Lead-salt tunable diode lasers (TDLs) have been used extensively in atmospheric trace gas measurement applications associated with global environmental studies. Recent development work in lead- salt TDL technology and support components relevant to atmospheric monitoring and related applications will be described. Specific topics include (1) progress toward molecular beam epitaxy (MBE) grown buried heterostructure lasers for long wavelength operation, (2) high temperature CW operation and packaging suitable for thermoelectric cooling, (3) miniaturized packaging for multiple wavelength arrays with individually controllable elements and design concepts for a fully automated, sequential multiple wavelength source assembly, and (4) design considerations and performance test results for a high-stability laser source dewar for field applications.

SRI International has designed and built several instruments that use tunable diode lasers and frequency modulation spectroscopy. These instruments have been used for flux measurements of trace gases, explosives detection, and environmental monitoring. A detection sensitivity of 2X10^-6 with a stability of 0.1% over 10 hours has been demonstrated using a GaAlAs laser and an oxygen absorption line at 760.56 nm.

A transputer-based platform has been developed which allows total control of a multicomponent FM-TDLAS spectrometer. Due to the modular structure flexible implementations of various signal processing algorithms can be checked. A need for self-adaptive algorithms, having some desirable learning capabilities, arises in the control of processes which are time varying, nonlinear, and have unknown dynamics with unknown disturbances acting upon them. For such a complex problem, no analytical solution can be found. Although a potential a priori control structure can be defined, it is generally not possible to specify, in advance, the parameters within this structure. In this paper some basic principles of various signal processing strategies for tunable diode laser spectroscopy will be presented and discussed.

We describe a new, dual-channel Tunable Diode Laser (TDL) field spectrometer for the rapid, simultaneous measurement of two species. The instrument uses two-tone frequency modulation (TTFM) with signal detection at approximately equals 12 MHz. Multiplexing is achieved using a dichroic optical element and a mechanical chopper which blocks each beam alternately. A control program running on a dedicated digital signal processor (DSP) allows the registration of the full absorption line shape each millisecond and simultaneous zero overhead on-line data reduction using a multiple linear regression algorithm. Gas exchange through the compact multireflection cell (2.71 volume, total path 53 m.) takes place in approximately equals 200 ms and thus determines the instrument response time. Applications of the spectrometer in airborne and ground based field measurement campaigns are described.

The goal of this paper is to present an overview of results obtained recently in General Physics Institute of Russian Academy of Sciences and collaborating organizations. The results are related to several topics of tunable diode laser spectroscopy such as development of tunable diode laser based systems and their applications. This paper discusses properties of diode lasers available in Russia and their importance for monitoring as well as several diode laser based systems developed in GPI. Some features enabling us to develop such systems under full computer control for long-term monitoring are touched upon.

The structures and electroluminescence characteristics of new two types of single mode A³B⁵ semiconductor tunable lasers in the 1.8-3.9 micrometers spectral range have been demonstrated. The first type of tunable diode laser based on quaternary solid solutions GaInAsSb and GaAlAsSb lattice matched to GaSb substrate covers 1.8-2.4 micrometers spectral range. Such tunable 1.8-2.4 micrometers lasers have single mode or quasi-single mode operation in the wide temperature range from 1.6 to 300K. The second type of tunable diode laser based on multiple component InPAsSb/InAsSb lattice matched or mismatched to InAs substrate covers 2.8-3.9 micrometers spectral range, which was not available for diode laser spectroscopy until nowadays. Such tunable 2.8-3.9 micrometers lasers have CW single mode operation up to 100K and pulse operation up to 180K. These lasers can be the key devices for diode laser spectroscopy and sensitive detection of pollutants.

Formaldehyde has been known as an air pollutant for some time. Most measurement techniques have been based upon wet chemistry. With the improvement of diode lasers, fast high-precision measurements can now be made using selected rotation-vibrational transitions in the IR region. It is very important to accurately measure the line intensity if you want to make reliable concentration measurements. Line strengths of several rotation- vibrational transitions of formaldehyde in the 2900 CM^-1 region have been measured with a tunable diode laser spectrometer system. The resulting line strengths are compared with the corresponding values in the 1992 version of HITRAN and are found to be a factor of 1.10+/- 0.03 larger. Measurements of the self- broadening constant of several transitions are also made with an average measured value of 6.6+/- 0.1 CM^-1 MPa^-1 (0.67+/- 0.01 CM^-1atm^-1). Intensities of multiple assigned formaldehyde transitions in the HITRAN compilation is also presented. Initial studies of permeation devices to deliver known concentration of formaldehyde is also investigated. The stability of these devices is presented.

