Optimizing Four Years of CO ₂ Biospheric Fluxes from OCO-2 and in situ data in TM5: Fire Emissions from GFED and Inferred from MOPITT CO data

Abstract. Column mixing ratio of carbon dioxide (CO2) data alone do not provide enough information for source attribution. Carbon monoxide (CO) is a product of inefficient combustion often used as a tracer of CO2. CO data can then provide a powerful constraint on fire emissions, supporting more accurate estimation of biospheric CO2 fluxes. In this framework and using the chemistry transport model TM5, a CO inversion using MOPITT v8 data is performed to estimate fire emissions which are then converted in CO2 fire emissions through the use of emission ratio. These CO2 fire emissions allow us, then, to estimate adjusted CO2 Net Ecosystem Exchange (NEE) and respiration which are then used as priors for CO2 inversions constrained either by the Orbiting Carbon Observatory 2 (OCO-2) v9 or by in situ data. For comparison purpose, we also balanced the respiration using fire emissions from the Global Fire Database Emissions (GFED) version 3 (GFED3) and version 4.1s (GFED4.1s). We hence study the impact of CO fire emissions in our CO2 inversions at global, latitudinal and regional scales over the period 2015–2018 and compare our results to the two other similar approaches using GFED3 and GFED4.1s, as well as with an inversion using CASA-GFED3 fire and NEE priors. After comparison at the different scales, the inversions are evaluated against TCCON data. Results show that variations in posterior flux are much smaller across different prior mean fluxes when compared with the data assimilated. However, at global scale and for most of the regions, while the net fluxes remain robust, we can observe differences in fire emissions among the priors, resulting in large adjustments in the Net Ecosystem Exchange (NEE) to match the fires and observations. Tropical flux estimates from in situ inversions are highly sensitive to the prior flux assumed, of which fires are a significant component. Slightly larger CO2 net sources are observed when using GFED4.1s and MOPITT CO prior in CO2 OCO-2 inversions than compared with the other priors, particularly during the 2015 El Niño event for most Tropical regions. Larger CO2 net sources with MOPITT CO and GFED4.1s priors are also observed in Tropical Asia in CO2 in situ inversions than compared with the other priors during the 2015–2016 El Niño period and shows large net emissions than compared to OCO-2 inversions. Evaluation with TCCON suggests that the re-balanced posterior simulated give biases and accuracy very close each other where biases have decreased and variability matches better the validation data than with the CASA-GFED3. Further work is needed to improve prior fluxes in Tropical regions where fires are a significant component.

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