NARSTO COMPENDIUM

R&D METHODS FOR PAMS/VOCS OZONE PRECURSORS

5. Differential Absorption LIDAR - DIAL

[Basis | Range | Min. Detection Level | Operating Temp. | Interferences | Notes of
Interests | References]

1. Basis: LIDAR stands for Light detection and ranging.  DIAL is an application of LIDAR but using powerful lasers directed into the atmosphere to measure content of the air in aerosols, dust, and gases.  This is achieved by the direct impingement of the laser beam on these materials, and its subsequent reflection and scattering.  Since the target substances vary in concentration along the axis (optical path) of the transmitted beam, the receiving telescope equipment analyzes the strength of the returning (reflected) beam continually during its reception (Ednar et al., 1995).  This reflected beam strength is reduced from the original transmission strength by some level which is proportional to the concentration of the target matter.

The DIAL technique is used to measure species concentrations in the lower few kilometer of the atmosphere.  The DIAL equipment uses two parallel laser beams, either from two separate lasers tuned slightly differently, or from a pulsing or switching system which separates a single beam into two beams with slightly different wavelengths (Fig. 1).  Fig. 1 is after Browell (1989).  One of the two beams is tuned to the absorption wavelength of the target chemical species, and the other beam is tuned to a slightly different wavelength so that it is not absorbed.  In other words, DIAL consists of two coordinated lidar beams.  The assumption is made that the difference in the strength of the two reflected beams will be a measure of the concentration of the target species (McGraw Hill, 1992).

2. Range: The analysis range refers to the typical span of species concentrations that are measured by lidar/DIAL instruments.  Various applications report the following concentrations, but they are NOT the concentration span that the instruments may be capble of.

a. Lidar: single ozone 15 to 75 ppbv 0 to 2700 m, laser with multi-beams,ground based (Zhao and Hardesty, 1996)

b. DIAL, ozone 20 to 65 ppbv at 950 m by aircraft (Senff et al., 1998)

c. UV-DIAL, ozone 42 to 82 ppbv, 450 to 1825 m by aircraft (Alvarez et al., 1998)

e. DIAL, SO2 approx. 100-200 mg/m3, 0 - 150 m by ground based equipment (Ednar et al., 1995)

f. DIAL, NO3 133 - 211 pptv, 0 - 7.5 km by ground based equipment (one part in 1012) combined with DOAS (Povey et al., 1998)

3. Minimum detection limit: Ozone, 15 ppbv (Alvarez et al., 1998); SO2 , 100 mgm/m3 (Ednar et al., 1995); NO3, 133 - 211 pptv (Povey et al., 1998)

4. Operating temperature:  The LIDAR equipment including DIAL optical housing, electronics, computer equipment, etc. typically is within an climate control enclosure (a trailer or an aircraft), therefore, the operating temperature is controlled to human comfort level ~ 70-72ºF.

5. Known Interference:

a. Use of a CO2 laser relies on existence of sufficient aerosols in the atmosphere to provide useful backscatter.  Visible and UV wavelengths must be used by molecular backscatter. The tuning difficulties of the CO2 laser make the task of searching among and eliminating non-target species difficult (Grant et al., 1992).

b. Chemical species absorption wavelengths may be temperature and pressure dependent, making it necessary to check these variables before tuning laser (Grant et al., 1992).

c. Wind speed and direction will also cause measurements to change rapidly, since the target mass could be shifting location and mixing ratio (concentration) rapidly (Ednar et al., 1995).

6. Notes of Interest:

a. The development of lidar and DIAL represents the hope that constituents of ambient air can become precisely measurable in some simple, remote sensing manner, over a wide range of altitudes, with greater convenience than any of the traditional techniques, such as the Fourier Transform Infrared (FTIR) open path technique which requires two pieces of equipment, one at each end of the beam pathway, or Laser Induced Fluorescence (LIF), which has not advanced beyond in situ applications.

b. The application of lasers has been seen as the key to the use of spectroscopic principles in achieving the goal of remote sensing.  Single laser beams with receivers (i.e., lidar alone) have been used to measure amounts of aerosols, dust, and molecular masses if they are dense enough to provide detectable reflection of the laser, called "backscatter" in the case of large volume substances (McGraw Hill, 1992).

c. DIAL has been used since the mid-1960’s to measure atmospheric constituent species, including O3, SO2, Cl2, NO, NO2, and Hg (Grant et al., 1992).

d. Due to advances in laser technology, lidars have also ramified into more or less distinct laser applications and design variations: Atmospheric lidars are sometimes divided into either aerosol/dust lidars and gas molecule lidars (Senff et al., 1998).

e. Other lidar variations include:

- Rayleigh lidars: designed to measure molecular scattering and thus determine atmospheric density and temperature profiles.

- Resonance fluorescence lidars: intended to identify atomic or molecular substances by the level of fluorescence detected at specific wavelengths.

- Raman lidars: intended to measure shifts in wavelength between the transmitted signal and the scattered signal, and have been used in the lower atmosphere to measure water vapor.

- Doppler lidars: designed to measure Doppler shifts in frequency in order to determine velocity of tropospheric winds.

7. References:

Alvarez II, R.J., C.J. Senff, R.M. Hardesty, D.D. Parrish, W.T. Luke, T.B. Watson, P.H. Daum, and N. Gillani. 1998. Comparisons of Airborne Lidar Measurements of Ozone with Airborne in Situ Measurements during the 1995 Southern Oxidants Study. J. Geophys. Res. 103:31155-31171.

Browell, E. V. 1989, Differential Absorption Lidar Sensing of Ozone, Proc. IEEE, 77: 419.

Ednar, H., P. Ragnarson, and E. Wallinder. 1995. Industrial Emission Control Using Lidar Techniques. Environ. Sci. Technol. 29:330-337.

Grant, W. B., R. H. Kagann, and W. A. McClenny. 1992. Optical Remote Measurement of Toxic Gases. J. Air Waste Manage. Assoc. 42:18-24.

McGraw Hill Encyclopedia of Science and Technology, 7th Edition. 1992. (Lidar.10:39-41.) McGraw-Hill Inc., New York.

Povey, I.M., A.M. South, A.t’K. De Roodenbeke, C. Hill, R.A. Freshwater, and R.L. Jonesl. 1998. A Broadband Lidar for the Measurement of Tropospheric Constituent Profiles from the Ground. J. Geophys. Res. 103:3369-3380.

Senff, C.J., R.M. Hardesty, R.J. Alvarez II, and S.D. Mayor. 1998. Airborne Lidar Characterization of Power Plant Plumes during the 1995 Southern Oxidants Study. J. Geophys. Res.103:31173-31189.