In this paper, we describe a new field diode laser system which incorporates a commercially-available astigmatic Herriott cell. This cell achieves a path length of 100 m while retaining a relative small sampling volume (approximately 3 l). This combination allows both high sensitivity and low cell residence times to be achieved. The optical system for effectively coupling the diode laser radiation into such a cell is briefly discussed. Line fitting procedures are also discussed for fitting second harmonic spectra in the presence of sloping and/or curved baseline structure. An initial application using the filed system for detection of formaldehyde is also described. Initial estimates place the instrumental detection sensitivity at approximately 50 pptv for formaldehyde using a 1Hz sampling bandwidth. This corresponds to a minimum detectable absorbance of about 2X10^-6.

Because of their very long atmospheric lifetimes the two perfluorocarbons, CF₄ and C₂F₆ have `global warming potentials' tens of thousands times greater than CO<sub>2</sub>. Aluminum smelting is believed to be the major source of these gases in the atmosphere although there is a great deal of uncertainty in the global emissions from this source. The emissions occur largely during `anode event' episodes. A tunable diode laser absorption spectrometer was used to measure the emissions of these gases from 8 smelters in the Province of Quebec which represent different technologies and contribute 11% to the total global production of aluminum. The results show that there is considerable variability in emissions between technologies, and in the intensity, the shape and the duration of the anode events with an apparent relationship between the duration and the fluxes during the anode events.

The reliable operation of a TDLAS instrument for measurement of trace gas species requires a unit providing calibration gases with mixing ratios in the range of measured concentrations. It is obvious that, especially at sub-ppb-levels, calibration becomes difficult. Therefore in this paper we focus on calibration procedures for an integrated tunable diode laser spectrometer. An automated calibration system based on permeation devices with a subsequent dilution will be presented. The system meets the ISO requirements and can be used as a secondary standard for field measurements. With this instrument multi-component calibration can be performed as well as multi stage calibration. According to the ISO regulations, procedures to check the linearity and to record the calibration function for a TDL system will be described. From this calibration function the detection and determination limit can be determined by an inverse regression procedure. Some measurements will be presented and discussed under quality control and quality assurance aspects.

Single-frequency near-infrared diode lasers are used to measure atmospheric methane and water vapor. Using high-frequency wavelength modulation methods, sensitive instrumentation with fast time response are designed. Communications lasers operating near 1310 nm probe weak overtone transitions of both molecules; lasers with custom wavelengths at present lack sophisticated packaging, but can achieve much higher sensitivity. We describe two field-tested instruments: an automated, airborne hygrometer with a sensitivity of 8 ppm (by volume) with a one second averaging time, and a fast response methane sensor with a sensitivity of 65 ppb. Improvements to these instruments are outlined, and the effects of laser nonlinearities are noted.

An industrialized computer-controlled fiber optic laser diode gas analysis system is described. A unique signal processing scheme completely eliminates both non-gas-related transmission variation and long-term drift. Sensitivities better than 1 ppm(DOT)m and 200 ppm(DOT)m is routinely achieved for field-installed systems for ammonia and oxygen, respectively. due to the superior selectivity of laser diodes, interference effects from coexisting gases, such as water vapor in ammonia measurements, are easily avoided.

Spectral Sciences has developed a family of automated remote gas sensors including a long-path absorption system, and a fiber- optic-based system utilizing multiple remote-sensing heads. Results are presented from initial field tests of the long-path sensor.

The need for innovative solutions in air compliance monitoring is becoming an increasing concern to Government and Industry due in large part to the regulations promulgated by the 1990 Amended Clean Air Act. Efforts are underway at CCNY in collaboration with the DuPont Company to develop a cost effective and practical solid-state laser based air pollution monitoring system for the control and measurement of hazardous air toxins in urban and industrial environments. A general discussion of the CCNY-DuPont project and the utilization of DIAL in air monitoring applications as required by the Clean Air Act are discussed.

