Monitoring atmospheric trace gases

Since there are only few - and expensive - tunable lasers available for the infrared range, they are not popular for routine applications. However, there are instances where CO2 lasers (with a high efficiency) are used to excite emission spectra (Belz et al., 1987). Semiconductor lasers have been developed for monitoring atmospheric trace gases (Grisar et al., 1987). 

Nelson D. D., Zahniser M. S., McManus J. B., Shorter J. H., Wormhoudt J. C., Kolb C. E., Recent Improvements in atmospheric trace gas monitoring using mid-infrared tunable diode lasers, SPIE Proc., 2834, 148-159, 1996. 

Measurements either from the ground or from satellites have been a major contribution to this effort, and satellite instruments such as LIMS (Limb Infrared Monitor of the Stratosphere) on the Nimbus 7 satellite (I) in 1979 and ATMOS (Atmospheric Trace Molecular Spectroscopy instrument), a Fourier transform infrared spectrometer aboard Spacelab 3 (2) in 1987, have produced valuable data sets that still challenge our models. But these remote techniques are not always adequate for resolving photochemistry on the small scale, particularly in the lower stratosphere. In some cases, the altitude resolution provided by remote techniques has been insufficient to provide unambiguous concentrations of trace gas species at specific altitudes. Insufficient altitude resolution is a handicap particularly for those trace species with large gradients in either altitude or latitude. Often only the most abundant species can be measured. Many of the reactive trace gases, the key species in most chemical transformations, have small abundances that are difficult to detect accurately from remote platforms. 

Solid samples can be a bulk material such as a polymer or a raw material, they can be a surface coating such as varnish buildup on a piston or a particle on a semiconductor wafer. Liquids can be pure or as a solution or mixture. Gases can be pure or a mixture, such as in stack gases they may also be very dilute, down to mg 1 (parts per million) level for trace gas analysis such as for atmospheric monitoring or breathing gas for divers. In addition to these possibilities, it may be necessary to carry out the analysis at temperature and pressure conditions well removed from ambient. A final complication is that pure samples are rarely encountered much more common are mixtures, often with the material of interest present as the minor component. 

In trace gas analysis, it is required to monitor compounds, frequently organics, in atmosphere at low concentrations (parts per million (mol) or lower). Mass spectrometry offers versatile instrumentation for this, particularly when sensitivity and selectivity are enhanced by special inlet techniques or ionization processes. 

From the above it is clear that quantitative measurements at high sensitivities are most useful for a variety of small polar molecules which are of concern from the atmospheric environmental pollution point of view. Thus a substantial amount of effort has been and continues to be placed upon the development of field operable, portable microwave spectrometers for trace gas monitoring using both CW and FT instrumentation. Although there are likely to be continued applications of microwave spectroscopy to pure analysis problems in the future, it seems likely that the microwave spectrometer will continue to find its most exciting applications in the chemistry and physics research laboratory. 

Y. Kamata, A. Matsunami, K. Kitagawa, N. Arai, EFT analysis of atmospheric trace concentration of N2O continuously monitored by gas chromatography and cross-correlation to climate parameters. Microchem. J. 71, 83-93 (2002). doi 10.1016/S0026-265X(01)00140-0... 

The first published report of a PTR-MS instrument based on TOP-MS was by Blake et al. in 2004 [58] and the experimental arrangement is shown in Pigure 3.21. This instrument was constructed with the aim of monitoring complex trace VOC gas mixtures in the atmosphere, a task that is better suited to TOP-MS than QMS because the former collects data across... 

As part of ITOP, measurements were carried out onboard the flying laboratory Falcon-20 which was able to monitor aerosol concentration and gas traces north of Paris in July-August 2004. Synchronous measurements were made with ground-based aerosol lidar which made it possible to assess the vertical structure of the atmosphere. During these measurements aerosols were detected from Canada and Alaska having crossed the North Atlantic, where at this time there was biomass burning. The Falcon-20 s equipment enabled us to... 

The measurements just described constitute extra-car/extra-engine monitoring of combustion trace gases, and how they may affect the atmosphere into which they are released, in contrast to the experiments highlighted at the beginning of this section (e.g. in-cylinder measurements). It is quite obvious that not only the ambient atmosphere is affected, but also that exhaust gas recirculation introduces combustion product gases into the air... 

There are two ways to monitor the concentrations of trace gases in the atmosphere by FT-IR spectrometry. The first is to draw the atmosphere in the region of interest into a long-path gas cell, and the second is to measure the spectrum of the atmosphere in situ. The first approach, which is known as extractive monitoring [1], is covered in this section, and the second, known as open-path FT-IR spectrometry (OP/FT-IR) [2], is covered in Section 22.2. 

The exposure of a sample to the reductive or oxidative gas, in the linear heating regime. The detector monitors the consumption of this active gas. Subsequently, the sample is cooled and purged in the inert atmosphere, in order to remove the traces of active gas If necessary, the sample is exposed to oxidative (if the sample was previously reduced) or reductive gas (if the sample was previously oxidized). The detector monitors the consumption of this active gas... 

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