Methane (CH₄) sources in Krakow, Poland: insights from isotope analysis

• Main Authors: M. Menoud, C. van der Veen, J. Necki, J. Bartyzel, B. Szénási, M. Stanisavljević, I. Pison, P. Bousquet, T. Röckmann
• Format: Article
• Language: English
• Published: Copernicus Publications 2021-09-01
• Series: Atmospheric Chemistry and Physics
• Online Access: https://acp.copernicus.org/articles/21/13167/2021/acp-21-13167-2021.pdf

<p>Methane (<span class="inline-formula">CH₄</span>) emissions from human activities are a threat to the resilience of our current climate system. The stable isotopic composition of methane (<span class="inline-formula"><i>δ</i>¹³</span>C and <span class="inline-formula"><i>δ</i>²</span>H) allows us to distinguish between the different <span class="inline-formula">CH₄</span> origins. A significant part of the European <span class="inline-formula">CH₄</span> emissions, 3.6 <span class="inline-formula">%</span> in 2018, comes from coal extraction in Poland, the Upper Silesian Coal Basin (USCB) being the main hotspot. <br/>Measurements of <span class="inline-formula">CH₄</span> mole fraction (<span class="inline-formula"><i>χ</i></span>(<span class="inline-formula">CH₄</span>)), <span class="inline-formula"><i>δ</i>¹³</span>C, and <span class="inline-formula"><i>δ</i>²</span>H in <span class="inline-formula">CH₄</span> in ambient air were performed continuously during 6 months in 2018 and 2019 at Krakow, Poland, in the east of the USCB. In addition, air samples were collected during parallel mobile campaigns, from multiple <span class="inline-formula">CH₄</span> sources in the footprint area of the continuous measurements. The resulting isotopic signatures from sampled plumes allowed us to distinguish between natural gas leaks, coal mine fugitive emissions, landfill and sewage, and ruminants. The use of <span class="inline-formula"><i>δ</i>²</span>H in <span class="inline-formula">CH₄</span> is crucial to distinguish the fossil fuel emissions in the case of Krakow because their relatively depleted <span class="inline-formula"><i>δ</i>¹³</span>C values overlap with the ones of microbial sources. The observed <span class="inline-formula"><i>χ</i></span>(<span class="inline-formula">CH₄</span>) time series showed regular daily night-time accumulations, sometimes combined with irregular pollution events during the day. The isotopic signatures of each peak were obtained using the Keeling plot method and generally fall in the range of thermogenic <span class="inline-formula">CH₄</span> formation – with <span class="inline-formula"><i>δ</i>¹³</span>C between <span class="inline-formula">−</span>59.3 <span class="inline-formula">‰</span> and <span class="inline-formula">−</span>37.4 <span class="inline-formula">‰</span> Vienna Pee Dee Belemnite (V-PDB) and <span class="inline-formula"><i>δ</i>²</span>H between <span class="inline-formula">−</span>291 <span class="inline-formula">‰</span> and <span class="inline-formula">−</span>137 <span class="inline-formula">‰</span> Vienna Standard Mean Ocean Water (V-SMOW). They compare well with the signatures measured for gas leaks in Krakow and USCB mines.</p> <p>The CHIMERE transport model was used to compute the <span class="inline-formula">CH₄</span> and isotopic composition time series in Krakow, based on two emission inventories. The magnitude of the pollution events is generally underestimated in the model, which suggests that emission rates in the inventories are too low. The simulated isotopic source signatures, obtained with Keeling plots on each simulated peak, indicate that a higher contribution from fuel combustion sources in the EDGAR v5.0 inventory would lead to a better agreement than when using CAMS-REG-GHG v4.2 (Copernicus Atmosphere Monitoring Service REGional inventory for Air Pollutants and GreenHouse Gases). The isotopic mismatches between model and observations are mainly caused by uncertainties in the assigned isotopic signatures for each source category and the way they are classified in the inventory. These uncertainties are larger for emissions close to the study site, which are more heterogenous than the ones advected from the USCB coal mines. Our isotope approach proves to be very sensitive in this region, thus helping to evaluate emission estimates.</p>