Recent progress in Lidar/DIAL technology has allowed to obtain 3-dimensional mappings of the concentration of air pollutants at highest sensitivity (ppbrange) and over large distances (10 km)13. This access to atmospheric dynamics allows direct correlation between emission and immission. Presently, it is possible to monitor on-line distributions of nitrogen oxides, sulfur dioxide, and ozone. Recently, also the detection of toluene and benzene in the near UV has been demonstrated4. Routine or fully automatic operation has been severely restricted, however, by the complexity and maintenance of the usually employed Nd:YAG or Excimer-pumped dye lasers. The advent of new tunable all-solid-state laser systems, such as vibronic lasers (Ti:Sapphire, LICAF, LISAF,. ..) or laser-pumped OPOs, opens a new era in the domain of userfriendly and fully automatic DIAL operation. Here, we present the first Lidar/DIAL systems which are based on a high energy flashlamp-pumped Ti:Sapphire laser. It combines wide tunability of the laser medium with reliability and ease of operation, characteristic for flashlamppumped solid state lasers. The wavelength range accessible by the laser and its extension by nonlinear optical devices makes it an ideal tool for both DIAL and meteorologic applications. This 'all solid state" laser is the heart of a new generation of stationary and mobile Lidar systems. The first of these systems, presented here, has been designed to replace the urban station in Leipzig, which routinely provides mappings of pollutants over the city since 1992. The advantages of such routine and long term Lidar measurements are presented in this paper, in comparison with former results obtained during campaigns in several large european cities. A second urban unit is being implemented in Berlin, while a mobile and very compact "all solid state" DIAL system is in construction. It will start the test phase by the end of the year, and first measurement campaigns in february.

This paper presents a summary of Optech's internal studies into the feasibility of developing lidar systems for gas detection and mapping for environmental monitoring. The paper summarizes our evaluations of the capabilities of two different lidar techniques to address this area of application. These two techniques are Raman scattering and differential absorption. Optech has experience in building lidar systems operating on either of these techniques, and in recent years has carried out several studies of the potential of these lidar technologies for pollution monitoring. The paper includes brief reviews of the basic measurement techniques for DIAL and Raman lidar and examples of currently operating systems are given. Some of the practical problems encountered in operating these systems are also discussed. The results of performance models for new system concepts are then presented for a number of important pollution gases. The discussion then addresses the limitations of the current technology, by considering issues such as limitations of the existing laser technology and factors affecting detection limits.

We describe a new lidar method capable of distinguishing between molecular and particulate backscattering. This method avoids the use of sophisticated high spectral resolution techniques by employing one laser, two transmitted wavelengths, and the observation of three different lidar return signals. For a system based on a frequency-doubled Nd:YAG laser emitting at 532 nm, Raman shifting in a N₂ gas cell generates a second wavelength at 607 nm. The transmission of laser pulses is alternated between these two wavelengths. When a 532 nm pulse is transmitted, both the backscattering near 532 nm and the Raman-shifted backscattering by atmospheric N₂ near 607 nm are observed. For the lidar transmissions at 607 nm, only the combined particulate and molecular signal is observed. If a region of aerosol-free air occurs at some distance within the lidar range, the product (or geometric mean) of the backscatter ratios can be determined as a function of range, directly from the three relevant lidar equations. Under minimal assumptions as to the change of the aerosol backscatter coefficient between the two transmitted wavelengths, the extraction of absolute extinction and backscatter coefficient range-profiles for both wavelengths is demonstrated. Initial error analysis suggests that the method has considerable promise for quantitative lidar applications. The technique will be useful in tropospheric studies, such as visibility measurements, and at higher altitudes where extinction by volcanic aerosols has to be taken into account in obtaining reliable temperature profiles throughout the stratosphere.

We have developed and operated an eyesafe lidar in support of an intensive set of air chemistry measurements in Atlanta, Georgia, which were part of the Southern Oxidants Research Program (SORP) during the summer of 1992. The lidar was used to monitor the thickness of the mixed layer by measuring the vertical distribution of boundary layer aerosols. The lidar system is based on a Raman-shifted Nd:YAG laser source at 1.54 microns wavelength with a pulse energy of 40 mJ and a pulse repetition frequency of 4 Hz. The receiver aperture was 46 mm in diameter and an InGaAs PIN diode was used as the detector. The lidar data was typically averaged over 1000 laser pulses, which required about 4 minutes. The lidar returns were range corrected to yield profiles of signal versus altitude in which the signal is proportional to the atmospheric backscatter coefficient. The profiles showed the vertical extent of boundary layer aerosols, and this was interpreted to find the mixed layer thickness. Data was acquired on nine days in July and August 1992. Measurements were typically made at 15-minute intervals from early morning until midafternoon. Mixed layer thicknesses provided by the lidar have been shown to be consistent with balloon sonde results, and they have proved to be useful in interpreting atmospheric chemistry results.

