Possibility of Hard-Target Lidar Detection of a Biogenic Volatile Organic Compound, nu-Pinene Gas, Over Forest Areas.

The absorption spectrum of alpha-pinene gas, a biogenic volatile organic compound, was directly measured with a pulsed mid-infrared laser. The maximum absorption wavelength was found to be ~3.42 mum, and an absorption cross section of 4.8 x 10(-23) m(2) molec(-1) was obtained. A simple theoretical calculation with the measured spectral data showed that several hundreds of parts in 10(12) (ppt) of alpha-pinene gas in forest-mountain areas over a range of ~10 km were detectable by a long-path-averaged hard-target absorption lidar. Requirements for system development were also discussed.

Fluorescence lidar imaging of historical monuments.

What is believed to be the first fluorescence imaging of the facades of a historical building, which was accomplished with a scanning fluorescence lidar system, is reported. The mobile system was placed at a distance of ~60 m from the medieval Lund Cathedral (Sweden), and a 355-nm pulsed laser beam was swept over the stone facades row by row while spectrally resolved fluorescence signals of each measurement point were recorded. By multispectral image processing, either by formation of simple spectral-band ratios or by use of multivariate techniques, areas with different spectral signatures were classified. In particular, biological growth was observed and different stone types were distinguished. The technique can yield data for use in facade status assessment and restoration planning.

Real-time gas-correlation imaging employing thermal background radiation.

Real-time imaging of gas leaks was demonstrated using an IR camera employing outdoor thermal background radiation. Ammonia, ethylene and methane detection was demonstrated in the spectral region 7-13 microm. Imaging was accomplished using an optical filter and a gas-correlation cell matching the absorption band of the gas. When two gases, such as ammonia and ethylene, are absorbing in the same wavelength region it is possible to isolate one for display by using gas-correlation multispectral imaging. Results from a field test on a leaking gas tanker are presented as QuickTime movies. A detection limit of 200 ppm x meter for ammonia was accomplished in this setup when the temperature difference between the background and the gas was 18 K and the frame rate was 15 Hz.

Gas imaging by infrared gas-correlation spectrometry.

We describe a new method for visualization of gas flows based on infrared absorption and gas-correlation techniques. This result is a gray-scale or false color-coded image showing the distribution of a specific gas in the area studied. A sequence of images showing the workplace test setup with a turbulent flowing gas is presented.

Fluorescence lidar multicolor imaging of vegetation.

Multicolor imaging of vegetation fluorescence following laser excitation is reported for distances of 50 m. A mobile laser-radar system equipped with a Nd:YAG laser transmitter and a 40-cm-diameter telescope was utilized. The laser light was Raman shifted to 397 nm with pulse energies of ˜ 30 mJ. An image-intensified CCD camera with a specially designed split-mirror Cassegrainian telescope was utilized for the simultaneous recording of fluorescence images of leaves and branches in four different spectral bands. Additionally, fluorescence spectra at selected points within the detection area were measured with an image-intensified diode array system. Image processing permits extraction of information related to the physiological status of the vegetation and might prove useful in forest decline research.

Differential optical absorption spectroscopy (DOAS) system for urban atmospheric pollution monitoring.

We describe a fully computer-controlled differential optical absorption spectroscopy system for atmospheric air pollution monitoring. A receiving optical telescope can sequentially tune in to light beams from a number of distant high-pressure Xe lamp light sources to cover the area of a medium-sized city. A beam-finding servosystem and automatic gain control permit unattended long-time monitoring. Using an astronomical code, we can also search and track celestial sources. Selected wavelength regions are rapidly and repetitively swept by a monochromator to sensitively record the atmospheric absorption spectrum while avoiding the detrimental effects of atmospheric turbulence. By computer fitting to stored laboratory spectra, we can evaluate the path-averaged concentration of a number of important pollutants such as NO(2), SO(2), and O(3). A measurement of NH(3) and NO close to the UV limit is also demonstrated.

Atmospheric atomic mercury monitoring using differential absorption lidar techniques.

