Open-path atmospheric monitoring

The alternative technique to extractive monitoring is open-path FT-IR spectrometry. In this approach, no gas cell is used to contain the air sample. Instead, the beam is passed through a path (typically, between 100 and 400 m in length) that crosses the 

Inlerterometer Receiving Optics % / IR Path Fransmitling Optics 

The retroreflector for monostatic measurements could— in principle at least— be a plane mirror. However, plane mirrors are very difficult to align, and slight misalignment (due, e.g., to a gust of wind) causes the reflected beam to miss the 

One of the major experimental difficulties with OP/FT-IR measurements is the need to minimize the effect of water vapor and carbon dioxide on the spectra. With pathlengths of over 100 m, essentially all radiation between about 4000 and 3500 cm and 2000 and 1300 cm is totally absorbed by atmospheric water vapor, and the radiation between 2150 and 2500 cm and between 700 cm and the cutoff of the MCT detector is lost because of absorption by CO2. Thus, useful analytical information can be obtained firom OP/FT-IR spectra only in the three atmospheric windows (700 to 1300, 2000 to 2150, and 2500 to 3000cm ). Even in these atmospheric windows, however, water lines can still be observed in the spectrum. 

There have been extensive discussions between practitioners of OP/FT-IR spectrometry as to the optimum resolution at which to measure the spectra [15-20]. By measuring the spectrum at relatively high resolution (Av = 0.5 or 1 cm ), the absorption lines of water can appear like a picket fence, and the absorption spectrum of analytes can sometimes be seen in the regions where the water lines do not totally absorb. It is for this reason that the EPA recommend that OP/FT-IR spectra be measured with relatively high resolution (e.g., Av = 1 cm ). 

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P R. Griffiths, R. L. Richardson, D. Qin, and C. Zhu, Open-path atmospheric monitoring with a low-resolution FT-IR spectrometer, in Optical Sensing for Envimnmental and Process Monitoring, O. A. Simpson, Ed., Society of Photo-Optical Instrumentation Engineers, Bellingham, WA, 1995, Vol. 2365, p. 274. 

TO-16 No sampling—field method Atmospheric gases Long, open-path FTIR monitoring... 

Applications The differential optical absorption spectrometer has been used to monitor concentrations of gases or intermediate compounds such as SO, NO, O5, HCHO, HNO, CS, NO, and OH in the atmosphere.In atmospheric measurements with open paths of 100 to 1000 m, a detection limit of about 1 ppb can be achieved. In the emission measurements, the path length across the duct or the plume can range in meters. 

The variability of hazardous air pollutants (HAPs) is an important factor in determining hnman exposure to such chemicals, and in designing HAP measurement programs. The factors that contribute to HAP variability in an urban area also affect their global impact. Temporal variation was the major contributor to HAP variability for 19 of the 39 frequently detected compounds (Spicer et al., 1996). In the future, more precise measurement tools will be available to determine HAPs. Open-path monitoring of the atmosphere using Fourier transform infrared spectrometry has recently become... 

The most important analytical applications of mulecular absorption spectroscopy with mid-lR DLs are environmental monitoring and atmospheric studies [9], Two different approaches are used (1) measurements in sample cells at reduced pressure, when the absorption linewidths are of ca. (1-3)x 10" cm , and (2) open-path measure-... 

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. 

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