Methane mapping in Greater Accra: Isotopic signatures and mobile detection techniques in a developing urban landscape

The study investigated the comprehensive mapping of methane mole fraction in Greater Accra Region, Ghana. It aims to quantify δ¹³C-CH₄, identify sources, and map methane levels, distinguishing between biogenic and thermogenic sources using isotopic analysis. On average, 300 study sites were spotted for each district in Greater Accra Region for methane concentrations mapping. A vehicle equipped with a G2201-i isotopic gas analyser was employed for isotopic measurements of δ¹³C-CH₄. The number of samples included in the Keeling plot for the plotting of the regression line in discerning isotopic signatures for the study data ranged from 4 – 19. The HYSPLIT (Hybrid Single Particle Lagrangian Integrated Trajectory) model was applied to help comprehend the dispersion patterns and tracing the origins of detected methane mole fraction in specific areas. Miller–Tans approach was used to estimate the average source signature across the dataset and obtained signature of source mix (‰) of −30 ± 1 ‰. The highest methane mole fraction was detected in Tema Metropolis, reaching 5.89 ppm with δ¹³C-CH₄ value of −24 ± 2 ‰. The Accra Metropolis also had elevated methane levels at 5.08 ppm with δ¹³C-CH₄ values of −24 ± 3 ‰. The isotopic signature of methane mixed-source in Greater Accra, derived using the Miller-Tans plot (-30 ± 1 ‰), indicating combustion and biomass burning emissions. Generally, the methane isotopic values in this region suggest a mix of natural gas leaks, wetlands, landfills, biomass burning, combustion process, and agricultural waste emissions. The data obtained provides a crucial primary representation of methane emissions in the Greater Accra-Region, and West African environment.