Carbon balance of South Asia constrained by passenger aircraft CO₂ measurements

Abstract. Quantifying the fluxes of carbon dioxide (CO₂) between the atmosphere and terrestrial ecosystems in all their diversity, across the continents, is important and urgent for implementing effective mitigating policies. Whereas much is known for Europe and North America for instance, in comparison, South Asia, with 1.6 billion inhabitants and considerable CO₂ fluxes, remained terra incognita in this respect. We use regional measurements of atmospheric CO₂ aboard a Lufthansa passenger aircraft between Frankfurt (Germany) and Chennai (India) at cruise altitude, in addition to the existing network sites for 2008, to estimate monthly fluxes for 64-regions using Bayesian inversion and transport model simulations. The applicability of the model's transport parameterization is confirmed using SF₆, CH₄ and N₂O simulations for the CARIBIC datasets. The annual amplitude of carbon flux obtained by including the aircraft data is twice as large as the fluxes simulated by a terrestrial ecosystem model that was applied to prescribe the fluxes used in the inversions. It is shown that South Asia sequestered carbon at a rate of 0.37 ± 0.20 Pg C yr⁻¹ (1 Pg C = 10¹⁵ g of carbon in CO₂) for the years 2007 and 2008. The seasonality and the strength of the calculated monthly fluxes are successfully validated using independent measurements of vertical CO₂ profiles over Delhi and spatial variations at cruising altitude over Asia aboard Japan Airlines passenger aircraft.