CLU-IN.ORG | Open Path Technologies

Open path technologies measure the concentrations of chemicals or particulates across an open path of air. They do this by emitting a concentrated beam of electromagnetic energy into the air and measuring its interactions with the air’s components. Some instruments (e.g., ultra-violet differential optical absorption UV-DOAS spectra) are capable of measuring only a few compounds simultaneously, while the Fourier transform-infrared FTIR spectra and others can measure a large number of compounds simultaneously. The most practical use of open path instruments provides average chemical concentrations over a set distance. This feature has an advantage over point-source measurements that may miss high-concentration plumes running between sampling devices or are too difficult to use in inaccessible areas. Conversely, open path instruments generally are not deployed to detect hot spots within a single line of measurement. An exception to this rule is the infrared video camera, which provides a real time visualization of fugitive gas plumes but cannot speciate or quantitate the gasses in the plume.

Differential absorption light detection and ranging (DIAL-LIDAR) instruments project a laser wavelength that is strongly absorbed by the target compound and a second nearby wavelength that is not absorbed by the target compound. The difference in intensity of the two return signals can be used to calculate the concentration of the target compound. These instruments are generally used to identify and quantitate one chemical at a time. However, they are capable of measuring the concentration of the target compound at any specified distance from the instrument; hence, they can be used to plot concentration contours that are used to identify high and low areas within the plume.

Although Raman open path instruments are capable of quantitating and speciating a large number of organic and inorganic compounds, they have relatively high detection limits. They are applied mostly to identifying unknown substances in emergency response situations and drug enforcement activities. The following table contains a summary of open path instrumentation characteristics and general uses. A thorough discussion of each instrument can be found at:

> UV-DOAS
> OP-FTIR
> LIDAR
> Raman Spectroscopy
> Tunable Diode Lasers (TDLs)

Summary of Open Path Instrumentation Characteristics and General Uses

Technology	Chemicals Detected	Detection Limits	Interferents	Effective Path Length	Advantages	Limitations	General Uses
DIAL LIDAR	Numerous organic and inorganic compounds	Low ppbv	Depends upon target chemical	Up to 10s of km	Gated reception allows for contaminant concentration contouring Operates only in point and shoot mode without retroreflectors	Measures one chemical or class of chemicals at a time	Contouring atmospheric gases and aerosols from earth, aircraft, or space-based platforms Contouring fugitive gas emissions at chemical plants and refineries Locating natural gas pipeline leaks from aircraft Industrial stack monitoring
OP-FTIR	Numerous organic and inorganic compounds, and acids (EPA/NIST spectral database contains 350 entries)	Low ppbv	CO₂, H₂O (high humidity)	1,000 m in bistatic mode; 500 m in mono-static mode	Quantitative measure-ment of multiple chemicals at one time Large linear range	For best detection limits requires cryogenic cooling Only molecular species can be detected Homonuclear diatomic species cannot be detected (e.g., Cl₂, N₂)	Passive mode stack emissions and flares Gas emissions from capped landfills Facility fence-line monitoring Cleanup perimeter monitoring Air quality monitoring Monitoring volcanic eruption gases Aircraft monitoring of airborne plumes [Click for more info]
Raman	Numerous organic and inorganic compounds	ppmv to several percent for open path; ppb for SERS	Depends upon target chemical; unlike FTIR, not sensitive to CO₂ or H₂O	Depending upon target concentration, portable instruments usually under 100 m and often under 50 m; 10 km or more for fixed LIDAR atmospheric equipment	Quantitative measurement of multiple chemicals at one time	High detection limits in open path mode	Fugitive emissions from chemical and refinery processes Point and shoot, normally at very close range, to identify hazardous materials, plastic composition, mineral composition, and drugs Identification of chemical warfare aerosols Used in conjunction with large fixed LIDAR unit to identify air pollutants and water vapor Chemical process monitoring [Click for more info]
TDLs	Low molecular-weight gases	Sub ppbv to ppmv (ppm for hand-held passive configuration)	Tunability eliminates or minimizes interferents	1 km	Light weight and easily deployed	Measures one chemical compound per tuning May require cryogenic cooling for best detection limits	Combustion gas monitoring of boiler incinerator efficiency Industrial stack monitoring Process tank and line monitor-ing for fugitive emissions Natural gas line monitoring Facility fence-line monitoring [Click for more info]
UV-DOAS	SO_x, NO_x, NH₃, HF, O₃, mono-aromatics (BTEX), and aldehydes (HCHO)	Low ppbv to sub ppbv	O₂ with BTEX	10 or more km	Continuous monitoring Relatively low detection limits	Detects limited number of chemical species	Urban air pollution monitoring Facility fence-line monitoring Airport plane exhaust monitoring Traffic exhaust monitoring In-plant storage/process area ambient monitoring Stratospheric pollutants Tropospheric pollutants [Click for more info]