Seasonal variability of tropospheric CO2 over India based on model simulation, satellite retrieval and in-situ observation.

In this study, investigation of the seasonal cycle of the tropospheric CO2 concentration over India was carried out using the GEOS-Chem atmospheric transport model, Greenhouse gas Observation SATellite (GOSAT) retrievals, and in-situ measurements. The model simulation is highly coherent with the satellite and in-situ datasets, and it shows a distinct seasonal cycle of the tropospheric CO2 tendency over India with a negative phase (decreasing concentration) during April–August and a positive phase (increasing concentration) during September–March. The model diagnostics were analyzed to estimate budgets of the surface layer CO2, up to 650 hPa pressure level, for the two-phases of the seasonal cycle. A mean tendency, equivalent to −0.70 ppmv month−1, observed during April–August, which results from the loss of CO2 content in the surface layer through horizontal advection (−2.25 ppmv month−1) and vertical diffusion (−0.20 ppmv month−1), that dominates the gain from vertical advection (1.53 ppmv month−1). The negative contribution of horizontal advection in this period comes from the transport of CO2 depleted air-parcels over the oceanic region to India by the southwest monsoon winds and the positive contributions of vertical advection comes from upwelling of CO2 enriched air-parcels. The mean tendency, equivalent to 1.01 ppmv month−1, during September–March results from the gain through vertical advection (0.78 ppmv month−1) and horizontal advection (0.37 ppmv month−1) and a small contribution of vertical diffusion (−0.15 ppmv month−1). In this period, positive contribution of horizontal advection is due to the transport of CO2 enriched air-parcels from the southeast Asian region to India by north-east monsoon winds. At the annual scale, CO2 content of the surface layer over India has a net gain of 0.75 GtC that comes from 14.31 GtC through vertical advection that exceeds the loss due to horizontal advection (−11.10 GtC) and vertical diffusion processes (−2.46 GtC). This net gain is almost 85% higher than the input of 0.4 GtC through surface fluxes, which composed of 0.61 GtC anthropogenic emission and −0.21 GtC net terrestrial ecosystem exchanges. Additional sensitivity experiment was carried out to elucidate the semi-annual features of the seasonal cycle of CO2 for north India, in contrast to the annual characteristics of the seasonal cycle for south India in relation to the GOSAT observation. Highlights: Greenhouse gas Observation SATellite (GOSAT) L3B and L4B retrievals and in situ flux tower measurements were analysed to describe seasonal cycle of tropospheric CO2 over India; and GEOS-Chem atmospheric transport model diagnostics were used to examine the causes of the variability. The seasonal cycle over north India is composed of mixed signature of annual and semi-annual frequencies while south India experiences dominance of annual oscillation. However, the surface layer CO2 seasonal tendency has a major negative phase during April–August and a positive phase during September–March. The net negative tendency during April–August results from the loss of CO2 from the surface layer through horizontal advection and vertical diffusion processes that dominates the gain from vertical advection; while the net positive tendency during September–March results from the gain through vertical advection and horizontal advection and a small negative contribution from vertical diffusion. At annual scale, the surface layer over India experiences net positive gain of CO2 concentration, which is 85% more than the net input from the surface fluxes, and is mostly contributed by large-scale transport processes. Sensitivity experiments were carried out to elucidate the semi-annual features of the seasonal cycle of CO2 over north India in relation to the GOSAT observation. It turns out that the secondary trough during October–December on the background of net positive tendency during September–March results from the drawdown of CO2 by the terrestrial ecosystem uptake. [ABSTRACT FROM AUTHOR]