Integrating radiocarbon measurements for CO2 source attribution at a suburban site upwind of the Delhi NCR

Atmospheric carbon dioxide (CO₂) is the most significant anthropogenic greenhouse gas, driving changes in Earth’s radiative balance and climate. The Indo-Gangetic Plain (IGP) is a global hotspot of CO₂ emissions due to dense population, intensive energy use, and widespread industrial activity; however, continuous and high-precision CO₂ observations across the region remain sparse. To address this gap, we conducted the first long-term in situ measurements of atmospheric CO₂ at a suburban site in Sonipat, located upwind of the Delhi National Capital Region. Using a state-of-the-art laser-based cavity ring-down spectrometer, continuous CO₂ observations were collected from February 2023 to February 2024, revealing an annual mean concentration of 422.3 ± 26.52 ppm. The seasonal cycle shows lower concentrations during the monsoon (404.9 ± 25.95 ppm) and elevated values in the post-monsoon season (438.8 ± 27.73 ppm), driven primarily by changes in boundary layer dynamics, regional emissions, and biospheric fluxes. Consistent diurnal patterns further highlight the influence of convective mixing, rush-hour traffic, and local industrial activity.

To quantify fossil fuel CO₂ (CO_2ff) contributions, we present the first radiocarbon (¹⁴C) measurements in CO₂ from Sonipat for April 2024 to March 2025. These data provide direct constraints on CO_2ff and enable separation of fossil and biospheric carbon components. By combining ¹⁴C-derived CO_2ff with collocated high-frequency carbon monoxide (CO) observations, we derive a constant CO–CO_2ff enhancement ratio (RCO). This ratio is then applied to reconstruct a continuous, high-resolution CO_2ff time series, capturing the strong seasonality linked to local emissions and meteorology. Subtracting fossil fuel and background contributions allows the isolation of regional biospheric CO₂ signals. 

Together, these integrated measurements demonstrate the value of continuous CO₂, CO, and ¹⁴C observations for improving carbon budget assessments over the IGP and highlight the critical role of multi-species atmospheric monitoring in constraining regional carbon fluxes.

Keywords: In-situ measurements, Indo-Gangetic Plain, Fossil fuel CO₂, Radiocarbon