Open Path Spectrophotometry

The use of open path monitoring is driven by either the need for probing an area without physical intrusion or by the need to monitor an area that is larger than one that can be cost-effectively monitored with a requisite num- 

Illustration of path-integrated concentrations and relationship to path average concentration for narrow plumes and widely dispersed vapor clouds. 

In applications in which FTIR does not have sufficient sensitivity, open path ultraviolet (OP-UV) spectroscopy is frequently employed. This methodology can be used for the detection of homonuclear diatomic molecules (chlorine, bromine, etc.), which have no infrared absorption, or molecules that absorb only weakly in the IR region, such as benzene, sulfur dioxide, and nitrogen oxides. 

OP-UV spectrometry can be used to measure vapors or gases that have weak absorption characteristics, and therefore, low sensitivities in the IR spectrum. These include such compounds as nitrogen oxides, formaldehyde, ozone, sulfur dioxide, benzene, toluene, and xylenes, and also homonuclear diatomic molecules, such as chlorine. The compounds that can be determined by UV are much fewer (see Table 3.43) than those that are absorbing in the IR spectra. 

OP-TDLAS has been applied commercially only in air monitoring. Instruments based on these lasers are capable of making very sensitive mea- 

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Open Path UV-Absorption Spectrophotometry (Including DOAS)... 

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