9 FOA = 87 FAO = −90 FSA = 64 FCO+OH = 1 Δ17Off = −0.39 FLA = 400 Fresp = 111 Fbb = 2 Δ17Obb = −0.23 Δ17Osoil = −0.01 Δ17Oleaf = −0.07 Δ17OCO = 5.0 FSIA = 30 FASI = −30 Δ17O isotope signatures in ‰ CO2 fluxes in PgC/yr Gerbrand Koren Wageningen University 11 • Many processes, but main source: stratosphere, and main sink: biosphere
- 13. Research objective • Previous studies for Δ17O in CO2 were done on the leaf-scale (Adnew et al., 2020) and the global scale (Koren et al., 2019) • This is the first study of the diurnal cycle for Δ17O in CO2 in a forest 1. Is there a measurable variation during the day? 2. If so, how does Δ17O in CO2 vary during the day? 3. Which are the main processes at the ecosystem level? Gerbrand Koren Wageningen University 12
- 14. TM5 model MXL model Loobos obs. Methods overview Gerbrand Koren Wageningen University 13
- 15. Measurement strategy • Loobos flux tower (NL-Loo), pine forest on sandy soil • 24 hours campaign in summer 2019, 30 flasks • Frequent measurements during morning transition • Air collected in flasks from 2 levels (~0.5m, ~25m) • Isotopic composition measured at Utrecht University • Dry sampling is crucial! Gerbrand Koren Wageningen University 14
- 16. Cloudy conditions • Cloudy on 15-16 August 2019, but no rain! Gerbrand Koren Wageningen University 15 MODIS
- 17. Boundary layer height Gerbrand Koren Wageningen University 16 • Good agreement of ABL height between observations, MXL model and TM5 meteo
- 18. Tower and surface obs. for CO2 and δ13C • Increase of CO2 and decrease of δ13C during night, largest signal close to surface Gerbrand Koren Wageningen University 17
- 19. Gerbrand Koren Wageningen University 18 • Tower Δ17O signal exceeds measurementuncertainty, but interpretation is difficult Tower and surface obs. for δ18O and Δ17O
- 20. Gerbrand Koren Wageningen University 19 • Systematicvariation of meteorological variables (humidity in free troposphere, cloud cover) and isotope parameters (degree of equilibration) MXL model allows to explore sensitivities (a, b) t = 9:00 (c, d) t = 15:00
- 21. Gerbrand Koren Wageningen University 20 Tracer tagging in TM5 model CO2CO2 CO2CO2 • For CO2 in the 1°×1° grid box over Loobos, the main processes are the biospheric exchange and fossil fuel combustion
- 22. Gerbrand Koren Wageningen University 21 • For Δ17O in CO2 the amplitude of the combined signal is smaller than the signal of some individual processes (since these are in anti-phase) Tracer tagging in TM5 model
- 23. Diurnal amplitude of Δ17O across the globe • Largest Δ17O amplitude in the tropics (~20 per meg) • Seasonality in diurnal Δ17O amplitude for NH • Good that we collected samples in the summer! • Model underestimates measured variability Gerbrand Koren Wageningen University 22
- 24. Conclusions Observations: • First measurementsof Δ17O in a forest: signal exceeds measurement uncertainty MXL model: • Reasonable agreement with observations • Ideal tool to explore sensitivity of Δ17O in CO2 TM5 model: • Timing of different processes leads to reduced amplitude in Δ17O in CO2 • Largest diurnal cycle expected in tropics, with highest biospheric contribution Gerbrand Koren Wageningen University 23