Variability of Atmospheric Methane Over The Indian Subcontinent From Two Decades (2003-2020) of Atmospheric Infrared Sounder (AIRS) Observations

Atmospheric methane 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. On a time scale of 20 years, its global warming potential is ~84 times more than that of CO2. Hence, managing and containing methane emissions is our shortcut towards arresting global warming. So far, most of the studies on methane variability are from the developed countries where the ground network is adequate. In lower-and-middle-income countries like India, very few monitoring sites are operational, prompting the use of satellite data in examining methane variability. Here we analyze data from Atmospheric Infrared Sounder (AIRS) on board AQUA satellite for 18 year-period (2003-2020) to examine the spatio-temporal variability over the Indian subcontinent. The geographical and vertical patterns of the atmospheric methane are analysed in view of the available emission inventory, topography and meteorology. High concentrations of methane are observed over the Indo-Gangetic Plain with a peak of ~1850 ppb during the post monsoon season (October-November) in the lower troposphere (LT) (1000hPa-600hPa). In the upper troposphere (UT) (500-30 hPa), the highest concentration (1772 ppb) is observed over central India in the same season. There is persistence of a strong latitudinal gradient between northern India and its southern counterpart in the LT throughout the year. This spatial variability is attributed to the regional differences of dynamical systems, southwest monsoon meteorology, agricultural practices and local emissions. However, in UT, homogeneous mixing of methane is observed over the entire Indian region. The time series analysis reveals that average methane concentration over India in the LT has increased from ~1800 ppb in 2003 to ~1863 ppb in 2020. Stark differences are detected in the rate of growth of methane concentration between LT (~3 ppb/year) and UT (~4.8 ppb/year), signalling a convective updraft of methane from LT to UT. The overall trend of methane increment has been between 2-5 ppb per year since 2003 as per the AIRS observations.This study serves as an important addition to the limited inverse modelling approaches to quantify the atmospheric methane over Indian subcontinent.