Measurement report: Regional characteristics of seasonal and long-term variations in greenhouse gases at Nainital, India and Comilla, Bangladesh
Authors: Nomura Shohei , Naja Manish , Ahmed Md. Kawser , Mukai Hitoshi , Terao Yukio , Machida Toshinobu , Sasakawa Motoki , Patra Prabir K. 
Published: May 2021

Abstract. Emissions of greenhouse gases (GHGs) from the Indian subcontinent have increased during the last 20 years along with rapid economic growth, however, there remains a paucity of GHG measurements for policy relevant research. In northern India and Bangladesh, agricultural activities are considered to play an important role on GHGs concentrations in the atmosphere. We performed weekly air sampling at Nainital (NTL) in northern India and Comilla (CLA) in Bangladesh from 2006 and 2012, respectively. Air samples were analyzed for dry-air gas mole fractions of CO2, CH4, CO, H2, N2O, and SF6, and carbon and oxygen isotopic ratios of CO2 (δ13C-CO2 and δ18O-CO2). Regional characteristics of these components over the Indo-Gangetic Plain are discussed compared to data from other Indian sites and Mauna Loa, Hawaii (MLO), which is representative of marine background air. We found that the CO2 mole fraction at both NTL and CLA had two seasonal minima in February‒March and September, corresponding to crop cultivation activities that depend on regional climatic conditions. The carbon isotopic signature also suggested that photosynthetic CO2 absorption by crops cultivated in each season contributes differently to lower CO2 mole fractions. The CH4 mole fraction of NTL and CLA in August‒October showed high values (i.e., sometimes over 4,000 ppb at CLA) due to the influence of CH4 emissions from the paddy fields in addition to the other sources due to the hot and humid climatic conditions. High CH4 mole fractions sustained over months at CLA were a characteristic feature in the Indo-Gangetic Plain. The CO mole fractions at NTL were also high and showed peaks in May and October, while CLA had much higher peaks in October‒March due to the influence of human activities such as emissions from biomass burning and brick production. The N2O mole fractions at NTL and CLA increased in June‒August and November‒February, which coincided with the application of nitrogen fertilizer and the burning of biomass such as the harvest residues and dung for domestic cooking. Based on H2 seasonal variation at both sites, it appeared that the emissions in this region were related to biomass burning in addition to production from the reaction of OH and CH4. The SF6 mole fraction was similar to that at MLO, suggesting that there were few anthropogenic emission sources in the district. The variability of CO2 growth rate at NTL was different from the variability in the CO2 growth rate at MLO, which is more closely linked with the El Niño Southern Oscillation (ENSO). In addition, the growth rates of the CH4 and SF6 mole fractions at NTL showed an anticorrelation with those at MLO, indicating that the frequency of southerly air masses strongly influenced these mole fractions. These finding showed that rather large regional climatic conditions considerably controlled interannual variations in GHGs, δ13C-CO2, and δ18O-CO2 through changes in precipitation and air mass.