Seasonal and annual variations of CO₂ and CH₄ at Shadnagar, a semi-urban site

Carbon dioxide (CO₂) and methane (CH₄) are the most important greenhouse gases (GHGs) due to their significant role in anthropogenic global climate change. The spatio-temporal variations of their concentration are characterized by the terrestrial biosphere, seasonal weather patterns and anthropogenic emissions. Hence, to understand the variability in regional surface GHG fluxes, high precision GHGs measurements were initiated by the National Remote Sensing Center (NRSC) of India. We report continuous CO₂ and CH₄measurements during 2014 to 2017 for the first time from Shadnagar, a suburban site in India. Annual mean CO₂ and CH₄ concentrations are 399.56 ± 5.46 ppm and 1.929 ± 0.09 ppm, respectively, for 2017. After the strong El Niño of 2015-2016, an abnormal rise in CO₂ growth rate of 5.5 ppm year^-1 was observed in 2017 at the study site, compared to 3.03 ppm year^-1 at Mauna Loa. Thus, the repercussion of the El Niño effect diminishes the net uptake by the terrestrial biosphere accompanied by increased soil respiration. Seasonal tracer to tracer correlation between CO₂ and CH₄ was also analyzed to characterize the possible source-sink relationship between the species. We compared CO₂ and CH₄ concentrations to simulations from an atmospheric chemistry transport model (ACTM). The seasonal phases of CH₄ were well captured by the ACTM, whereas the seasonal cycle amplitude of CO₂ was underestimated by about 30%.