Calibration procedures for lidar and DIAL systems are discussed. Resulting requirements for the standardization of the technique are reviewed. Possible schemes for on-line calibration, based on a standard reference cell, are presented.

Spatial distribution of aerosol density in the boundary layer can be measured with a scanning lidar system. These measurements can be used to visualize, through color coded R-H maps, the inversion layer height and the dispersion of aerosols from natural and manmade sources. Lidar inversion height measurements were verified by taking simultaneously temperature profiles measurements with a meteorological balloon. The results obtained were in good agreement, suggesting lidar as a powerful tool to investigate aerosol dispersion in boundary layer.

Battelle has designed and fabricated an upward-looking atmospheric backscatter lidar for high-altitude airborne applications. The compact, rugged system was assembled and integrated into a cupola on top of a Lear 36 aircraft to provide particle backscatter data and aerosol profile distributions of cirrus clouds occurring between 50,000 and 100,000 ft ASL. The high altitude airborne lidar system consists of a laser transmitter operating at 532 and 1064 nm simultaneously with output energy of 75 mJ at both wavelengths and a collecting telescope aperture of 10 inches in diameter. Laser backscatter energy is collected and directed via a dichroic beamsplitter to two avalanche photodetectors (APD) through narrow bandpass optical filters at 532 and 1064 nm. The outputs of the APDs are digitized by a 10-bit, 100-MHz transient digitizer before being recorded to a 1.2-Gbyte hard disk with IRIG timing for data analysis. This paper describes the lidar system design, predicted performance, and some of the operational challenges.

The University of Michigan's Space Physics Research Laboratory has constructed a mobile high-spectral-resolution Doppler lidar capable of measuring wind and aerosol loading profiles in the troposphere and lower stratosphere. The system uses a 3-W pulsed frequency-doubled Nd:YAG laser operating at 532 nm as the active source. Backscattered signal is collected by a 44.4-cm-diameter Newtonian telescope. A two axis mirror scanning system allows the instrument to achieve full sky coverage. A pair of Fabry-Perot interferometers in combination with a narrowband (0.1nm) interference filter are used to filter daylight background and provide a high spectral resolving element to measure the Doppler shift. In addition, the aerosol and molecular scattered components of the signal can be separated, giving a measure of the relative aerosol loading. Measurements have been made day and night in the boundary layer with vertical resolution of 100 m and a temporal resolution of approximately 5 minutes. Accuracy of the wind velocity is on the order of 1 to 2 m/s in the boundary layer.

A self-contained mobile differential absorption lidar (DIAL) system intended for measuring SO₂ and NO₂ concentrations from stationary combustion sources has been completed for enforcement use. The system uses tunable Ti:sapphire laser technology, with nonlinear conversion to the blue and UV absorption wavelengths. Separate tunable laser oscillators at slightly offset wavelengths are pumped on alternate pulses of a 20 Hz doubled Nd:YAG pump laser; the outputs are amplified in a common amplifier, doubled or tripled, and transmitted toward a target region via a two-mirror beam director. Scattered atmospheric returns are collected in a 0.27-m-diameter telescope, detected with a filtered photomultiplier, and digitized and stored for analysis. Extensive software-based control and display windows are provided for operator interaction with the system. The DIAL system is built into a small motor coach. Gasoline- powered electrical generation, laser cooling, and air conditioning services are present. Separate computers are provided for simultaneous data collection and data analysis activities, with shared data base access. A laser printer supplies hardcopy output. The system includes the capability for automatic data collection at a series of scanner angles, and computer processing to present results in a variety of formats. Plumes from coal-fired and mixed-fuel-fired combusters have been examined for NO₂ and SO₂ content. Noise levels of a few parts per million are reached with averaging times of less than one minute.

