Abstract
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 a mid-to-upper tropospheric content of methane (CH4) in clear-sky conditions, in the tropics, over sea, for the first 16 months of operation of MetOp (July 2007–October 2008). With its high spectral resolution, IASI provides nine channels in the 7.7 μm band highly sensitive to CH4 with reduced sensitivities to other atmospheric variables. These channels, sensitive to both CH4 and temperature, are used in conjunction with AMSU channels, 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 methane seasonality, trend, or geographical patterns. The precision of the retrieval is estimated to be about 16 ppbv (~0.9%). Features of the retrieved methane space-time distribution include: (1) a strong seasonal cycle of 30 ppbv in the northern tropics with a maximum in January–March and a minimum in July–September, and a flat seasonal cycle in the southern tropics, in agreement with in-situ measurements; (2) a latitudinal decrease of 30 ppbv from 20° N to 20° S, in boreal spring and summer, lower than what is observed at the surface but in excellent agreement with tropospheric aircraft measurements; (3) geographical patterns in good agreement with simulations from the atmospheric transport and chemistry model MOZART-2, but with a higher variability and a higher concentration in boreal winter; (4) signatures of CH4 emissions transported to the middle troposphere such as a large plume of elevated tropospheric methane south of the Asian continent, which might be due to Asian emissions from rice paddies uplifted by deep convection during the monsoon period and then transported towards Indonesia. In addition to bringing a greatly improved view of methane distribution, these results from IASI should provide a means to observe and understand atmospheric transport pathways of methane from the surface to the upper troposphere.
Highlights
Knowledge of today’s methane (CH4) sources and sinks, their spatial distribution and their variability in time is essential for predicting the future CH4 atmospheric concentration levels, which is second only to carbon dioxide (CO2) as an important human-caused greenhouse gas (IPCC, 2007)
The tropics play a major role in CH4 and carbon monoxide (CO) chemistry, due to the considerable photochemical production of OH in these regions (e.g., Spivakovsky et al, 2000), which is the main sink of methane by globally removing about 85% of methane molecules emitted to the atmosphere (e.g., Logan et al, 1981)
Based on the non-linear inference scheme described in Sect. 3, 16 months, from July 2007 to October 2008, of coupled Infrared Atmospheric Sounding Interferometer (IASI) and Advanced Microwave Sounding Unit (AMSU) observations have been interpreted in terms of a tropospheric integrated content of CH4 in the tropical belt (20◦ N:20◦ S), over sea, at night, for clear-sky only
Summary
Knowledge of today’s methane (CH4) sources and sinks, their spatial distribution and their variability in time is essential for predicting the future CH4 atmospheric concentration levels, which is second only to carbon dioxide (CO2) as an important human-caused greenhouse gas (IPCC, 2007). The tropics are crucial to global budgets of methane. Tropical wetlands are believed to be a considerable CH4 source (e.g., Cicerone and Oremland, 1988; Prentice et al, 2001; Mikaloff-Fletcher et al, 2004a; Chen and Prinn, 2006; Bousquet et al, 2006), as well as emissions from rice paddies (Huang et al, 2004) and termites. Tropical deforestation is associated with considerable emissions of CH4, since the bulk of this deforestation is accomplished through fires. The tropics play a major role in CH4 and carbon monoxide (CO) chemistry, due to the considerable photochemical production of OH in these regions (e.g., Spivakovsky et al, 2000), which is the main sink of methane by globally removing about 85% of methane molecules emitted to the atmosphere (e.g., Logan et al, 1981). The exact location, intensity and nature of methane sources and sinks are still not fully elucidated
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