Keywords:GOSAT, methane, South Asia
Methane (CH4) is the second most important anthropogenic greenhouse gas (GHG), and plays critical role in air pollution chemistry in the troposphere. With the availability of satellite observations from space, variabilities in CH4 have been captured for most parts of the global land with major emissions. The satellite observations however do not allow us to derive emission information directly, unlike the in-situ measurements, without separating the role of transport and chemistry in the columnar dry-air mole fractions (XCH4). Here we analyze XCH4 variability over different regions of India, Arabian Sea and Bay of Bengal, measured by the GHGs Observation SATellite (GOSAT) using an atmospheric chemistry-transport model (ACTM). We show that the peak in observed XCH4 over the Indo-Gangetic Plain (IGP) during the southwest (SW) monsoon season (July-September) is produced mainly from the emissions on the surface (50% in 1000-600hPa layer) and uplifted high-CH4 air mass in the upper troposphere (30% in the 400-0 hPa layer) using the ACTM simulations. These contributions are, in contrast, generated mostly from the upper troposphere over the semi-arid western India, up to 70% from the 600-0 hPa layers. This is because the signal from high CH4 emissions during SW monsoon season is confined to a smaller region of the IGP, while the large-scale deep convection coupled with the anticyclonic wind during the SW monsoon lead to widespread CH4 enhancement covering the whole South Asia and extending through the East Asia.