With open-path systems, sophisticated, meteorological modeling software is necessary to integrate information about regional topography and moment-to-moment changes in atmospheric conditions. Such modeling programs are still in the early stages. Therefore, discrete-point sampling is needed to confirm optical sensing results. Another shortcoming of using open-path systems is the assumption that the distribution of molecules in the beam is homogeneous. Path-averaging assumptions can underestimate a localized problem.

Despite some remaining problems, the use of perimeter monitoring is rising for documenting cumulative emissions over process facilities. The EPA is developing procedures to demonstrate equivalence with reference test methods for use with some toxic gases (Schwartz, Sample, and McIlvaine 1994). These procedures are required before regulatory agencies and industry can adopt optical techniques.

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