23 research outputs found
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Processes contributing to cloud dissipation and formation events on the North Slope of Alaska
Clear-sky periods across the high latitudes have profound impacts on the surface energy budget and lower atmospheric stratification; however an understanding of the atmospheric processes leading to low-level cloud dissipation and formation events is limited. A method to identify clear periods at Utqiaġvik (formerly Barrow), Alaska, during a 5-year period (2014–2018) is developed. A suite of remote sensing and in situ measurements from the high-latitude observatory are analyzed; we focus on comparing and contrasting atmospheric properties during low-level (below 2 km) cloud dissipation and formation events to understand the processes controlling clear-sky periods. Vertical profiles of lidar backscatter suggest that aerosol presence across the lower atmosphere is relatively invariant during the periods bookending clear conditions, which suggests that a sparsity of aerosol is not frequently a cause for cloud dissipation on the North Slope of Alaska. Further, meteorological analysis indicates two active processes ongoing that appear to support the formation of low clouds after a clear-sky period: namely, horizontal advection, which was dominant in winter and early spring, and quiescent air mass modification, which was dominant in the summer. During summer, the dominant mode of cloud formation is a low cloud or fog layer developing near the surface. This low cloud formation is driven largely by air mass modification under relatively quiescent synoptic conditions. Near-surface aerosol particles concentrations changed by a factor of 2 around summer formation events. Thermodynamic adjustment and increased aerosol presence under quiescent atmospheric conditions are hypothesized as important mechanisms for fog formation.
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Assessing the vertical structure of Arctic aerosols using balloon-borne measurements
The rapidly warming Arctic is sensitive to perturbations in the surface energy budget, which can be caused by clouds and aerosols. However, the interactions between clouds and aerosols are poorly quantified in the Arctic, in part due to (1) limited observations of vertical structure of aerosols relative to clouds and (2) ground-based observations often being inadequate for assessing aerosol impacts on cloud formation in the characteristically stratified Arctic atmosphere. Here, we present a novel evaluation of Arctic aerosol vertical distributions using almost 3 years' worth of tethered balloon system (TBS) measurements spanning multiple seasons. The TBS was deployed at the U.S. Department of Energy Atmospheric Radiation Measurement Program's facility at Oliktok Point, Alaska. Aerosols were examined in tandem with atmospheric stability and ground-based remote sensing of cloud macrophysical properties to specifically address the representativeness of near-surface aerosols to those at cloud base. Based on a statistical analysis of the TBS profiles, ground-based aerosol number concentrations were unequal to those at cloud base 86 % of the time. Intermittent aerosol layers were observed 63 % of the time due to poorly mixed below-cloud environments, mostly found in the spring, causing a decoupling of the surface from the cloud layer. A uniform distribution of aerosol below cloud was observed only 14 % of the time due to a well-mixed below-cloud environment, mostly during the fall. The equivalent potential temperature profiles of the below-cloud environment reflected the aerosol profile 89 % of the time, whereby a mixed or stratified below-cloud environment was observed during a uniform or layered aerosol profile, respectively. In general, a combination of aerosol sources, thermodynamic structure, and wet removal processes from clouds and precipitation likely played a key role in establishing observed aerosol vertical structures. Results such as these could be used to improve future parameterizations of aerosols and their impacts on Arctic cloud formation and radiative properties.
