Ground-Based Mobile Measurements to Track Urban Methane Emissions from Natural Gas in 12 Cities across Eight Countries

To mitigate methane emission from urban natural gas distribution systems, it is crucial to understand local leak rates and occurrence rates. To explore urban methane emissions in cities outside the U.S., where significant emissions were found previously, mobile measurements were performed in 12 cities across eight countries. The surveyed cities range from medium size, like Groningen, NL, to large size, like Toronto, CA, and London, UK. Furthermore, this survey spanned across European regions from Barcelona, ES, to Bucharest, RO. The joint analysis of all data allows us to focus on general emission behavior for cities with different infrastructure and environmental conditions. We find that all cities have a spectrum of small, medium, and large methane sources in their domain. The emission rates found follow a heavy-tailed distribution, and the top 10% of emitters account for 60-80% of total emissions, which implies that strategic repair planning could help reduce emissions quickly. Furthermore, we compare our findings with inventory estimates for urban natural gas-related methane emissions from this sector in Europe. While cities with larger reported emissions were found to generally also have larger observed emissions, we find clear discrepancies between observation-based and inventory-based emission estimates for our 12 cities.

The fingerprint of the summer 2018 drought in Europe on ground-based atmospheric CO2 measurements

During the summer of 2018, a widespread drought developed over Northern and Central Europe. The increase in temperature and the reduction of soil moisture have influenced carbon dioxide (CO₂) exchange between the atmosphere and terrestrial ecosystems in various ways, such as a reduction of photosynthesis, changes in ecosystem respiration, or allowing more frequent fires. In this study, we characterize the resulting perturbation of the atmospheric CO₂ seasonal cycles. 2018 has a good coverage of European regions affected by drought, allowing the investigation of how ecosystem flux anomalies impacted spatial CO₂ gradients between stations. This density of stations is unprecedented compared to previous drought events in 2003 and 2015, particularly thanks to the deployment of the Integrated Carbon Observation System (ICOS) network of atmospheric greenhouse gas monitoring stations in recent years. Seasonal CO₂ cycles from 48 European stations were available for 2017 and 2018. Earlier data were retrieved for comparison from international databases or national networks. Here, we show that the usual summer minimum in CO₂ due to the surface carbon uptake was reduced by 1.4 ppm in 2018 for the 10 stations located in the area most affected by the temperature anomaly, mostly in Northern Europe. Notwithstanding, the CO₂ transition phases before and after July were slower in 2018 compared to 2017, suggesting an extension of the growing season, with either continued CO₂ uptake by photosynthesis and/or a reduction in respiration driven by the depletion of substrate for respiration inherited from the previous months due to the drought. For stations with sufficiently long time series, the CO₂ anomaly observed in 2018 was compared to previous European droughts in 2003 and 2015. Considering the areas most affected by the temperature anomalies, we found a higher CO₂ anomaly in 2003 (+3 ppm averaged over 4 sites), and a smaller anomaly in 2015 (+1 ppm averaged over 11 sites) compared to 2018. This article is part of the theme issue 'Impacts of the 2018 severe drought and heatwave in Europe: from site to continental scale'.

The use of a custom mode electron capture detector to determine mixing ratios of environmental tracers: Sulfur hexafluoride, chlorotrifluoromethane and bromotrifluoromethane

Atmospheric concentrations of anthropogenic trace gases, such as sulfur hexafluoride, SF6, chlorotrifluoromethane, CF3Cl, and bromotrifluoromethane, CF3Br, are increasing. Their long lifetimes and limited chemical reactivity make them attractive environmental tracers for hydrology and oceanography. While ambient SF6 concentrations can be readily measured using GC-ECD, the simultaneous analysis of CF3Cl and CF3Br is hampered due to their low ECD sensitivity. The response of a commercial ECD for those gases was enhanced using the resonance detection mode which is based on shifting the mean energy of electrons in the ECD detector towards the region where the electron-capture reaction reveals a distinct maximum. A custom electronic system enabled operation of a commercial ECD in the resonance detection mode. An approximately 50-fold amplification of the ECD signal was obtained for CF3Cl by application of high-frequency electric field (amplitude of 50V and frequency of 40MHz). For CF3Br, a 3.5-fold increase of the ECD signal was obtained, with a lower HF field (20-30V). In the case of SF6 the application of the HF field reduces the magnitude of ECD signal by a factor of 40. The electron-capture coefficients for SF6, CF3Cl and CF3Br were determined from 453 to 633K in the standard and the resonance modes. The electron-capture coefficients for CF3Cl and CF3Br increase with increasing temperature for both modes, while that for SF6 decreases slightly with increasing temperature. The application of the resonance detection mode to a commercial ECD provides an attractive and cost-effective alternative to GCMS for high-quality quantitative analyses of CF3Cl and CF3Br as environmental tracers.