CO–CO2 Correlations over the Tropics during the 2015 El Niño Event Observed with Two Flux Inversions

Carbon monoxide (CO) and carbon dioxide (CO₂) are both emitted from combustion. CO is a product of inefficient combustion often used as a tracer of CO₂ for combustion. Most of the previous studies used CO measurements to constrain and improve CO₂ flux inversions. Observed CO-CO₂ correlation slopes thus provide useful constraints to characterize emissions sources. The CO-CO₂ correlation can also be used to analyze the variability of CO and CO₂ during biomass burning emissions. Satellites measuring CO and CO₂ by Infrared remote sensing allow with their high spatio-temporal coverage to improve our ability to map CO and CO₂ and to understand their respective variability. MOPITT and OCO-2 have been already used and validated in several studies for CO and CO₂ inversions respectively.

The goal of this study is to observe and analyze CO₂ and CO correlations with two independent inversions during the intense biomass burning of 2015 over the tropics related with the strongest El Niño event recorded. Two independent inversions are compared during 2015 and 2016 : a CO₂ inversion with the OCO-2 (Orbiting Carbon Observatory-2) measurements and a CO inversion with the MOPITT (Measurements Of Pollution in The Troposphere) data. We apply a four-dimensional variational (4D-VAR) data assimilation system to optimize CO and CO₂ emissions in the two separate inversions using the chemistry transport model TM5. The 6°x4° global version of TM5 with 25 vertical sigma hybrid levels is used.

In 2015, the strongest El Niño event has been recorded since 1950. It has been shown by some previous studies that El Niño event influenced carbon cycle variations and concentrations globally particularly in the tropics due to severe droughts. We analyze in this study CO-CO₂ correlation slopes as well as CO and CO₂ variability over the Northern Africa, Southern Africa, Indonesia and Southern America to observe the large emissions of biomass burning linked with the severe El Niño event of 2015. We will particularly observe strong correlation between the two independent inversions over the tropics and the influence of El Niño on the CO and CO₂ correlation and variability.