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.

Sources and atmospheric processing of size segregated aerosol particles revealed by stable carbon isotope ratios and chemical speciation.

Size-segregated aerosol particles were collected during winter sampling campaigns at a coastal (55°37' N, 21°03'E) and an urban (54°64' N, 25°18' E) site. Organic compounds were thermally desorbed from the samples at different temperature steps ranging from 100 °C to 350 °C. The organic matter (OM) desorbed at each temperature step is analysed for stable carbon isotopes using an isotope ratio mass spectrometer (IRMS) and for individual organic compounds using a Proton Transfer Reaction Time-of-Flight Mass Spectrometer (PTR-MS). The OM desorbed at temperatures <200 °C was classified as less refractory carbon and the OM desorbed at temperatures between 200 °C and 350 °C was classified as more refractory carbon. At the coastal site, we identified two distinct time periods. The first period was more frequently influenced by marine air masses than the second time period, which was characterized by Easterly wind directions and continental air masses. During the first period OM contained a large fraction of hydrocarbons and had a carbon isotopic signature typical of liquid fossil fuels in the region. Organic mass spectra provide strong evidence that shipping emissions are a significant source of OM at this coastal site. The isotopic and chemical composition of OM during the second period at the coastal site was similar to the composition at the urban site. There was a clear distinction in source contribution between the less refractory OM and the more refractory OM at these sites. According to the source apportionment method used in this study, we were able to identify fossil fuel burning as predominant source of the less refractory OM in the smallest particles (D50 < 0.18 µm), and biomass burning as predominant source of the more refractory OM in the larger size range (0.32 < D50 < 1 µm).

Statistical clumped isotope signatures.

High precision measurements of molecules containing more than one heavy isotope may provide novel constraints on element cycles in nature. These so-called clumped isotope signatures are reported relative to the random (stochastic) distribution of heavy isotopes over all available isotopocules of a molecule, which is the conventional reference. When multiple indistinguishable atoms of the same element are present in a molecule, this reference is calculated from the bulk (≈average) isotopic composition of the involved atoms. We show here that this referencing convention leads to apparent negative clumped isotope anomalies (anti-clumping) when the indistinguishable atoms originate from isotopically different populations. Such statistical clumped isotope anomalies must occur in any system where two or more indistinguishable atoms of the same element, but with different isotopic composition, combine in a molecule. The size of the anti-clumping signal is closely related to the difference of the initial isotope ratios of the indistinguishable atoms that have combined. Therefore, a measured statistical clumped isotope anomaly, relative to an expected (e.g. thermodynamical) clumped isotope composition, may allow assessment of the heterogeneity of the isotopic pools of atoms that are the substrate for formation of molecules.

Natural and anthropogenic variations in methane sources during the past two millennia.

Methane is an important greenhouse gas that is emitted from multiple natural and anthropogenic sources. Atmospheric methane concentrations have varied on a number of timescales in the past, but what has caused these variations is not always well understood. The different sources and sinks of methane have specific isotopic signatures, and the isotopic composition of methane can therefore help to identify the environmental drivers of variations in atmospheric methane concentrations. Here we present high-resolution carbon isotope data (δ(13)C content) for methane from two ice cores from Greenland for the past two millennia. We find that the δ(13)C content underwent pronounced centennial-scale variations between 100 BC and AD 1600. With the help of two-box model calculations, we show that the centennial-scale variations in isotope ratios can be attributed to changes in pyrogenic and biogenic sources. We find correlations between these source changes and both natural climate variability--such as the Medieval Climate Anomaly and the Little Ice Age--and changes in human population and land use, such as the decline of the Roman empire and the Han dynasty, and the population expansion during the medieval period.

CO2 isotope analyses using large air samples collected on intercontinental flights by the CARIBIC Boeing 767.

Analytical details for 13C and 18O isotope analyses of atmospheric CO2 in large air samples are given. The large air samples of nominally 300 L were collected during the passenger aircraft-based atmospheric chemistry research project CARIBIC and analyzed for a large number of trace gases and isotopic composition. In the laboratory, an ultra-pure and high efficiency extraction system and high-quality isotope ratio mass spectrometry were used. Because direct comparison with other laboratories was practically impossible, the extraction and measurement procedures were tested in considerable detail. Extracted CO2 was measured twice vs. two different working reference CO2 gases of different isotopic composition. The two data sets agree well and their distributions can be used to evaluate analytical errors due to isotope measurement, ion corrections, internal calibration consistency, etc. The calibration itself is based on NBS-19 and also verified using isotope analyses on pure CO2 gases (NIST Reference Materials (RMs) and NARCIS CO2 gases). The major problem encountered could be attributed to CO2-water exchange in the air sampling cylinders. This exchange decreased over the years. To exclude artefacts due to such isotopic exchange, the data were filtered to reject negative delta18O(CO2) values. Examples of the results are given.

Influence of flooding on delta15N, delta18O, 1delta15N and 2delta15N signatures of N2O released from estuarine soils--a laboratory experiment using tidal flooding chambers.