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The Pilatus unmanned aircraft system for lower atmospheric research
This paper presents details of the University of Colorado (CU) “Pilatus” unmanned research aircraft, assembled to provide measurements of aerosols, radiation and thermodynamics in the lower troposphere. This aircraft has a wingspan of 3.2 m and a maximum take-off weight of 25 kg, and it is powered by an electric motor to reduce engine exhaust and concerns about carburetor icing. It carries instrumentation to make measurements of broadband up- and downwelling shortwave and longwave radiation, aerosol particle size distribution, atmospheric temperature, relative humidity and pressure and to collect video of flights for subsequent analysis of atmospheric conditions during flight. In order to make the shortwave radiation measurements, care was taken to carefully position a high-quality compact inertial measurement unit (IMU) and characterize the attitude of the aircraft and its orientation to the upward-looking radiation sensor. Using measurements from both of these sensors, a correction is applied to the raw radiometer measurements to correct for aircraft attitude and sensor tilt relative to the sun. The data acquisition system was designed from scratch based on a set of key driving requirements to accommodate the variety of sensors deployed. Initial test flights completed in Colorado provide promising results with measurements from the radiation sensors agreeing with those from a nearby surface site. Additionally, estimates of surface albedo from onboard sensors were consistent with local surface conditions, including melting snow and bright runway surface. Aerosol size distributions collected are internally consistent and have previously been shown to agree well with larger, surface-based instrumentation. Finally the atmospheric state measurements evolve as expected, with the near-surface atmosphere warming over time as the day goes on, and the atmospheric relative humidity decreasing with increased temperature. No directional bias on measured temperature, as might be expected due to uneven heating of the sensor housing over the course of a racetrack pattern, was detected. The results from these flights indicate that the CU Pilatus platform is capable of performing research-grade lower tropospheric measurement missions
Fundamental optical processes in armchair carbon nanotubes
Single-wall carbon nanotubes provide ideal model one-dimensional (1-D) condensed matter systems in
which to address fundamental questions in many-body physics, while, at the same time, they are
leading candidates for building blocks in nanoscale optoelectronic circuits. Much attention has been
recently paid to their optical properties, arising from 1-D excitons and phonons, which have been
revealed via photoluminescence, Raman scattering, and ultrafast optical spectroscopy of semiconducting
carbon nanotubes. On the other hand, dynamical properties of metallic nanotubes have been poorly
explored, although they are expected to provide a novel setting for the study of electronヨhole pairs in
the presence of degenerate 1-D electrons. In particular, (n,n)-chirality, or armchair, metallic nanotubes
are truly gapless with massless carriers, ideally suited for dynamical studies of TomonagaヨLuttinger
liquids. Unfortunately, progress towards such studies has been slowed by the inherent problem of
nanotube synthesis whereby both semiconducting and metallic nanotubes are produced. Here, we use
post-synthesis separation methods based on density gradient ultracentrifugation and DNA-based ion-exchange chromatography to produce aqueous suspensions strongly enriched in armchair nanotubes.
Through resonant Raman spectroscopy of the radial breathing mode phonons, we provide macroscopic
and unambiguous evidence that density gradient ultracentrifugation can enrich ensemble samples in
armchair nanotubes. Furthermore, using conventional, optical absorption spectroscopy in the nearinfrared
and visible range, we show that interband absorption in armchair nanotubes is strongly
excitonic. Lastly, by examining the G-band mode in Raman spectra, we determine that observation of
the broad, lower frequency (G!) feature is a result of resonance with non-armchair “metallic”
nanotubes. These !ndings regarding the fundamental optical absorption and scattering processes in
metallic carbon nanotubes lay the foundation for further spectroscopic studies to probe many-body
physical phenomena in one dimension
Ramanstudien an einzelnen Nanoröhren und Nanoröhrenensemble - Schwingungseigenschaften und Streueffizienzen
