A View of Atmospheric Methane Variability Over the Indian Subcontinent From Multi-sensor Data

Atmospheric methane (CH4) is the second largest anthropogenic contributor to global warming after CO2. Its concentration has almost tripled since the pre-industrial period, owing to an increase in anthropogenic sources. CH4 has a lifetime of ~10 years in the atmosphere. Hence, managing and mitigating methane emissions is our shortest route to arrest the increase in rate of global warming.

We analyze a maximum of 11 years (2009-2020) of three observational datasets of columnar dry-air mole fractions (XCH4) of methane, from the Greenhouse gases Observation SATellite (GOSAT) GOSAT-1 (2009-2020), GOSAT-2 (2019-2020) on board the IBUKI satellites, and TROPOspheric Monitoring Instrument (TROPOMI) (2018-2020) on board the Sentinel 5 Precursor (S5-P) satellite. The XCH4 variability over the Indian subcontinent is also used from the JAMSTEC's MIROC4 atmospheric chemistry transport model (ACTM) from 2009-2020, after convolution with GOSAT-1, GOSAT-2 and TROPOMI a priori profiles and averaging kernels. We have investigated several unique perspectives of the long-term (11 years+) record of XCH4 observations from GOSAT-1 along with the measurements with high accuracy, exceptional temporal and spatial resolution from TROPOMI. GOSAT-2 expands upon the capabilities of its predecessor GOSAT-1 and serves as a good intermediate between the GOSAT-1 and TROPOMI.

Under clear skies, all three sensors detect methane by its absorption in shortwave infrared zone (1.63-1.70 µm and 2.2-2.4µm) with near-unit sensitivity down to the surface. GOSAT-1 operates at 1.65µm and TROPOMI operates at 2.3µm band while GOSAT-2 operates at both. GOSAT-1 and GOSAT-2 provide global coverage every 3 days while TROPOMI has a return time of 1 day. Our time series analysis of GOSAT-1 shows that the annual mean XCH4 has increased by 100 ppb from 2009 to 2020 over the Indian subcontinent. All three data sets show strong seasonality in the XCH4 with a peak of 1916 ppb in the post-monsoon (October-November) season in 2020.

Using a combination of the ACTM and GOSAT-2-TROPOMI datasets, new mega emitters of methane have been identified, such as Gujrat-Maharashtra region on the western ghats and the Andhra Pradesh-Telangana region on the eastern ghats. This study tries to isolate the meteorology, transport and chemical loss signals from the local emissions.