Atoms of Metals and LIDAR - Essay Example

?Atoms of Metals and LIDAR Introduction Every day, one hundred metric tons of meteor debris enters the Earth’s atmosphere – generally in the form of metallic sodium, iron, silicon, potassium, magnesium, and calcium particles less than 1mm in diameter. When combined with high temperature in the upper atmosphere, these particles vaporize and form the Atomic Metal Layers located 80 to 105 kilometres above the Earth’s surface (ATHENA Research and Innovation Centre n. pag.). The Sodium Layer was discovered in 1929 by Vesto Melvin Slipher. Since then, other layers have been discovered such as iron, potassium, and calcium (Keese n. pag). Majority of observations on atomic metal layers have been through ground-based LIDAR or Light Detection and Ranging instruments (ATHENA Research and Innovation Centre n. pag). LIDAR is a remote sensing technology that can measure distances or properties of a material by laser illumination (Sanderson n. pag.). Origin of metal atoms in the atmosphere Normally, meteoroids enter the Earth’s atmosphere between 11 to 72 kilometres per second. Friction between the meteor and air molecules in the upper atmosphere generates enough heat to disintegrate it to particles less than 1mm in diameter. These particles are deposited at an altitude of 70 to 140 km in the upper atmosphere (Von Zahn, Hoffner and McNeil, 149). McNeil, Lai and Murad acknowledged the fact that meteoric material is the most significant source of metal atoms in the upper atmosphere. However, the amount of metal atoms in the atmosphere differs from the amount present in the meteor itself. This is attributed to the process known as differential ablation. Differential ablation suggests that more volatile metals vaporize earlier than less volatile metal components in a meteorite. Comparison between sodium, magnesium, and calcium ablation revealed that sodium is the most volatile of the three elements. Thus, sodium ablates at a much higher altitude. On the other hand, calcium ablates at a much lower altitude. Therefore, the rate of conversion of elemental calcium into compounds in the lower atmosphere is the primary cause of calcium ion depletion (10899-10911). Aside from sodium, metallic atoms of potassium, lithium, calcium and iron make up the rest of the metal atom layer in the mesosphere. The discovery of other metal constituents started in 1973. Potassium ions detected through the use of ruby-laser-pumped dye laser components calibrated at 769.9 and 766.5 nanometres. Lithium ions were identified using a flashlamp-pumped dye laser calibrated at 670.8 nanometres with 800 millijoules output pulse energy. Calcium was detected with a dye laser calibrated at 422.7 nanometres (Abo 315). Thin layers of sodium, calcium and iron have been detected by LIDAR in the upper atmosphere. These layers range from 100 metres to several kilometres thick and usually superimpose on a background layer 10 kilometres thick. Despite efforts to ascertain the origins of sporadic layers, no single scientific explanation was accepted (Clemesha 725). Chemical role of metal atoms in the atmosphere The formation of metal layers in the upper atmosphere has been instrumental in several vital atmospheric processes. Rapp and Thomas have confirmed the role of mesospheric metal layers in the formation of noctilucent clouds through the nucleation of ice particles in the upper atmosphere (715-744). Murphy, Thomson and Mahoney investigated the composition of aerosol particles in the stratosphere and discovered the existence of meteoric particles in stratospheric aerosol. Thus a link was suggested between mesospheric metal particles and the condensation of stratospheric sulphate aerosols (1664-1669). Atmospheric observation over the South Pole was initiated through LIDAR to measure the seasonal variations of the mesospheric sodium and iron layer. The polar annual mean abundance is almost equal to mid-latitude readings while the mean centroid height is 100 metres higher for sodium and 450 metres higher for iron when compared to readings collected at 40?N. However, it has been noted that during midsummer, the sodium and iron layers located below 90 kilometres almost disappears from the readings. The measurements collected were analysed and compared with chemical models developed at the University of East Anglia. The results revealed that in order to prevent the sodium and iron layer below 85 kilometres from disappearing, mesospheric vertical downwelling in winter should be less than 1 cm s-1. Also, an additional source of gas-phase metallic species must be present during the winter months to maintain mesospheric sodium and iron abundances (Gardner et al. n. pag.) LIDAR: Its role in detecting presence and concentration of metal atoms LIDAR is capable of detecting distance, speed, rotation, and chemical composition and concentration of substances by illuminating the target with a light source such as laser. LIDAR has been put into