First year of upper tropospheric integrated content of CO₂ from IASI hyperspectral infrared observations

Abstract. Simultaneous observations from the Infrared Atmospheric Sounding Interferometer (IASI) and from the Advanced Microwave Sounding Unit (AMSU), launched together onboard the European MetOp platform in October 2006, are used to retrieve an upper tropospheric content of carbon dioxide (CO₂) covering the range 11–15 km (100–300 hPa), in clear-sky conditions, in the tropics, over sea, for the first year of operation of MetOp (January 2008–December 2008). With its very high spectral resolution, IASI provides fourteen channels in the 15 μm band highly sensitive to CO₂ with reduced sensitivities to other atmospheric variables. IASI observations, sensitive to both CO₂ and temperature, are used in conjunction with AMSU observations, only sensitive to temperature, to decorrelate both signals through a non-linear inference scheme based on neural networks. A key point of this approach is that no use is made of prior information in terms of CO₂ seasonality, trend, or geographical patterns. The precision of the retrieval is estimated to be about 2.0 ppmv (~0.5%) for a 5°×5° spatial resolution on a monthly time scale. Features of the retrieved CO₂ space-time distribution include: (1) a strong seasonal cycle of 4 ppmv in the northern tropics with a maximum in June–July and a minimum in September–October. This cycle is characterized by a backward two-months lag as compared to the surface, by a backward one-month lag as compared to measurements performed at 11 km, and by a forward one-month lag as compared to observations performed at the tropopause (16 km). This is likely due to the time-lag of CO₂ cycle while transported from the surface to the upper troposphere; (2) a more complex seasonal cycle in the southern tropics, in agreement with in-situ measurements; (3) a latitudinal variation of CO₂ shifting from a South-to-North increase of 3.5 ppmv in boreal spring to a South-to-North decrease of 1.5 ppmv in the fall, in excellent agreement with tropospheric aircraft measurements; (4) signatures of CO₂ emissions transported to the upper troposphere. In addition to bringing an improved view of CO₂ distribution, these results from IASI should provide an additional means to observe and understand atmospheric transport pathways of CO₂ from the surface to the upper troposphere.