| 1. |
- Fisher, Rebecca E., et al.
(författare)
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Measurement of the C-13 isotopic signature of methane emissions from northern European wetlands
- 2017
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Ingår i: Global Biogeochemical Cycles. - 0886-6236 .- 1944-9224. ; 31:3, s. 605-623
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Tidskriftsartikel (refereegranskat)abstract
- Isotopic data provide powerful constraints on regional and global methane emissions and their source profiles. However, inverse modeling of spatially resolved methane flux is currently constrained by a lack of information on the variability of source isotopic signatures. In this study, isotopic signatures of emissions in the Fennoscandian Arctic have been determined in chambers over wetland, in the air 0.3 to 3m above the wetland surface and by aircraft sampling from 100m above wetlands up to the stratosphere. Overall, the methane flux to atmosphere has a coherent delta C-13 isotopic signature of -71 +/- 1%, measured in situ on the ground in wetlands. This is in close agreement with delta C-13 isotopic signatures of local and regional methane increments measured by aircraft campaigns flying through air masses containing elevated methane mole fractions. In contrast, results from wetlands in Canadian boreal forest farther south gave isotopic signatures of -67 +/- 1%. Wetland emissions dominate the local methane source measured over the European Arctic in summer. Chamber measurements demonstrate a highly variable methane flux and isotopic signature, but the results from air sampling within wetland areas show that emissions mix rapidly immediately above the wetland surface and methane emissions reaching the wider atmosphere do indeed have strongly coherent C isotope signatures. The study suggests that for boreal wetlands (>60 degrees N) global and regional modeling can use an isotopic signature of -71 parts per thousand to apportion sources more accurately, but there is much need for further measurements over other wetlands regions to verify this.
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| 2. |
- Flynn, Michael J., et al.
(författare)
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Modelling cloud processing of aerosol during the ACE-2 HILLCLOUD experiment
- 2000
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Ingår i: Tellus. Series B: Chemical and Physical Meteorology. - : Stockholm University Press. - 0280-6509. ; 52:2, s. 779-800
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Tidskriftsartikel (refereegranskat)abstract
- A numerical model has been used to simulate the conditions observed during the ACE-2 Hillcloud experiment and to study the processes which may be taking place. The model incorporates gas phase chemistry of sulphur and nitrogen compounds upstream of the cloud, and the interaction of aerosol, precursor trace gases and oxidants within the cloud. Gas phase and aerosol inputs to the model have been provided from measurements made in the field. Dynamics of the air flow over the hill consisted of simple prescribed dynamics based on wind speed measurements, and also for some cases modelled dynamics. In this modelling study, it was found that during clean case studies particles down to 40-55 nm diameter were activated to form cloud droplets, the total number of droplets formed ranging from 200 to 400 drops/cm3. Significant modification of the aerosol spectra due to cloud processing was observed. In polluted cases particles down to 65-80 nm diameter were activated to form cloud droplets, the total number of droplets ranging from 800 to 2800 drops/cm3. Modification of the aerosol spectra due to cloud processing was slight. In all cases, changes in the aerosol spectra were due to both the uptake of HNO3, HCl, NH3 and SO2 from the gas phase, (the SO2 being oxidised to sulphate) and the repartitioning of species such as HNO3, HCl, and NH3 from larger particles onto smaller ones. Modelling results have been compared with observations made. Modelled droplet numbers are typically within 20% of the best measured values. The mode of the droplet distribution typically around 10-20 μm for clean cases and 4-8 μm for polluted cases was found to be in good agreement with the measured values of 10-25 μm for clean cases, but not in such good agreement for polluted cases. Measurements of upwind and interstitial aerosol distributions showed that the smallest particles activated were 30 and 50 nm for clean and polluted cases respectively, slightly smaller than the model values quoted above. Measured upwind and downwind aerosol spectra showed similar modification to that predicted by the model in eight out of the eleven model runs carried out. Chemistry measurements also give general evidence for both the uptake of species from the gas phase, and repartitioning of species from large particles onto smaller ones, though comparisons for individual cases are more difficult. From this modelling study, it can be concluded that in general, in the remote environment the exchange of hydrochloric acid, nitric acid and ammonia between aerosol particles and take up from the gas phase in the vicinity of cloud may be a very important mechanism in regulating the evolution of the aerosol spectrum. Further, the much more linear relationship between cloud droplet and accumulation mode aerosol number, which was observed in the measurements made during the ACE-2 HILLCLOUD project is supported by these modelling results. The implications of this for the indirect effect will be explored in future work.
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| 3. |
- Fox, Stuart, et al.
