Measurement report: Insights into the high temporal variability of atmospheric carbon dioxide (CO ₂ ) at a suburban station in the Indo-Gangetic Plain

Abstract. The unusual weather patterns and large anthropogenic emissions over the Indo-Gangetic Plain (IGP) make it a significant hotspot of greenhouse gases like carbon dioxide (CO2). Given the significance of the IGP and highly populated Delhi National Capital Region (Delhi-NCR), a GHG observatory was established at a suburban monitoring station in Sonipat, Haryana (28.95° N, 77.10° E; 228 m a.s.l.), about 45 km north of the Delhi state boundary. Using a laser-based cavity ring-down spectroscopy (CRDS) technique, we measured CO2 mole fraction from February 2023 to January 2025. An annual average CO2 mole fraction of 440.8 ± 19.7 parts per million (ppm) was recorded in 2024, which includes a strong seasonal variability, ranging from 422.6 ± 23.3 ppm during the monsoon (June–September) to 456.4 ± 30.8 ppm in post-monsoon (October–November). A strong CO2 diurnal amplitude of 29 ppm in May and 63 ppm in October was observed mainly due to seasonal changes in boundary layer mixing (faster in May than October) and biospheric activity (weaker in May than October). Further investigation of the drivers of strong seasonal and diurnal CO2 variability over IGP revealed a strong contrast to other global monitoring stations in the same latitude band. A strong correlation between CO2 and methane (CH4) indicated a co-located emission source, while the strong positive correlation between CO2 and carbon monoxide (CO) during post-monsoon emerges due to emissions from biomass burning. We demonstrated that the high temporal CO2 variability in the IGP region is driven by the complex interplay of local anthropogenic and biomass burning emissions, biospheric fluxes, and prevailing meteorology.