Abstract. Characterizing and attributing methane (CH4) emissions across varying scales are important from environmental, safety, and economic perspectives and are essential for designing and evaluating effective mitigation strategies. Mobile real-time measurements of CH4 in ambient air offer a fast and effective method to identify and quantify local CH4 emissions in urban areas. We carried out extensive campaigns to measure CH4 mole fractions at the street level in Utrecht, the Netherlands (2018 and 2019), and Hamburg, Germany (2018). We detected 145 leak indications (LIs; i.e., CH4 enhancements of more than 10 % above background levels) in Hamburg and 81 LIs in Utrecht. Measurements of the ethane-to-methane ratio (C2:C1), methane-to-carbon dioxide ratio (CH4:CO2), and CH4 isotope composition (δ13C and δD) show that in Hamburg about 1∕3 of the LIs, and in Utrecht 2∕3 of the LIs (based on a limited set of C2:C1 measurements), were of fossil fuel origin. We find that in both cities the largest emission rates in the identified LI distribution are from fossil fuel sources. In Hamburg, the lower emission rates in the identified LI distribution are often associated with biogenic characteristics or (partly) combustion. Extrapolation of detected LI rates along the roads driven to the gas distribution pipes in the entire road network yields total emissions from sources that can be quantified in the street-level surveys of 440±70 t yr−1 from all sources in Hamburg and 150±50 t yr−1 for Utrecht. In Hamburg, C2:C1, CH4:CO2, and isotope-based source attributions show that 50 %–80 % of all emissions originate from the natural gas distribution network; in Utrecht more limited attribution indicates that 70 %–90 % of the emissions are of fossil origin. Our results confirm previous observations that a few large LIs, creating a heavy tail, are responsible for a significant proportion of fossil CH4 emissions. In Utrecht, 1∕3 of total emissions originated from one LI and in Hamburg >1/4 from two LIs. The largest leaks were located and fixed quickly by GasNetz Hamburg once the LIs were shared, but 80 % of the (smaller) LIs attributed to the fossil category could not be detected and/or confirmed as pipeline leaks. This issue requires further investigation.

Abstract. In regions where there are multiple sources of methane (CH4) in close proximity, it can be difficult to apportion the CH4 measured in the atmosphere to the appropriate sources. In the Surat Basin, Queensland, Australia, coal seam gas (CSG) developments are surrounded by cattle feedlots, grazing cattle, piggeries, coal mines, urban centres and natural sources of CH4. The characterization of carbon (δ13C) and hydrogen (δD) stable isotopic composition of CH4 can help distinguish between specific emitters of CH4. However, in Australia there is a paucity of data on the various isotopic signatures of the different source types. This research examines whether dual isotopic signatures of CH4 can be used to distinguish between sources of CH4 in the Surat Basin. We also highlight the benefits of sampling at nighttime. During two campaigns in 2018 and 2019, a mobile CH4 monitoring system was used to detect CH4 plumes. Sixteen plumes immediately downwind from known CH4 sources (or individual facilities) were sampled and analysed for their CH4 mole fraction and δ13CCH4 and δDCH4 signatures. The isotopic signatures of the CH4 sources were determined using the Keeling plot method. These new source signatures were then compared to values documented in reports and peer-reviewed journal articles. In the Surat Basin, CSG sources have δ13CCH4 signatures between −55.6 ‰ and −50.9 ‰ and δDCH4 signatures between −207.1 ‰ and −193.8 ‰. Emissions from an open-cut coal mine have δ13CCH4 and δDCH4 signatures of -60.0±0.6 ‰ and -209.7±1.8 ‰ respectively. Emissions from two ground seeps (abandoned coal exploration wells) have δ13CCH4 signatures of -59.9±0.3 ‰ and -60.5±0.2 ‰ and δDCH4 signatures of -185.0±3.1 ‰ and -190.2±1.4 ‰. A river seep had a δ13CCH4 signature of -61.2±1.4 ‰ and a δDCH4 signature of -225.1±2.9 ‰. Three dominant agricultural sources were analysed. The δ13CCH4 and δDCH4 signatures of a cattle feedlot are -62.9±1.3 ‰ and -310.5±4.6 ‰ respectively, grazing (pasture) cattle have δ13CCH4 and δDCH4 signatures of -59.7±1.0 ‰ and -290.5±3.1 ‰ respectively, and a piggery sampled had δ13CCH4 and δDCH4 signatures of -47.6±0.2 ‰ and -300.1±2.6 ‰ respectively, which reflects emissions from animal waste. An export abattoir (meat works and processing) had δ13CCH4 and δDCH4 signatures of -44.5±0.2 ‰ and -314.6±1.8 ‰ respectively. A plume from a wastewater treatment plant had δ13CCH4 and δDCH4 signatures of -47.6±0.2 ‰ and -177.3±2.3 ‰ respectively. In the Surat Basin, source attribution is possible when both δ13CCH4 and δDCH4 are measured for the key categories of CSG, cattle, waste from feedlots and piggeries, and water treatment plants. Under most field situations using δ13CCH4 alone will not enable clear source attribution. It is common in the Surat Basin for CSG and feedlot facilities to be co-located. Measurement of both δ13CCH4 and δDCH4 will assist in source apportionment where the plumes from two such sources are mixed.

