Methane sources from waste and natural gas sectors detected in Pune, India, by concentration and isotopic analysis

Methane (CH₄) is a potent greenhouse gas and also plays a significant role in tropospheric chemistry. High-frequency (sub-hourly) measurements of CH₄ and carbon isotopic ratio (δ¹³CH₄) have been conducted at Pune (18.53°N, 73.80°E), an urban environment in India, during 2018-2020. High CH₄ concentrations were observed, with a mean of 2100 ± 196 ppb (1844-2749 ppb), relative to marine background concentrations. The δ¹³CH₄ varied between -45.11 and -50.03 ‰ for the entire study period with an average of -47.41 ± 0.94 ‰. The diurnal variability of CH₄ typically showed maximum values in the morning (08:00-09:00 local time) and minimum in the afternoon (15:00 local time). The deepest diurnal amplitude of ~500 ppb was observed during winter (December-February), which was reduced to less than half, ~200 ppb, during the summer (March-May). CH₄ concentration at Pune showed a strong seasonality (470 ppb), much higher than that at Mauna Loa, Hawaii. On the other hand, δ¹³CH₄ records did not show distinct seasonality at Pune. The δ¹³CH₄ values revealed that the significant sources of CH₄ in Pune were from the waste sector (enhanced during the monsoon season; signature of depleted δ¹³CH₄), followed by the natural gas sector with a signature of enriched δ¹³CH₄. Our analysis of Covid-19 lockdown (April to May 2020) effect on the CH₄ variability showed no signal in the CH₄ variability; however, the isotopic analysis indicated a transient shift in the CH₄ source to the waste sector (early summer of 2020).