Die vorgelegte Dissertationsschrift befasst sich mit den optischen Eigenschaften von Kohlenstoffnanoröhren. Kohlenstoffnanoröhren (KNR) sind Nanometer dicke hohle Zylinder, deren Wände aus Kohlenstoffatomen aufgebaut sind. Das große Interesse an KNRs beruht unter anderem auf den vielversprechenden elektronischen Eigenschaften wie etwa dem ballistischem Transport und einer großen Anzahl möglicher Bandlücken. Die Charakterisierung von KNRs geschieht meist mithilfe der Raman-Spektroskopie. So lässt sich anhand der radialen Atmungsmode (RAM) die atomare Struktur – gegeben durch die chiralen Indices (n1, n2) – der in einer Probe vorhanden Röhren bestimmen. Anhand der hochenergetischen Moden lässt sich überprüfen, ob eine Probe metallische Röhren enthält. Die Arbeit besteht aus zwei Teilen. Im ersten Teil wird die maximale Raman-Intensität der RAM für unterschiedliche (n1, n2) untersucht. Diese Intensitäten lassen Rückschlüsse auf die Elektron-Phonon-Kopplung zu, welche wiederum relevant für das Verständnis von Transporteigenschaften ist. Außerdem verspricht man sich, die Häufigkeiten der einzelnen (n1, n2) in einer Probe mithilfe der RAM-Intensitäten bestimmen zu können. Wir beobachten große Unterschiede zwischen den Raman-Intensitäten der verschiedenen (n1, n2), welche wir entweder auf Unterschiede der natürlichen Linienbreiten oder auf Variationen der Elektron-Phonon-Kopplung zurückführen können. Der zweite Teil der Arbeit behandelt die Form und den Ursprung der Moden im hochenergetischen Bereich des Raman-Spektrums von KNRs. Wie oben erwähnt, kann dieser Bereich des Spektrums zum Nachweis von metallischen Röhren in einer Probe benutzt werden. Entspricht die Anregungsenergie der Resonanzenergie einer metallischen Röhre, so verschiebt und verbreitert sich der sogenannte G− Peak. Hierzu gibt es zwei Theorien, welche das Phänomen auf sehr unterschiedliche physikalische Prozesse zurückführen. Gewöhnliche Nanoröhrenproben enthalten viele verschieden Röhrensorten, metallische sowie halbleitende, sodass sich die Raman-Signale überlagern und daher keine der genannten Theorien zu favorisieren ist. In den letzten Jahren sind zunehmend Experimente an scheinbar einzelnen Röhren durchgeführt worden, deren Interpretationen sich je nach zurate gezogener Theorie widersprachen. In dieser Arbeit werden Raman-Experimente an einem winzigen Bündel von einer metallischen und einer halbleitenden Nanoröhre präsentiert. Wir zeigen, dass der verbreiterte und verschobene Peak in metallischen Röhren dem durch eine sehr starke Elektron- Phonon-Kopplung verschobenen und verbreitert LO Phonon zuzuordnen ist. Die Beobachtung eines weiteren Peaks bei der unveränderten Frequenz des LO Phonons ist folglich nur durch die Anwesenheit einer halbleitenden Röhre zu erklären. Außerdem bestätigen unsere Messungen, dass der verschobene und verbreiterte Peak ein intrinsisches Merkmal von metallischen KNRs ist, er resultiert also nicht aus der Bündelung von Röhren. Des Weiteren können wir durch Raman-Experimente an Proben, die alle denkbaren (n1, n2) enthalten, die Durchmesserabhängigkeit der TO Phononen in halbleitenden Röhren zu kleineren Röhrendurchmessern erweitern.In this work we present Raman scattering experiments on ensembles of nanotubes in solution and individual suspended nanotubes. In the first part of this work we study the maximum Raman intensities for a large number of (n1, n2) including semiconducting and metallic nanotubes. We show that the strong differences between the RBM Raman intensities of the first and second optical transition of semiconducting nanotubes can be related to a larger broadening parameter of the second transition compared to the first transition. This is in accordance with a shorter live times of carriers excited into the second optical transition compared to carriers in the first transition. Also intensity differences between metallic and semiconducting nanotubes can be related to a stronger broadening of metallic transitions rather than differences in the electron-phonon coupling. On the other hand intensity variations as a function of nanotube family, chiral angle and diameter are related to variations in the Raman matrix elements, primarily the electron-phonon coupling. The dependence of the RBM intensity on the family and the chiral angle can be correlated to the position of the electronic transition with respect to the K point. We find a small maximum of the electron-phonon coupling for tubes with the transition close to K-Gamma symmetry line and a large maximum for tubes with the transition close to the K-M direction. Close to K-K line we find a minimum, where the electron-phonon coupling is close to zero. Due to the trigonal warping of the graphene bandstructure the minimum electron-phonon coupling is obtained for nanotubes with a chiral angle of ≈ 20° and with a family index of either ν = +1 or ν = −1 depending on the electronic transition. The dependence of the RBM Raman intensity on the nanotube diameter is related to three effects. First, with increasing diameter the broadening of the electronic transitions decreases which causes the intensity to rise. Second, with increasing diameter the RBM frequency decreases, which reduces the distance between incoming and outgoing resonance. This again causes the intensity to rise with diameter. The observed overall decrease of the intensities with diameter can therefore only be explained by a decrease of the electron phonon coupling. In the second part of this work we study the lineshape of the high-energy mode. From experiments on a tiny bundle of one metallic and one semiconducting nanotube we conclude that the broad and downshifted G− peak is related to the LO phonon in metallic nanotubes. We show that