Urban Methane Surveys – A Case Study Where Isotope Measurements Guided Source Attribution

Methane (CH₄) is the second most important anthropogenic greenhouse gas after carbon dioxide (CO₂) in the atmosphere¹. Human activities are estimated to contribute ~50% of total CH₄ emissions globally². With 68% of the population projected to live in urban areas by 2050³, there is a need to better quantify CH₄ emissions from urban sources and to develop mitigation plans. Major potential urban sources include: the gas distribution network, landfills, the sewerage network, appliances in houses (heaters, stoves, hot-water systems), wood burning heaters, and urban wetlands.

This study aimed to determine the major CH₄ sources in Melbourne (Australia’s second largest city with a population approaching 5 million people). Melbourne has grown rapidly since it was founded ~200 years ago and this has left legacy potential CH₄ sources. For example, the gas distribution system has piping ranging from modern to 100 years old (common in unrenovated houses from early last century); landfills that use to be on the city fringe are now surrounded by new housing developments. 

To map the location of major CH₄ sources throughout Melbourne we conducted a mobile survey, measuring the CH₄ mole fraction ([CH₄]) at a height of 3 m using a Los Gatos Research ultra-portable greenhouse gas analyser. An air inlet was attached to the roof of the car and the location of the measurements were georeferenced using a Hemisphere GPS system as we drove around the city. The day and night-time surveys were undertaken from the 26^th – 27^th July 2019 (winter).  When a major CH₄ plume was detected 10 air samples were collected and stored 3 litre FlexFoil bags. These samples were analysed for [CH₄], δ¹³C-CH₄, [CO₂], δ¹³C-CO₂ using a Picarro G2201-i cavity ring-down spectrometer (CRDS). To determine the δ¹³C-CH₄ signature of the plume each set of bags was analysed using a Miller-Tans plot and Bayesian regression. The combination of potential observable sources and the δ¹³C-CH₄ signature was then used to attribute the source of the CH₄ plume. We show that the δ¹³C-CH₄ signatures of the CH₄ plumes are needed to reduce the risk of attributing a plume to the wrong source. We present an example of separating domestic wood fires from a ‘super-emitter’ leak from the gas distribution system, and we also show how we traced a landfill plume for a distance of over 5 km using δ¹³C-CH₄ measurements.

Our research demonstrates that mobile [CH₄] surveys coupled with δ¹³C-CH₄ analyses is a cost-effective workflow for mapping both diffuse CH₄ emissions and ‘super-emitters’. Such surveys could be systematically undertaken in cities worldwide to delineate and prioritise targets for CH₄ emission reduction. 

 

References

(1)      Myhre, G. et al. Cambridge University Press, 2013; Vol. 9781107057, pp 659–740.

(2)      Saunois, M. et al. Earth Syst. Sci. Data Discuss. 2019, 1–138.

(3)      United Nations. World Urbanization Prospects The 2018 Revision (ST/ESA/SER.A/420). 2019.