A view of 3D variability of atmospheric methane over India using MIROC4-ACTM simulations

In this study, we examine a maximum of 13 years (2009-2021) of the observational datasets of columnar dry-air mole fractions of methane (XCH₄), from the Greenhouse gases Observation SATellite (GOSAT) GOSAT-1 (2009-2021) on board the IBUKI satellite and vertical profiles of CH₄ from the Atmospheric Infrared Sounder (AIRS) on board AQUA satellite. We also use the XCH₄ and vertical distributions over the Indian subcontinent from the JAMSTEC’s MIROC4 atmospheric chemistry transport model (ACTM) from 2009-2021, after convolution with GOSAT-1 and AIRS a priori profiles and averaging kernels. 

A comparison of observed and modeled CH₄ and XCH₄ reveals that MIROC4-ACTM provides explicit insights into the spatio-temporal variability of atmospheric methane over the Indian region. Global CH₄  emission inventory EDGARv7.0 reports a total of ~30 Mt of anthropogenic emissions in India in 2020 while GAINS emissions stand at ~33Mt for the same year. The ACTM simulations from this study are also examined to retrieve preliminary information on the quality of these constructed bottom-up fluxes of methane (EDGAR and GAINS).      

Our time series analysis of GOSAT-1 shows the annual mean XCH₄ has increased by 100 ppb from 2009 to 2021 over the Indian subcontinent, compared to 85 ppb globally in the latitude band of 6.25 °N-41.25 °N. Observations from both AIRS and GOSAT-1 show distinct seasonality in the vertical profiles of CH₄ and XCH₄ over the entire region, respectively. Seasonality in the CH₄  concentration over India in boundary layer heights (>850 hPa) is affected majorly by the local emission strengths while in the middle (400- 600 hPa) and upper (< 200 hPa) troposphere it is governed by the convective transport of surface emissions signals and redistribution in the monsoon winds in the upper troposphere. GOSAT-1 shows highest XCH₄ of ~1900 ppb (2021) in northern India (North of 20° N) in winter season  (November) while AIRS shows highest CH4 ~1910 ppb in summer month of May at 200 hPa for the same year. Based on the findings from the vertical (AIRS) and total column (GOSAT-1) distribution of CH₄, our analysis suggests that 3D variability of CH₄ over the entire Indian region is governed by a diverse spread of surface emissions and global monsoon divergent wind circulations.  

Through this study, the reasons for these observed patterns of CH₄ are explored in light of forward model results using an ACTM by combining and comparing the strengths of AIRS and GOSAT-1.