Despite the importance of methane for climate change mitigation, uncertainties regarding the temporal and spatial variability of the emissions remain. Measurements of CH 4 isotopic composition are used to partition the relative contributions of different emission sources. We report continuous isotopic measurements during 5 months at the Lutjewad tower (north of the Netherlands). Time-series of v(CH 4 ), d 13 C-CH 4 , and dD-CH 4 in ambient air were analysed using the Keeling plot method. Resulting source signatures ranged from À67.4 to À52.4& vs V-PDB and from À372 to À211& vs V-SMOW, for d 13 C and dD respectively, indicating a prevalence of biogenic sources. Analysis of isotope and wind data indicated that (i) emissions from off-shore oil and gas platforms in the North Sea were not detected during this period, (ii) CH 4 from fossil fuel related sources was usually advected from the east, pointing towards the Groningen gas field or regions further east in Germany. The results from two atmospheric transport models, CHIMERE and FLEXPART-COSMO, using the EDGAR v4.3.2 and TNO-MACC III emission inventories, reproduce v(CH 4 ) variations relatively well, but the isotope signatures were over-estimated by the model compared to the observations. Accounting for geographical variations of the d 13 C signatures from fossil fuel emissions improved the model results significantly. The difference between model and measured isotopic signatures was larger when using TNO-MACC III compared to EDGAR v4.3.2 inventory. Uncertainties in the isotope signatures of the sources could explain a significant fraction of the discrepancy, thus a better source characterisation could further strengthen the use of isotopes in constraining emissions.

Coral reefs are degrading from the effects of anthropogenic activities, including climate change. Under these stressors, their ability to survive depends upon existing phenotypic plasticity, but also transgenerational adaptation. Parental effects are ubiquitous in nature, yet empirical studies of these effects in corals are scarce, particularly in the context of climate change. This study exposed mature colonies of the common reef-building coral Stylophora pistillata from the Gulf of Aqaba to seawater conditions likely to occur just beyond the end of this century during the peak planulae brooding season (Representative Concentration Pathway 8.5: pH −0.4 and +5°C beyond present day). Parent and planulae physiology were assessed at multiple time points during the experimental incubation. After 5 weeks of incubation, the physiology of the parent colonies exhibited limited treatment-induced changes. All significant time-dependent changes in physiology occurred in both ambient and treatment conditions. Planulae were also resistant to future ocean conditions, with protein content, symbiont density, photochemistry, survival and settlement success not significantly different compared with under ambient conditions. High variability in offspring physiology was independent of parental or offspring treatments and indicate the use of a bet-hedging strategy in this population. This study thus demonstrates weak climate-change-associated carryover effects. Furthermore, planulae display temperature and pH resistance similar to those of adult colonies and therefore do not represent a larger future population size bottleneck. The findings add support to the emerging hypothesis that the Gulf of Aqaba may serve as a coral climate change refugium aided by these corals' inherent broad physiological resistance.