Three-dimensional mapping of atmospheric atomic mercury has been performed with lidar techniques, to our knowledge, for the first time. Industrial pollution monitoring, as well as measurements of background concentrations, is reported. High-efficiency frequency doubling of narrowband pulsed dye laser radiation was employed to generate intense radiation at the mercury UV resonance line. Field measurements were supplemented with extensive laboratory investigations of absorption cross sections and interfering lines of molecular oxygen.

Rotational CARS: a comparison of different techniques with emphasis on accuracy in temperature determination.

Different rotational CARS techniques have been evaluated in terms of single-shot temperature accuracy and signal intensity in room temperature nitrogen and in flames. The different techniques include both dual broadband techniques, using one or two broadband dye lasers, and conventional rotational CARS with different dye lasers. These techniques are also compared with vibrational CARS concerning temperature accuracy and with theoretical predictions. The results indicate that the dual broadband techniques are to be preferred over conventional rotational CARS and also over vibrational CARS at room temperature. At flame temperatures the vibrational CARS technique seems to be the technique yielding highest temperature accuracy. The experimental results are also generally in good agreement with the calculated values.

NO plume mapping by laser-radar techniques.

Mapping of NO plumes by using laser-radar techniques has been demonstrated with a mobile differential absorption lidar system. The system was equipped with a narrow-linewidth Nd:YAG-pumped dye laser that, with doubling and mixing, generated pulse energies of 3-5 mJ at 226 nm, with a linewidth of 1pm. This permitted range-resolved measurements of NO, with a range of about 500 m. The detection limit was estimated to 3 microg/m(3), with an integration interval of 350 m. Spectroscopic studies on the gamma(0, 0) bandhead near 226.8 nm were performed with 1-pm resolution, and the differential absorption cross section was determined to be (6.6 +/- 0.6) x 10(-22) m(2), with a wavelength difference of 12 pm.

Differential optical absorption spectroscopy system used for atmospheric mercury monitoring.

A high-resolution differential optical absorption spectroscopy (DOAS) system for long-path atmospheric pollution monitoring is described. The system, consisting of a broadband lamp and a dispersive, fast-scanning optical receiver, separated by a few kilometers, was used in measurements of different pollutants, highlighted by the monitoring of the local concentration of atomic mercury. Mercury levels in the ppt (1:10(12)) range were assessed by comparisons with laboratory measurements.

Simultaneous spatially resolved NO and NO(2) measurements using one- and two-photon laser-induced fluorescence.

We demonstrate how spatially resolved distributions of NO and NO(2) can be simultaneously detected by using a single laser pulse at 452 nm. The laser-induced fluorescence from NO was achieved by a two-photon transition in the gamma band at 226 nm followed by UV detection, whereas NO(2) was detected by a one-photon transition followed by Stokes fluorescence.

Gas-correlation lidar.

Basic principles for the extension of gas-correlation techniques to the lidar situation are discussed. Favorable signal-to-noise ratios and relaxed laser requirements characterize the technique. Preliminary experiments on atomic mercury are reported.

Remote measurement of atmospheric mercury using differential absorption lidar.

Atomic mercury in the atmosphere was detected by the differential-absorption lidar technique. Laser light at the mercury resonance wavelength of 253.65 nm was generated by anti-Stokes shifting in H(2) of the frequency-doubled output from a Nd:YAG-pumped dye laser. The measurements demonstrate that a concentration of 4 ng/m(3), corresponding to a typical background value, can be detected if a path length of 2 x 1 km is used.

Laser monitoring of atmospheric NO using ultraviolet differential-absorption techniques.

Atmospheric NO was detected in a long-path absorption experiment using a frequency-doubled dye laser, twice Raman shifted in H(2) to 227 nm. Apart from measurements employing a distant retroreflector, the potential of range-resolved lidar measurements was investigated.

Single-pulse laser-induced OH fluorescence in an atmospheric flame, spatially resolved with a diode array detector.

Laser-induced fluorescence measurements of OH have been performed in an atmospheric stoichiometric CH(4)/air flame and in a highly sooting propane flame. The measurements were realized with a single 6-nsec pulse from a frequency-doubled dye laser pumped by a Nd:YAG laser and with a spatial resolution of ~25 microm. This was achieved by imaging through a suitably chosen filter set a section of the laser beam onto a gated and intensified diode array.