The influence of flooding on N2O fluxes, denitrification rates, dual isotope (delta18O and delta15N) and isotopomer (1delta15N and 2delta15N) ratios of emitted N2O from estuarine intertidal zones was examined in a laboratory study using tidal flooding incubation chambers. Five replicate soil cores were collected from two differently managed intertidal zones in the estuary of the River Torridge (North Devon, UK): (1) a natural salt marsh fringing the estuary, and 2 a managed retreat site, previous agricultural land to which flooding was restored in summer 2001. Gas samples from the incubated soil cores were collected from the tidal chamber headspaces over a range of flooding conditions, and analysed for the delta18O, delta15N, 1delta15N and 2delta15N values of the emitted N2O. Isotope signals did not differ between the two sites, and nitrate addition to the flooding water did not change the isotopic content of emitted N2O. Under non-flooded conditions, the isotopic composition of the emitted N2O displayed a moderate variability in delta18O and 2delta15N delta values that was expected for microbial activity associated with denitrification. However, under flooded conditions, half of the samples showed strong and simultaneous depletions in 1delta15N and delta18O values, but not in 2delta15N. Such an isotope signal has not been reported in the literature, and it could point towards an unidentified N2O production pathway. Its signature differed from denitrification, which was generally the N2O production pathway in the salt marsh and the managed retreat site.

The position dependent 15N enrichment of nitrous oxide in the stratosphere.

The position dependent 15N fractionation of nitrous oxide (N2O), which cannot be obtained from mass spectrometric analysis on molecular N2O itself, can be determined with high precision using isotope ratio mass spectrometry on the NO+ fragment that is formed on electron impact in the source of an isotope ratio mass spectrometer. Laboratory UV photolysis experiments show that strong position dependent 15N fractionations occur in the photolysis of N2O in the stratosphere, its major atmospheric sink. Measurements on the isotopic composition of stratospheric N2O indeed confirm the presence of strong isotope enrichments, in particular the difference in the fractionation constants for 15N14NO and 14N15NO. The absolute magnitudes of the fractionation constants found in the stratosphere are much smaller, however, than those found in the lab experiments, demonstrating the importance of dynamical and also additional chemical processes like the reaction of N2O with O(1D).

Mass spectrometry of the intramolecular nitrogen isotope distribution of environmental nitrous oxide using fragment-ion analysis

Mass spectrometry is applied to measure the intramolecular distribution of (15)N in N(2)O samples of near natural isotopic composition. The method is relatively straightforward and based on the analysis of the (14)NO and (15)NO fragment ion beams at mass 30 and 31, respectively, in combination with the standard analysis of the masses 44, 45, and 46 of the non-fragmented N(2)O. Various complications in the application, not all of which are resolved at present, are discussed. Copyright 1999 John Wiley & Sons, Ltd.

Mass-independent oxygen isotope fractionation in atmospheric CO as a result of the reaction CO + OH

Atmospheric carbon monoxide (CO) exhibits mass-independent fractionation in the oxygen isotopes. An 17O excess up to 7.5 per mil was observed in summer at high northern latitudes. The major source of this puzzling fractionation in this important trace gas is its dominant atmospheric removal reaction, CO + OH --> CO2 + H, in which the surviving CO gains excess 17O. The occurrence of mass-independent fractionation in the reaction of CO with OH raises fundamental questions about kinetic processes. At the same time the effect is a useful marker for the degree to which CO in the atmosphere has been reacting with OH.

Russian doll type cryogenic traps: improved design and isotope separation effects.

An improved cryogenic trap for removing micromole quantities of condensables from gas mixtures is described. It is based on the so-called Russian doll design, with which extraordinary trapping efficiencies at flow rates of up to 10 L/min are obtained. The active element consists of one or more nested glass fiber thimbles. Despite the large fiber area, quantitative retrieval of condensed CO(2) is obtained. The new design is demountable and incorporates a heat exchange section; trapping efficiency depends little on the level of coolant, as has been tested for CO(2). When fractionation is induced by incomplete trapping, a small isotopic enrichment in (13)C occurs. It also has been discovered that when He is used as carrier gas, a gas chromatographic effect for CO occurs in Russian doll traps, accompanied by a large isotope separation, in which the elution sequence corresponds to the vapor pressure ratios of the isotopomers.

Experiental evaluation of brain spect using d, 1 - HMPAO 99m Tc in dogs. Anatomo-physiological implications

O d, 1-Hexametilpropileno oxima marcado com o tecnécio-99m (potência 99m Tc-HMPAO) é um importante radiofármaco usado tanto para imagens tomográficas cerebrais quanto na marcaçäo "in vitro" de leucócitos. A crescente utilizaçäo deste radiofármaco no estudo de doenças neurológicas no homem levou-nos à obtençäo de "kits" de HMPAO para ser marcado pelo tecnécio-99m. Neste trabalho apresentamos o estudo experimental efetuado com "kits" liofilizados de d, 1-HMPAO elevado à 99mTc desenvolvidos em nosos laboratórios, comparando inclusive com aqueles encontrados no comércio. As imagens cerebrais inicialmente foram feitas em coelhos, entretanto os melhores resultados foram obtidos em cäes. Assim, selecionaram-se seis cäes sem raça definida, considerados clinicamente normais para obtençäo das imagens cintilográficas do cérebro e o "washout" das regiöes de interesse (ROIs). Os estudos experimentais em animais e a considerável reduçäo do custo do radiofármaco mostrou-nos a possibilidade da utilizaçäo destes exames na clínica veterinária, especialmente de pequenos animais. Por outro lado, a perfusäo do d, 1-HMPAO elevado à 99mTc mostra que a concentraçäo do radiofármaco se faz ao mesmo tempo na regiäo do cérebro e nas fossas nasais, demonstrando a existência de estruturas iguais às da barreira hemo-encefálica nesta última regiäo