(författare)
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Airborne validation of radiative transfer modelling of ice clouds at millimetre and sub-millimetre wavelengths
- 2019
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Ingår i: Atmospheric Measurement Techniques. - : Copernicus GmbH. - 1867-1381 .- 1867-8548. ; 12:3, s. 1599-1617
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Tidskriftsartikel (refereegranskat)abstract
- The next generation of European polar orbiting weather satellites will carry a novel instrument, the Ice Cloud Imager (ICI), which uses passive observations between 183 and 664 GHz to make daily global observations of cloud ice. Successful use of these observations requires accurate modelling of cloud ice scattering, and this study uses airborne observations from two flights of the Facility for Airborne Atmospheric Measurements (FAAM) BAe 146 research aircraft to validate radiative transfer simulations of cirrus clouds at frequencies between 325 and 664 GHz using the Atmospheric Radiative Transfer Simulator (ARTS) and a state-of-the-art database of cloud ice optical properties. Particular care is taken to ensure that the inputs to the radiative transfer model are representative of the true atmospheric state by combining both remote-sensing and in situ observations of the same clouds to create realistic vertical profiles of cloud properties that are consistent with both observed particle size distributions and bulk ice mass. The simulations are compared to measurements from the International Submillimetre Airborne Radiometer (ISMAR), which is an airborne demonstrator for ICI. It is shown that whilst they are generally able to reproduce the observed cloud signals, for a given ice water path (IWP) there is considerable sensitivity to the cloud microphysics, including the distribution of ice mass within the cloud and the ice particle habit. Accurate retrievals from ICI will therefore require realistic representations of cloud microphysical properties.
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| 4. |
- Martinsson, Bengt G., et al.
(författare)
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Droplet nucleation and growth in orographic clouds in relation to the aerosol population
- 1999
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Ingår i: Atmospheric Research. - 0169-8095. ; 50:3-4, s. 289-315
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Tidskriftsartikel (refereegranskat)abstract
- The formation and development of orographic clouds was studied in a field experiment comprising several measurement sites at a mountain ridge. The influence of the aerosol population present on the cloud microstructure was studied in relation to the dynamics in the cloud formation. Droplet nucleation scavenging was investigated by the introduction of a non-dimensional particle diameter related to the process, and it was found that the scavenging rose rapidly in a relatively narrow particle size interval. The size dependency of the scavenging could partly be explained by external mixture of the aerosol. The large particles in the cloud interstitial aerosol was found to be of a chemical nature which allows for only a very weak uptake of water, implying that the chemical composition of these particles rather than entrainment of dry air prevented the droplet nucleation. The aerosol particle number concentration was found to strongly influence the cloud microstructure. Droplet number concentrations up to approximately 2000 cm-3 were observed together with a substantially reduced effective droplet diameter. The observed effect of elevated particle number concentrations in orographic clouds was generalised to the climatologically more important stratiform clouds by the use of a cloud model. It was found that the microstructure of stratiform clouds was strongly dependent on the aerosol population present as well on the dynamics in the cloud formation.
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| 5. |
- Martinsson, Bengt G., et al.
(författare)
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Validation of very high cloud droplet number concentrations in air masses transported thousands of kilometres over the ocean
- 2000
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Ingår i: Tellus. Series B: Chemical and Physical Meteorology. - 0280-6509. ; 52:2, s. 801-814
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Tidskriftsartikel (refereegranskat)abstract
- The microstructure of orographic clouds related to the aerosol present was studied during the second Aerosol Characterisation Experiment (ACE-2). Very high cloud droplet number concentrations (almost 3000 cm -3 ) were observed. These high concentrations occurred when clouds formed on a hill slope at Tenerife in polluted air masses originating in Europe that had transported the order of 1000 km over the Atlantic Ocean. The validity of the measured droplet number concentrations was investigated by comparing with measurements of the aerosol upstream of the cloud and cloud interstitial aerosol. Guided by distributions of the ratios between the measurements, three criteria of typically 30% in maximum deviation were applied to the measurements to test their validity. Agreement was found for 88% of the cases. The validated data set spans droplet number concentrations of 150-3000 cm -3 . The updraught velocity during the cloud formation was estimated to 2.2 m s -1 by model calculations, which is typical of cumuliform clouds. The results of the present study are discussed in relation to cloud droplet number concentrations previously reported in the literature. The importance of promoting the mechanistic understanding of the aerosol/cloud interaction and the use of validation procedures of cloud microphysical parameters is stressed in relation to the assessment of the indirect climatic effect of aerosols.
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| 6. |
- Middleton, Matthew J., et al.
