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Measuring Mole Fraction of Atmospheric Greenhouse and Trace Gases

Mole fraction determination is pervasive in measurements used in bottom-up and top-down quantification both for greenhouse gases and other atmospheric trace gases. Two approaches are used, either direct in situ observation or determination via samples collected in the field and measured in a laboratory. The latter is accomplished with a variety of well-known laboratory analytical methods involving gas chromatography and mass spectrometry' and the combination of the two. Some field applications use research-grade instruments that har e been modified for field usage, particularly for quantification of the very low atmospheric concentrations of the fluorinated gases (MIT, 2019; Prinn, 2018). Various methods based on infrared spectroscopy have been developed and widely used in both laboratory and field applications.

Infrared spectroscopy

Molecular spectroscopy involves the study of the motions of the atoms comprising molecules. It provides a means of identifying and measuring mole fraction of molecular species in gas mixtures such as the atmosphere. Internal molecular motions, and their resulting spectra, are well-described by a branch of quantum mechanics developed and verified over much of the 20tli century and with spectroscopic research continuing today. Spectroscopic properties of molecules are rooted in the number and kind of atoms comprising a molecule. Motions of these atoms, rotation and vibration (bending and stretching motions) about the molecule’s center of mass, give rise to quantized energy levels comprising absorption bands that occur primarily across the infrared (IR) region of the optical spectrum. Absorption of a photon whose frequency is the same as that of an energy level raises the total energy of the molecule resulting in change of rotational or vibrational motion. The molecule is then said to be in an excited state above that it normally occupies. It will revert to its original state, its ground state, upon emission of a photon of similar energy or through collisions with nearby molecules.

Molecules having different constituent atoms have spectroscopic absorption bands that cover different wavelength regions. Internal chemical bonds between atoms har e characteristic spectra depending mainly on the mass values of a molecule’s atoms. For example, the three atom molecules of water vapor (H,0) and CO, have different chemical bond strengths, resulting in absoiptiou bands occurring differently in wavelength regions of the IR. Bands characteristic of different molecules may or may not overlap in wavelength. An example of a CO, band structure in the 1.56 to 1.59 micrometer (pm) wavelength region is shown in Figure 7. The data are from the widely-used, high-resolution transmission molecular absorption database, HITRAN (HITRAN. 2016; Gordon, 2017). The 1.57 pm CO, baud is one of several CO, IR bands. Others occur near 2.1 pm, 4.23 pm and at longer wavelengths in the thermal IR, each with differing structure and absoiptiou strength. At the longer wavelengths, line strength increases significantly. As discussed below, measuring instrument design strategies utilize infrared absorption bauds, or some portion of them, as a means of determining mole fraction values. Depending upon factors such as instrument target performance accuracy and range, application, cost, and available technology, absoiptiou strength, either of a band or single line within a band, may also be a design factor.

Energy absorbed by a GHG molecule in a collision with a photon whose energy corresponds to that of one of the molecule's absorption band transitions changes its motion state to a higher energy level. Depending upon photon energy and details of the absoiptiou process, this energy is partitioned between the rotational and vibrational states of the GHG molecule. It sheds excess energy through collisions with nearby molecules in the atmosphere. This quickly transfers energy to the collision partner allowing the GHG molecule to return to its pre-absorption state. Since oxygen or nitrogen molecules make up 99% of the atmosphere and neither har e energy level strtictrues that allow them to accept the GHG molecule’s excess energy as rotational or vibrational motions, the added energy becomes kinetic energy or heat. By rapidly absorbing thermal energy radiated from the earth and transferring that energy to the main

Tlie 1.57 gm CO, absoiptiou band and smgle line near 1.5796 gin

Figure 7. Tlie 1.57 gm CO, absoiptiou band and smgle line near 1.5796 gin.

atmospheric constituents via the molecular collisions continuously occurring in the atmosphere, GHG molecules are highly efficient in raising the atmosphere’s temperature.

 
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