use on several fields such as geography, forestry, astronomy, and atmospheric physics (Cracknell & Hayes, 74). It has been the most widely used instrument for the observation of atmospheric phenomena such as hurricanes, ozone depletion, and greenhouse gases. In addition, LIDAR has been instrumental in the discovery of metallic atoms in the mesosphere. Earlier experiments in measuring air density in the upper atmosphere utilized a continuous beam of light from searchlights. The development of LIDAR technology started during the invention of the laser in the 1960s. Further improvements in optical and electronic technology paved way for the development of lasers that are custom-design for LIDAR such as laser power, wavelength, pulse width, beam shape, and spectral unity. In addition, optical filters which feature high transmissivity, narrow bandwidth, etc. provided added range and versatility for advanced LIDAR systems. Finally, computer hardware development enabled LIDAR systems to process more data than before (Wandinger 1-6). The basic LIDAR system consists of a transmitter and receiver. Short light pulses are generated by the laser located in the transmitter. These pulses of light are beamed out into the atmosphere while a telescope in the receiver component detects it. The collected data is then routed to an optical analyzer which processes and filters out specific wavelengths and polarization states. The filtered data is then fed into an optical detector which converts the optical signal into an electronic signal. Finally, the electronic signal is stored in a computer for further analysis. To reduce divergence of the laser pulse, beam expanders are utilized to prevent background light from the atmosphere from interfering with the receiver telescope. This also causes the amount of photons that were produced through multiple scattering to go down, which in turn, improves detection rates. Moreover, a highly collimated beam ensures that signal detection with high spectral resolution is made possible. This is due to the small acceptance angles available for wavelength-selective devices. The primary telescope used in the receiver component of LIDAR measures from 0.1 to a few meters, which is dictated on the type of application the LIDAR will be utilized. Generally, LIDAR systems use mirror telescopes since lenses are compatible with small-aperture receivers. Photomultiplier tubes (PMTs) or avalanche photodiodes (APDs) are utilized for the detection of light pulses by counting photons generated by the laser. The rate at which the laser pulse is generated is usually set to intervals of a few seconds to minutes. This is done to improve computer processing efficiency and reduces the space needed for data storage (Wandinger 1-6). Fig. 1. Basic setup of a LIDAR system from Wandinger, U. LIDAR: Range Resolved Optical Remote Sensing of the Atmosphere. Ed. Claus Weitkamp. New York: Springer, 2005. Print. The basic operations involved in LIDAR involve the following steps. First, a pulse of light, usually a laser beam is emitted into the atmosphere. The laser pulse interacts with particles in the atmosphere which cause changes in the intensity, polarization, and wavelength of the backscattered light. The backscattered light is then measured and analyzed to determine the properties of the particles in the atmosphere. LIDAR can be used to generate a vertical profile of the atmosphere and its constituents by determining the distance of the scattering medium and calculating the time delay of the return signal (Veerabuthiran 33-34). By utilizing different types of lasers, a LIDAR system is able to operate on a broad range of wavelengths, from ultraviolet to infrared. Through the process of scattering, absorption, resonance, and fluorescence, LIDAR is able to calculate the composition and density of solid particulates such as aerosols, ozone, water vapour, and metal atoms such as sodium and iron. LIDAR may utilize Ruby, Nd:YAG, CO2, CO, and dye flash lamp pumped lasers depending on the atmospheric parameters to be studied (Veerabuthiran 34 & 37). LIDAR applications in the observation of atmospheric phenomena, specifically metallic atoms in the atmosphere, paved the way for the development of Resonance Fluorescence LIDAR (Wandinger 17). Resonance Fluorescence LIDAR is described as a type of LIDAR technology wherein the wavelength of the laser generated is matched to the absorption wavelength of a specific molecular species. The cross-section of the resonant backscatter generated is measured to determine the density of the molecular specie studied (Farlex n. pag.). Fig. 2. Metals and their resonant transitions used in LIDAR from Von Zahn, U., Hoffner, J. & McNeil, W.J. Meteors in the Earth’s Atmosphere. Eds. Edmond Murad & Iwan Prys Williams. Cambridge: Cambridge University Press, 2002. Print. The Purple Crow LIDAR or PCL at the University of Western Ontario features a sodium resonance-fluorescence LIDAR system which operates simultaneously