Although I consider myself a “climate change” skeptic – at least so far as the phrase is interpreted today – I do believe in making accurate measurements: “Science” is never … settled.
In my career, many of my projects involved some form of “environmental monitoring (which led to my PhD in geophysics rather than electronics). My most recent projects were supported by NASA, one of which involved measurements of CO2 by means of laser absorption from an aircraft based instrument.
This discussion gets into some of the aspects of the environment from within which measurements are made.
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Atmospheric CO2 – Introduction
An example of one reason I chose geophysics for my PhD rather than electronics is demonstrated in the following discussion. The work early in my career focused my attention towards what I call “data acquisition” – how do I acquire the information of interest? Not only was I interested in the “how” of taking measurements,…
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Atmospheric CO2 – Abstract
It is useful to have a “best-case” expectation of measurement parameters and uncertainties in order to optimize instrumentation methods and limiting parameters. While the proposed method is well established, it is possible to refine the measurement instrumentation to optimize the data integrity when the experimental bounds are established. The uncertainties related to beam divergence angle…
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Atmospheric CO2 – Beer-Lambert & Optical Depth
Expression Development: Beer-Lambert Law* Consider a section of optical path having length dz as shown below. The photon flux is considered uniformly distributed and cylindrical in nature. The flux is normal to the disk and the media is homogeneous. A differential illustration of the optical path geometry is shown The cylinder has volume Adz and…
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Atmospheric CO2 – Geometrical Construction
While many approaches to collecting the desired information of interest are under consideration, the structure of the measurement environment is identical for each. The basic approach is that a small diameter laser beam of the desired wavelength(s) is transmitted normal to the reflecting surface (ground). The beam is reflected back and is gathered by a…
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Atmospheric CO2 – Atmosphere Elements
Density is a measure of the number of particles per volume. Recall that one form of the Beer-Lambert defines attenuation constant as the product of absorption or scattering cross-section of each particle and the number of particles per volume … particle density. A uniform particle distribution can not be assumed along a vertical atmospheric optical…
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Atmospheric CO2 – Atmospheric Carbon Dioxide
This discussion of atmosphere is based on the idealized Standard Atmosphere Model parameters. Adding physical variations to this ideal representation will not decrease the uncertainty of measurement. Both density and temperature are of interest. According to data from the Standard Atmosphere model, the atmospheric density for altitude up to 10 km is plotted below. The…
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Atmospheric CO2 – Absorption Cross-Section
The effective absorption cross-section of the species of interest: CO2 is very highly dependent on wavelength as shown in the following figures. Carbon Dioxide alone Including major atmospheric gas constituents: the only significant gas in this vicinity is carbon monoxide. Among the primary atmospheric gases of interest, only CO2 and CO have significant spectral responses[1]…
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ORIG – Atmospheric CO2 – Attenuation Elements
Particles that are transparent to the beam are not of concern here. This leaves only attenuation elements – those elements that cause a loss of photons from the beam regardless of the cause of loss. Recall that the laser transmits a constant number of photons per second; the receiver converts the number of returning photon…
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Atmospheric CO2 – Plots
All plots use parameters as listed unless noted [1]: CO2 = 380 CO2 = 1.4e-23 HCO2 = 5552To = 288.15 T[z] = To – 650e-6 z H is altitude (10 km) ; z is length of total path: z = 2 h 1) Optical path volume dependency: beam divergence ; lens radius r2 for…
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Atmospheric CO2 – Basic Reflectivity & Range
This discussion is based on a relative “best-case” experiment – expected variations if everything is “ideal”. A worst-case analysis obviously has a different objective. One of the major disruptions in this analysis is the effect of the reflecting surface. Even considering atmospheric effects such as variations in scattering elements, the most significant loss of beam…
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Atmospheric CO2 – Photodiodes & Signal Range
A receiving photodiode operates in the same manner as any reverse-biased diode (pn-junction) except for being optimized for such operation. The diode is used in the “OFF” state and only very small leakage current flows. This leakage current is called dark current in a photodiode; it is the current which flows absent any optical stimulus…
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Atmospheric CO2 – Avalanche Photodiodes (APD)
Photodiode Operation The photodiode detector provides the conversion from optical power to electrical current; it is this electrical current which is eventually turned into “numbers” to be analyzed. Consider the basic pn-junction (diode). An electric field establishes a depletion region in which no free carriers exist. When a photon of the correct energy correctly impacts…
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Atmospheric CO2 – References
from Optical Depth GeometryB – May2014.docx Abshire, J. B., Riris, H., Allan, G. R., Weaver, C. J., Mao, J., Sun, X., Hasselbrack, W. E., Kawa, S. R. and Biraud, S., “Pulsed airborne lidar measurements of atmospheric CO2 column absorption.“ Tellus B, (2010). Abshire J, Riris H, Allan G, Weaver C, Mao J, Sun X, Hasselbrack…
