142 research outputs found
Numerical simulations of homogeneous freezing processes in the aerosol chamber AIDA
The homogeneous freezing of supercooled H<sub>2</sub>SO<sub>4</sub>/H<sub>2</sub>O aerosols in an aerosol chamber is investigated with a microphysical box model using the activity parameterization of the nucleation rate by Koop et al. (2000). The simulations are constrained by measurements of pressure, temperature, total water mixing ratio, and the initial aerosol size distribution, described in a companion paper Möhler et al. (2003). Model results are compared to measurements conducted in the temperature range between 194 and 235 K, with cooling rates in the range between 0.5 and 2.6 K min<sup>-1</sup>, and at air pressures between 170 and 1000 hPa. The simulations focus on the time history of relative humidity with respect to ice, aerosol size distribution, partitioning of water between gas and particle phase, onset times of freezing, freezing threshold relative humidities, aerosol chemical composition at the onset of freezing, and the number of nucleated ice crystals. The latter four parameters can be inferred from the experiments, the former three aid in interpreting the measurements. Sensitivity studies are carried out to address the relative importance of uncertainties of basic quantities such as temperature, total H<sub>2</sub>O mixing ratio, aerosol size spectrum, and deposition coefficient of H<sub>2</sub>O molecules on ice. The ability of the numerical simulations to provide detailed explanations of the observations greatly increases confidence in attempts to model this process under real atmospheric conditions, for instance with regard to the formation of cirrus clouds or polar stratospheric ice clouds, provided that accurate temperature and humidity measurements are available
Construction and Performance of a Micro-Pattern Stereo Detector with Two Gas Electron Multipliers
The construction of a micro-pattern gas detector of dimensions 40x10 cm**2 is
described. Two gas electron multiplier foils (GEM) provide the internal
amplification stages. A two-layer readout structure was used, manufactured in
the same technology as the GEM foils. The strips of each layer cross at an
effective crossing angle of 6.7 degrees and have a 406 um pitch. The
performance of the detector has been evaluated in a muon beam at CERN using a
silicon telescope as reference system. The position resolutions of two
orthogonal coordinates are measured to be 50 um and 1 mm, respectively. The
muon detection efficiency for two-dimensional space points reaches 96%.Comment: 21 pages, 17 figure
Head-mounted Sensory Augmentation Device: Comparing Haptic and Audio Modality
This paper investigates and compares the effectiveness of haptic and audio modality for navigation in low visibility environment using a sensory augmentation device. A second generation head-mounted vibrotactile interface as a sensory augmentation prototype was developed to help users to navigate in such environments. In our experiment, a subject navigates along a wall relying on the haptic or audio feedbacks as navigation commands. Haptic/audio feedback is presented to the subjects according to the information measured from the walls to a set of 12 ultrasound sensors placed around a helmet and a classification algorithm by using multilayer perceptron neural network. Results showed the haptic modality leads to significantly lower route deviation in navigation compared to auditory feedback. Furthermore, the NASA TLX questionnaire showed that subjects reported lower cognitive workload with haptic modality although both modalities were able to navigate the users along the wall
Experimental investigation of homogeneous freezing of sulphuric acid particles in the aerosol chamber AIDA
The homogeneous freezing of supercooled H<sub>2</sub>SO<sub>4</sub>/H<sub>2</sub>O solution droplets was investigated in the aerosol chamber AIDA (Aerosol Interactions and Dynamics in the Atmosphere) of Forschungszentrum Karlsruhe. 24 freezing experiments were performed at temperatures between 189 and 235 K with aerosol particles in the diameter range 0.05 to 1 ”m. Individual experiments started at homogeneous temperatures and ice saturation ratios between 0.9 and 0.95. Cloud cooling rates up to -2.8 K min<sup>-1</sup> were simulated dynamically in the chamber by expansion cooling using a mechanical pump. Depending on the cooling rate and starting temperature, freezing threshold relative humidities were exceeded after expansion time periods between about 1 and 10 min. The onset of ice formation was measured with three independent methods showing good agreement among each other. Ice saturation ratios measured at the onset of ice formation increased from about 1.4 at 231 K to about 1.75 at 189 K. The experimental data set including thermodynamic parameters as well as physical and chemical aerosol analysis provides a good basis for microphysical model applications
Observation of microwave induced resistance and photovoltage oscillations in MgZnO/ZnO heterostructures
Microwave induced resistance and photovoltage oscillations were investigated in Mg_xZn_(1âx)O/ZnO heterostructures. The physics of these oscillations is controlled significantly by scattering mechanisms, and therefore these experiments were motivated by the recently achieved high quality levels in this material and the apparent dominance of large angle, short-range scattering, which is distinct from the prevailing small angle scattering in state-of-the-art GaAs structures. Within the studied frequency range of 35â120 GHz, up to four oscillations were resolved at 1.4 K temperature, but only in high density samples. This allowed us to extract the value of the effective electron mass m^â = (0.35 ± 0.01)mâ, which is enhanced over the bare band mass, and estimate a local quantum scattering time of about 5 ps
Observation of microwave induced resistance and photovoltage oscillations in MgZnO/ZnO heterostructures
Microwave induced resistance and photovoltage oscillations were investigated in Mg_xZn_(1âx)O/ZnO heterostructures. The physics of these oscillations is controlled significantly by scattering mechanisms, and therefore these experiments were motivated by the recently achieved high quality levels in this material and the apparent dominance of large angle, short-range scattering, which is distinct from the prevailing small angle scattering in state-of-the-art GaAs structures. Within the studied frequency range of 35â120 GHz, up to four oscillations were resolved at 1.4 K temperature, but only in high density samples. This allowed us to extract the value of the effective electron mass m^â = (0.35 ± 0.01)mâ, which is enhanced over the bare band mass, and estimate a local quantum scattering time of about 5 ps