the observation of a sharp peak at 1590 cm−1 indicates the presence of an additional semiconducting nanotube. The peak at 1590 cm−1 in semiconducting nanotubes is again related to the LO phonon. The energy of the LO phonon in metallic nanotubes is broadened and downshifted in comparison to the LO in semiconducting nanotubes due to the effect of a Kohn anomaly. The fact that we see the downshifted and broadened G− peak of metallic nanotubes in the tiny bundle as well as in the nanotube separated in solution clarifies that this peak is an intrinsic feature of metallic carbon nanotubes. On nanotube ensembles dispersed in solution we observe several features in addition to the major G− and G+ features. We can assign a serious of peaks on the low energy side of the HEM to particular (n1, n2) or groups of nanotubes. This leads to a diameter dependence of this feature. By comparison to theoretical prediction we assign this peak to the TO phonon in semiconducting nanotubes. In order to strengthen this assignment the measurements should be extended to higher and lower excitation energies. Furthermore we find peaks on the high energy side of the HEM which we tentatively assign to the second-order Raman modes of the infrared-active phonon. To clarify this assignment a detailed study of this feature and the intermediate frequency modes is necessary. The frequency of the first-order phonon falls into this region and might be visible due to imperfections in the nanotubes
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Processes contributing to Arctic cloud dissipation and formation events that bookend clear sky periods
Abstract. The Arctic is predominantly cloudy with intermittent clear sky periods. These clear periods have profound impacts on the surface energy budget and lower atmospheric stratification, connected to a lack of downwelling longwave radiation in the absence of cloud. Despite the importance of clear sky conditions, an understanding of the atmospheric processes leading to low-level cloud dissipation and formation events is relatively limited. A strict definition to identify clear periods at Utqiagvik (formerly Barrow), Alaska, during a five-year period (2014–2018) is developed. A suite of remote sensing and in situ instrumentation from the high-latitude observatory are analysed; we focus on comparing and contrasting atmospheric properties during low-level cloud dissipation and formation events to understand the processes controlling clear sky periods. Vertical profiles of lidar backscatter suggest that aerosol presence across the lower atmosphere is relatively invariant around the clear period bookends, which suggests that a sparsity of aerosol is not frequently a cause for cloud dissipation. Further meteorological analysis indicates two active processes ongoing that appear to support the formation of low clouds after a clear sky period and have a link to surface aerosol concentrations; namely, horizontal advection which was dominant in winter and early spring and quiescent air mass modification which was dominant in the summer. During summer, the dominant mode of cloud formation is a low cloud/fog layer developing near the surface. This low cloud formation is driven largely by air mass modification and pooling of aerosol particles near the surface under lower-atmosphere stratification
Recommended from our members
Processes contributing to Arctic cloud dissipation and formation events that bookend clear sky periods
Abstract. The Arctic is predominantly cloudy with intermittent clear sky periods. These clear periods have profound impacts on the surface energy budget and lower atmospheric stratification, connected to a lack of downwelling longwave radiation in the absence of cloud. Despite the importance of clear sky conditions, an understanding of the atmospheric processes leading to low-level cloud dissipation and formation events is relatively limited. A strict definition to identify clear periods at Utqiagvik (formerly Barrow), Alaska, during a five-year period (2014–2018) is developed. A suite of remote sensing and in situ instrumentation from the high-latitude observatory are analysed; we focus on comparing and contrasting atmospheric properties during low-level cloud dissipation and formation events to understand the processes controlling clear sky periods. Vertical profiles of lidar backscatter suggest that aerosol presence across the lower atmosphere is relatively invariant around the clear period bookends, which suggests that a sparsity of aerosol is not frequently a cause for cloud dissipation. Further meteorological analysis indicates two active processes ongoing that appear to support the formation of low clouds after a clear sky period and have a link to surface aerosol concentrations; namely, horizontal advection which was dominant in winter and early spring and quiescent air mass modification which was dominant in the summer. During summer, the dominant mode of cloud formation is a low cloud/fog layer developing near the surface. This low cloud formation is driven largely by air mass modification and pooling of aerosol particles near the surface under lower-atmosphere stratification