(författare)
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Bright radio emission from an ultraluminous stellar-mass microquasar in M 31
- 2013
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Ingår i: Nature. - : Springer Science and Business Media LLC. - 0028-0836 .- 1476-4687. ; 493:7431, s. 187-190
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Tidskriftsartikel (refereegranskat)abstract
- A subset of ultraluminous X-ray sources (those with luminosities of less than 10(40) erg s(-1); ref. 1) are thought to be powered by the accretion of gas onto black holes with masses of similar to 5-20M(circle dot), probably by means of an accretion disk(2,3). The X-ray and radio emission are coupled in such Galactic sources; the radio emission originates in a relativistic jet thought to be launched from the innermost regions near the black hole(4,5), with the most powerful emission occurring when the rate of infalling matter approaches a theoretical maximum (the Eddington limit). Only four such maximal sources are known in the Milky Way(6), and the absorption of soft X-rays in the interstellar medium hinders the determination of the causal sequence of events that leads to the ejection of the jet. Here we report radio and X-ray observations of a bright new X-ray source in the nearby galaxy M 31, whose peak luminosity exceeded 10(39) erg s(-1). The radio luminosity is extremely high and shows variability on a timescale of tens of minutes, arguing that the source is highly compact and powered by accretion close to the Eddington limit onto a black hole of stellar mass. Continued radio and X-ray monitoring of such sources should reveal the causal relationship between the accretion flow and the powerful jet emission.
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| 7. |
- Svenningsson, Birgitta, et al.
(författare)
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Cloud droplet nucleation scavenging in relation to the size and hygroscopic behaviour of aerosol particles
- 1997
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Ingår i: Atmospheric Environment. - 1352-2310. ; 31:16, s. 2463-2475
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Tidskriftsartikel (refereegranskat)abstract
- The size distributions and hygroscopic growth spectra of aerosol particles were measured during the GCE cloud experiment at Great Dun Fell in the Pennine Hills in northern England. Hygroscopic growth is defined as the particle diameter at 90% RH divided by the particle diameter at 10% RH. The fraction of the aerosol particles scavenged by cloud droplets as a function of particle size was also measured. The general aerosol type was a mixture of marine and aged anthropogenic aerosols. The Aitken and accumulation mode numbers (average ± 1 S.D.) were 1543 ± 1078 and 1023 ± 682 cm-3, respectively. The mean diameters were in the range 30-100 nm and 100-330 nm. The hygroscopic growth spectra were bimodal about half the time. The less-hygroscopic particles had average growth factors of 1.06, 1.06; 1.03, 1.03, and 1.03 for particle diameters of 50, 75, 110, 165, and 265 nm, respectively. For the more-hygroscopic particles of the same sizes, the average hygroscopic growth was 1.34, 1.37, 1.43, 1.47, and 1.53. The effects of ageing on the aerosol particle size distribution and on hygroscopic behaviour are discussed. The scavenged fraction of aerosol particles was a strong function of particle diameter. The diameter with 50% scavenging was in the range 90-220 nm. No tail of smaller particles activated to cloud drops was observed. A small tail of larger particles that remained in the interstitial aerosol can be explained by there being a small fraction of less-hygroscopic particles. A weak correlation between the integral dry particle diameter and the diameter with 50% scavenging was seen.
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| 8. |
- Swietlicki, Erik, et al.
(författare)
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A closure study of sub-micrometer aerosol particle hygroscopic behaviour
- 1999
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Ingår i: Atmospheric Research. - 0169-8095. ; 50:3-4, s. 205-240
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Tidskriftsartikel (refereegranskat)abstract
- The hygroscopic properties of sub-micrometer aerosol particles were studied in connection with a ground-based cloud experiment at Great Dun Fell, in northern England in 1995. Hygroscopic diameter growth factors were measured with a Tandem Differential Mobility Analyser (TDMA) for dry particle diameters between 35 and 265 nm at one of the sites upwind of the orographic cloud. An external mixture consisting of three groups of particles, each with different hygroscopic properties, was observed. These particle groups were denoted less-hygroscopic, more-hygroscopic and sea spray particles and had average diameter growth factors of 1.11-1.15, 1.38-1.69 and 2.08-2.21 respectively when taken from a dry state to a relative humidity of 90%. Average growth factors increased with dry particle size. A bimodal hygroscopic behaviour was observed for 74-87% of the cases depending on particle size. Parallel measurements of dry sub-micrometer particle number size distributions were performed with a Differential Mobility Particle Sizer (DMPS). The inorganic ion aerosol composition was determined by means of ion chromatography analysis of samples collected with Berner-type low pressure cascade impactors at ambient conditions. The number of ions collected on each impactor stage was predicted from the size distribution and hygroscopic growth data by means of a model of hygroscopic behaviour assuming that only the inorganic substances interacted with the ambient water vapour. The predicted ion number concentration was compared with the actual number of all positive and negative ions collected on the various impactor stages. For the impactor stage which collected particles with aerodynamic diameters between 0.17-0.53 μm at ambient relative humidity, and for which all pertinent data was available for the hygroscopic closure study, the predicted ion concentrations agreed with the measured values within the combined measurement and model uncertainties for all cases but one. For this impactor sampling occasion, the predicted ion concentration was significantly higher than the measured. The air mass in which this sample was taken had undergone extensive photochemical activity which had probably produced hygroscopically active material other than inorganic ions, such as organic oxygenated substances.
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