Bounding the role of black carbon in the climate system: A scientific assessment
Black carbon aerosol plays a unique and important role in Earth's climate system. Black carbon is a type of carbonaceous material with a unique combination of physical properties. This assessment provides an evaluation of blackâcarbon climate forcing that is comprehensive in its inclusion of all known and relevant processes and that is quantitative in providing best estimates and uncertainties of the main forcing terms: direct solar absorption; influence on liquid, mixed phase, and ice clouds; and deposition on snow and ice. These effects are calculated with climate models, but when possible, they are evaluated with both microphysical measurements and field observations. Predominant sources are combustion related, namely, fossil fuels for transportation, solid fuels for industrial and residential uses, and open burning of biomass. Total global emissions of black carbon using bottomâup inventory methods are 7500 Gg yr â1 in the year 2000 with an uncertainty range of 2000 to 29000. However, global atmospheric absorption attributable to black carbon is too low in many models and should be increased by a factor of almost 3. After this scaling, the best estimate for the industrialâera (1750 to 2005) direct radiative forcing of atmospheric black carbon is +0.71 W m â2 with 90% uncertainty bounds of (+0.08, +1.27) W m â2 . Total direct forcing by all black carbon sources, without subtracting the preindustrial background, is estimated as +0.88 (+0.17, +1.48) W m â2 . Direct radiative forcing alone does not capture important rapid adjustment mechanisms. A framework is described and used for quantifying climate forcings, including rapid adjustments. The best estimate of industrialâera climate forcing of black carbon through all forcing mechanisms, including clouds and cryosphere forcing, is +1.1 W m â2 with 90% uncertainty bounds of +0.17 to +2.1 W m â2 . Thus, there is a very high probability that black carbon emissions, independent of coâemitted species, have a positive forcing and warm the climate. We estimate that black carbon, with a total climate forcing of +1.1 W m â2 , is the second most important human emission in terms of its climate forcing in the presentâday atmosphere; only carbon dioxide is estimated to have a greater forcing. Sources that emit black carbon also emit other shortâlived species that may either cool or warm climate. Climate forcings from coâemitted species are estimated and used in the framework described herein. When the principal effects of shortâlived coâemissions, including cooling agents such as sulfur dioxide, are included in net forcing, energyârelated sources (fossil fuel and biofuel) have an industrialâera climate forcing of +0.22 (â0.50 to +1.08) W m â2 during the first year after emission. For a few of these sources, such as diesel engines and possibly residential biofuels, warming is strong enough that eliminating all shortâlived emissions from these sources would reduce net climate forcing (i.e., produce cooling). When open burning emissions, which emit high levels of organic matter, are included in the total, the best estimate of net industrialâera climate forcing by all shortâlived species from blackâcarbonârich sources becomes slightly negative (â0.06 W m â2 with 90% uncertainty bounds of â1.45 to +1.29 W m â2 ). The uncertainties in net climate forcing from blackâcarbonârich sources are substantial, largely due to lack of knowledge about cloud interactions with both black carbon and coâemitted organic carbon. In prioritizing potential blackâcarbon mitigation actions, nonâscience factors, such as technical feasibility, costs, policy design, and implementation feasibility play important roles. The major sources of black carbon are presently in different stages with regard to the feasibility for nearâterm mitigation. This assessment, by evaluating the large number and complexity of the associated physical and radiative processes in blackâcarbon climate forcing, sets a baseline from which to improve future climate forcing estimates.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/99106/1/jgrd50171.pd
The observation of nitric acid-containing particles in the tropical lower stratosphere
Airborne in situ measurements over the eastern Pacific Ocean in January 2004 have revealed a new category of nitric acid (HNO<sub>3</sub>)-containing particles in the tropical lower stratosphere. These particles are most likely composed of nitric acid trihydrate (NAT). They were intermittently observed in a narrow layer above the tropopause (18±0.1 km) and over a broad geographic extent (>1100 km). In contrast to the background liquid sulfate aerosol, these particles are solid, much larger (1.7-4.7 µm vs. 0.1µm in diameter), and significantly less abundant (<10<sup>-4</sup> cm<sup>-3</sup> vs. 10 cm<sup>-3</sup>). Microphysical trajectory models suggest that the NAT particles grow over a 6-14 day period in supersaturated air that remains close to the tropical tropopause and might be a common feature in the tropics. The small number density of these particles implies a highly selective or slow nucleation process. Understanding the formation of solid NAT particles in the tropics could improve our understanding of stratospheric nucleation processes and, therefore, dehydration and denitrification
Aviation Fuel Tracer Simulation: Model Intercomparison and Implications
An upper limit for aircraft-produced perturbations to aerosols and gaseous exhaust products in the upper troposphere and lower stratosphere (UT/LS) is derived using the 1992 aviation fuel tracer simulation performed by eleven global atmospheric models. Key Endings are that subsonic aircraft emissions: (1) have not be responsible for the observed water vapor trends at 40 deg N; (2) could be a significant source of soot mass near 12 km, but not at 20 km; (3) might cause a noticeable increase in the background sulfate aerosol surface area and number densities (but not mass density) near the northern mid-latitude tropopause; and (4) could provide a global, annual mean top of the atmosphere radiative forcing up to +0.006 W/sq m and -0.013 W/sq m due to emitted soot and sulfur, respectively
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