Urban Methane Surveys &#8211; A Case Study Where Isotope Measurements Guided Source Attribution
Authors: Kelly Bryce F.J. , Lu Xinyi (Lexie) , Loh Zoë M. , Fisher Rebecca E. 
Published: March 2020

&lt;p&gt;Methane (CH&lt;sub&gt;4&lt;/sub&gt;) is the second most important anthropogenic greenhouse gas after carbon dioxide (CO&lt;sub&gt;2&lt;/sub&gt;) in the atmosphere&lt;sup&gt;1&lt;/sup&gt;. Human activities are estimated to contribute ~50% of total CH&lt;sub&gt;4&lt;/sub&gt; emissions globally&lt;sup&gt;2&lt;/sup&gt;. With 68% of the population projected to live in urban areas by 2050&lt;sup&gt;3&lt;/sup&gt;, there is a need to better quantify CH&lt;sub&gt;4&lt;/sub&gt; 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.&lt;/p&gt;&lt;p&gt;This study aimed to determine the major CH&lt;sub&gt;4&lt;/sub&gt; sources in Melbourne (Australia&amp;#8217;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 CH&lt;sub&gt;4 &lt;/sub&gt;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.&amp;#160;&lt;/p&gt;&lt;p&gt;To map the location of major CH&lt;sub&gt;4&lt;/sub&gt; sources throughout Melbourne we conducted a mobile survey, measuring the CH&lt;sub&gt;4 &lt;/sub&gt;mole fraction ([CH&lt;sub&gt;4&lt;/sub&gt;]) 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&lt;sup&gt;th&lt;/sup&gt; &amp;#8211; 27&lt;sup&gt;th&lt;/sup&gt; July 2019 (winter).&amp;#160; When a major CH&lt;sub&gt;4&lt;/sub&gt; plume was detected 10 air samples were collected and stored 3 litre FlexFoil bags. These samples were analysed for [CH&lt;sub&gt;4&lt;/sub&gt;], &amp;#948;&lt;sup&gt;13&lt;/sup&gt;C-CH&lt;sub&gt;4&lt;/sub&gt;, [CO&lt;sub&gt;2&lt;/sub&gt;], &amp;#948;&lt;sup&gt;13&lt;/sup&gt;C-CO&lt;sub&gt;2&lt;/sub&gt; using a Picarro G2201-i cavity ring-down spectrometer (CRDS). To determine the &amp;#948;&lt;sup&gt;13&lt;/sup&gt;C-CH&lt;sub&gt;4 &lt;/sub&gt;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 &amp;#948;&lt;sup&gt;13&lt;/sup&gt;C-CH&lt;sub&gt;4&lt;/sub&gt; signature was then used to attribute the source of the CH&lt;sub&gt;4 &lt;/sub&gt;plume. We show that the &amp;#948;&lt;sup&gt;13&lt;/sup&gt;C-CH&lt;sub&gt;4 &lt;/sub&gt;signatures of the CH&lt;sub&gt;4&lt;/sub&gt; 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 &amp;#8216;super-emitter&amp;#8217; leak from the gas distribution system, and we also show how we traced a landfill plume for a distance of over 5 km using &amp;#948;&lt;sup&gt;13&lt;/sup&gt;C-CH&lt;sub&gt;4&lt;/sub&gt; measurements.&lt;/p&gt;&lt;p&gt;Our research demonstrates that mobile [CH&lt;sub&gt;4&lt;/sub&gt;] surveys coupled with &amp;#948;&lt;sup&gt;13&lt;/sup&gt;C-CH&lt;sub&gt;4&lt;/sub&gt; analyses is a cost-effective workflow for mapping both diffuse CH&lt;sub&gt;4&lt;/sub&gt; emissions and &amp;#8216;super-emitters&amp;#8217;. Such surveys could be systematically undertaken in cities worldwide to delineate and prioritise targets for CH&lt;sub&gt;4 &lt;/sub&gt;emission reduction.&amp;#160;&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;References&lt;/p&gt;&lt;p&gt;(1) &amp;#160;&amp;#160;&amp;#160;&amp;#160; Myhre, G. et al. Cambridge University Press, 2013; Vol. 9781107057, pp 659&amp;#8211;740.&lt;/p&gt;&lt;p&gt;(2) &amp;#160;&amp;#160;&amp;#160;&amp;#160; Saunois, M. et al. Earth Syst. Sci. Data Discuss. 2019, 1&amp;#8211;138.&lt;/p&gt;&lt;p&gt;(3) &amp;#160;&amp;#160;&amp;#160;&amp;#160; United Nations. World Urbanization Prospects The 2018 Revision (ST/ESA/SER.A/420). 2019.&lt;/p&gt;