In 2004, a mobile research-grade lidar instrument for smoke measurements was assembled and delivered to the Fire Sciences Lab (FSL), Missoula, MT from the University of Iowa.

The elements of the lidar are shown in the figure 1 (right). A short-pulsed Nd:YAG laser (1) operating at two wavelengths, 355 nm and 1064 nm, simultaneously, is used as the light source. The laser is attached directly to the top of a telescope (2). The laser beam is emitted parallel to the telescope after going through a periscope (3); the periscope simplifies the alignment of the laser beam and increases the total measurement range. The telescope-laser system is able to turn rapidly through 180° horizontally and 90° vertically using computer-controlled motors incorporated into the telescope mount (4). The operating range of the lidar extends from the minimal measurement range of approximately 1 km up to a maximum distance of 20 km, depending on atmospheric conditions. The lidar can be operated during day and night.

Due to its compactness and portability, the lidar can easily be deployed to the field site of interest. The lidar is transported by a specially designed van, and having an independent power supply (a generator) can be utilized in the remote wilderness areas (figure 2, below).

PRINCIPLE INVESTIGATOR

Vladimir Kovalev, Atmospheric Physicist

FUNCTION AND OBJECTIVES

Historically, the lidar measurements were focused on investigating the vertical distribution of gases and aerosols and studying atmospheric processes within the boundary layer. However, during the last years, the lidar technique has been increasingly implemented to investigate smoke polluted atmospheres. Lidar is an invaluable tool, which can effectively provide the following types of measurements in such smoke-polluted atmospheres:
(1) Vertical scans of smoke plumes and smoke layering can give height information about the top and bottom of the plume and vertical transport of the smoke particulates (figure 3).
(2) Time series measurements of smoke layers in the vicinity of strong wildfires allow monitoring of the behavior of these layers and their evolution in the evening, overnight, and in the morning.
(3) Measurement of the vertical profiles of the smoke optical characteristics (figure 4) to provide an estimate of vertical smoke concentration and its temporal changes during day and night.

Fig. 3. Vertical lidar scans obtained in the vicinity of the Montana I-90 Fire in August 2005 with a temporal interval of three hours. The colored scale shows the relative level of backscattering. The blue colors show clear-air areas, the green, brown, and red colors show areas and layers polluted by smoke particulates. Note a significant transport of the smoke particulates down to heights below 2000 m in the right plot.

Fig. 4. Example of the vertical profile of the extinction coefficient and the backscatter-to-extinction ratio (BER), the inverse of the column-integrated lidar ratio at the wavelength 355 nm obtained in the vicinity of the I-90 Fire. The intense smoke layer is located at the heights ~2000 -3000 m.

Lidar instrumentation is able to provide the most practical remote-sensing technology for the measurement of smoke particulate characteristics and the monitoring of smoke-plume dynamics in real-time. The instrument allows obtaining and documenting information on the smoke plume rise and dispersion and the investigation of the three-dimensional distribution of smoke particulate concentration in areas around and close to prescribed fires and wildfires. The information on changes in particulate levels enables fire and air quality managers to assess smoke effects on visibility and public health in near real-time. Such measurements can also provide critical information on plume heights and aerosol levels to validate smoke dispersion models operated by the Forest Service Consortia for Advanced Modeling of Meteorology and Smoke (FCAMMS).

LIDAR Specifications

Design and construction: William E. Eichinger, University of Iowa
Azimuth range: 0 to 180°, Min. step size: 0.001°
Elevation range: -3 to 90°, Min. step size: 0.002°

LASER
Manufacturer: Big Sky Laser
Model: CFR 400
Type: Nd:YAG
Pulse Width: 8 ns
Pulse Repetition Frequency: 30 Hz
355 nm Near Field Beam Diameter: 5.77 mm
355 nm Divergence: 0.86 mrad
Beam orientation: Co-axial

Wavelength Energy Polarization
1064 nm 100 mJ Elliptical
355 nm 45 mJ Vertical

Receiver
Optics
10” Schmidt-Cassegrain Telescope, UV enhanced

Channel 1 Detector
Wavelength: 1064 nm
Type: Photo Diode
Detector size: 1.5 mm dia.
Responsivity @ 1060 nm: 34 A/W typ., QE=38%
Integrated Peltier cooler and temp. controller
Detector temperature:+0°C, Temperature stability: <0.5 K

Channel 2 Detector
Wavelength: 355 nm
Type: Photo Multiplier Tube (PMT)
Cathode diameter: 8 mm
Gain: 10^5-10^6

Digitizer
Analog Inputs
Full scale voltage : 100 mV p-p to 3.0 V p-p
Maximum sample rate: 125 Mhz
Impedance : 50 ohms
Bandwidth : DC to 50 MHz
Coupling : AC or DC

Frequency Response
Gain flatness, dc - 10 Mhz : ± 0.1 dB
-3 dB bandwidth : 50 MHz

Voltage Ranges
Full scale value (peak-peak) : 100mV, 300mV, 1.0V, 3.0V

DC Offset Voltage
8 bit DAC full scale

Digitizer
Resolution : 12 bits
Clock oscillator : 62.5 or 50 MHz
Clock divider : 1 to 128
Aperture jitter : 1pS RMS