with a Rayleigh and Raman-scatter LIDAR. The PCL is able to perform measurements on metallic sodium composition and concentration in the 83 to 100 kilometre range. In addition, the PCL can measure temperature profiles from the tropopause to the lower thermosphere (Argall, Vassiliev, Sica & Mwangi 2393-2400). The resonance LIDAR located at Gadanki, India utilizes a YAG pumped dye laser as transmitter calibrated to the sodium D2 line at 589.0 nm. The LIDAR system was set up to explore the sodium layer in the atmosphere between 80 to 110 kilometres. The system became operational in 2005 and provided the first sodium layer readings from India. The system made initial measurements on the mesospheric sodium layer, including concentration profiles. Analysis of the initial atmospheric data revealed sodium structures similar to gravity waves. Also, a significant variability of the sodium distribution in the mesosphere during sodium layer event was observed (Kumar, Rao, Murthy & Krishnaiah 86203-86213). Conclusion LIDARs have been the scientists’ most potent tool in the study of metal atoms in the atmosphere. However, there is still much work to be done in the field of atmospheric observations. For instance, due to a shortage of LIDAR sites, the worldwide distribution of metallic layers in the atmosphere remains undetermined. Another important issue that needs to be addressed is the need for more accurate models on differential ablation. Moreover, the link between metal layers in the atmosphere and the formation of noctilucent clouds is still unclear (ATHENA Research and Innovation Centre n.pag). Suggested improvements on LIDAR research on the upper atmosphere includes a proposed space-based LIDAR system designed to record and analyze temperature and horizontal wind readings. The system design also enables ascertain gravity wave characteristics, and heat and momentum wave flux. Furthermore, the space-based LIDAR will be able to determine the effects on the background atmosphere, formation of noctilinear clouds, and global distribution and variability of sodium atoms in the mesosphere (European Space Agency n. pag.). Future directions in LIDAR research include ground, rocket, and satellite observations, laboratory experiments, and improved theoretical models (Thomas 553-557). Works Cited Abo, M. Resonance Scattering LIDAR. Ed. Claus Weitkamp. New York: Springer, 2005. Print. Argall, P. S., Vassiliev, O. N., Sica, R. J. & Mwangi, M. M. “Lidar Measurements Taken With a Large-Aperture Liquid Mirror. 2. Sodium Resonance-Fluorescence System." Applied Optics 39 (2000): 2393-2400. Print. ATHENA Research and Innovation Centre. Metallic Layers, 2009. Web. 21 February 2011. Clemesha, B. R. “Sporadic Neutral Metal Layers in the Mesosphere and Lower Thermosphere.” Journal of Atmospheric and Terrestrial Physics 57.7 (1995): 725-735. Print. Cracknell, A. P. & Hayes, L. Introduction to Remote Sensing. London: Taylor and Francis, 2007. Print. European Space Agency. GLEME: Global LIDAR Exploration of the Mesosphere, 2011. Web. 21 February 2011. Farlex. Resonance Fluorescence LIDAR, 2011. Web. 21 February 2011. Gardner, C. S., Plane, J. M. C., Pan, W., Vondrak, T., Murray, B. J. & Chu, X. “Seasonal variations of the Na and Fe layers at the South Pole and their implications for the chemistry and general circulation of the polar mesosphere.” Journal of Geophysical Research 110 (2005). Print. Keese, R. G. Metallic Vapour Layers, 2010. Web. 21 February 2011. Kumar, Y. B., Rao, D. N., Murthy, M. S. & Krishnaiah, M. “Resonance LIDAR system for mesospheric sodium measurements.” Optical engineering: The journal of the Society of Photo-optical Instrumentation Engineers 46 (2007): 86203-86213. Print. McNeil, W. J., Lai, S. T. & Murad, E. “Differential ablation of cosmic dust and implications for the relative abundances of atmospheric metals.” Journal of Geophysical Research 103.D9 (1998): 10899-10911. Print. Murphy, D. M., Thomson, D. S. & Mahoney, M. J. “In Situ Measurements of Organics, Meteoric Material, Mercury, and Other Elements in Aerosols at 5 to 19 kilometres.” Science 282.5394 (1998): 1664-1669. Print. Rapp, M. & Thomas, G. E. “Modelling the Microphysics of Mesospheric Ice Particles: Assessment of Current Capabilities and Basic Sensitivities.” Journal of Atmospheric and Solar-Terrestrial Physics 68.7 (2005): 715-744. Print. Sanderson, R. Introduction to Remote Sensing, 2010. Web. 21 February 2011. Thomas, G. E. “Mesospheric Clouds and the Physics of the Mesopause Region.” Reviews of Geophysics 29.4 (1991): 553-575. Print. Veerabuthiran, S. “Exploring the Atmosphere with LIDAR.” Journal of Science Education 8.4 (2003): 33-43. Print. Von Zahn, U., Hoffner, J. & McNeil, W.J. Meteors in the Earth’s Atmosphere. Eds. Edmond Murad & Iwan Prys Williams. Cambridge: Cambridge University Press, 2002. Print. Wandinger, U. LIDAR: Range Resolved Optical Remote Sensing of the Atmosphere. Ed. Claus Weitkamp. New York: Springer, 2005. Print. Read More