Lidar has been used by SRI for making lower atmospheric measurements since 1963, just three years following the introduction of the ruby laser. To commemorate thirty years of conducting environmental lidar programs and to acquaint new investigators with application of lidar techniques, this paper reviews both early and recent SRI ground-based and airborne systems and their measurements of pollution plumes and boundary layers. Literature references are given for more detailed discussion than can be presented here.

This paper presents and interprets continuous profile measurements of ozone and particulate density of urban origin obtained remotely with the EPA airborne ultraviolet differential absorption lidar (UV-DIAL) from studies conducted in southeastern Michigan (greater Detroit area) during May 1992 and in the Houston, Texas, Gulf Coast area in summer 1993.

As a result of the development of a new air pollution measurement capability, we present observations of air pollution parameters in the Baltimore/Washington, D.C. area for the months of August and September 1992, requested by the Air and Radiation Management Administration, State of Maryland Department of the Environment (MDE). The measured parameters are (1) the height of the atmospheric boundary layer (BL) as determined by lidar (laser backscatter by aerosols) and (2) the concentration of the major gaseous pollutants (O₃, SO₂, and NO₂) as determined by long-path differential optical absorption spectroscopy. Selected results are presented in pictorial, graphical, and tabular forms, and are compared with each other and with observations taken with other pollutant and meteorological instruments operating in the same region and time period. Complete data have been provided to the MDE for Urban Airshed Model calculations. Early results are promising on correlations between BL parameters and pollutant concentration; this relatively recent study is still underway.

A replacement of point-detector network or its combination with an optical remote sensor is considered. The advantages achieved as well as some unavoidable difficulties, are discussed. The main difficulties arise due to inherent differences in spatial and temporal resolution of detectors considered and the different physical units in which their output data is given. Experimental results to illustrate the main points are presented.

SRI International has designed and developed a fully automated frequency-agile CO₂ DIAL (differential absorption lidar) system. The system sensor head consists of a single, frequency- agile, CO₂, TEA laser; a 10-inch receiver telescope, a liquid-nitrogen-cooled HgCdTe detector; and a transmit energy monitor. The sensor head and its auxiliary equipment (including the data acquisition and processing system, laser power supply, and water cooler) are mounted in a Grumman-Olson 11-ft step van. The self-contained, mobile system can be used to detect and quantify many volatile organic compounds (VOCs) at parts per million sensitivities over open-path ranges to 5 km. Characterization and demonstration of the system is ongoing. However, data collected on benzene, toluene, xylene, methanol, ethyl acetate, acetic anhydride, and other VOCs will be described herein. The system could be used by industry and government agencies in stand-off monitoring to map VOC emission sources and transport patterns into surrounding communities. A single mobile system could be used for several locations to verify compliance with environmental regulations such as the 1990 Clean Air Act Amendments.

The Southern Oxidants Research Program on Ozone Non-Attainment (SORP-ONA) field study was held in Atlanta, Georgia, during the summer of 1992. SORP-ONA was the first in a series of intensive studies to characterize urban ozone in the South as a part of the Southern Oxidants Study (SOS). The University of Michigan Doppler Lidar was stationed on the Georgia Tech Campus during the study and measured aerosol profiles with approximately 15-minute temporal resolution. A study of mixing in the urban boundary layer determined that aerosol and presumably chemical constituents are not always well-mixed as expected and that some structure does exist. A technique for separation of aerosol and molecular scattered signal for retrieval of aerosol profiles is described. Additionally, a technique is introduced to estimate boundary layer mixing height which shows excellent correlation with rawinsonde potential temperature profile estimates of mixing height.

In this paper we discuss the potential advantages of the alexandrite laser as a light source for long range DOAS monitoring of air quality. The tunable alexandrite output can be frequency converted to provide essentially continuous coverage of the near UV spectrum. The wide tuning range and spectrally narrow output achieved by injection seeding are nearly ideal for several DOAS applications.

A very compact, transportable differential absorption lidar (DIAL) for ozone and aerosol profiling in the lower troposphere (from near surface to about 3 km) has been developed at the National Oceanic and Atmospheric Administration's Environmental Technology Laboratory. The ozone lidar has been employed in two field experiments in California. the first was in intercomparison experiment of the lidar and an airborne ozone analyzer carried out in the Sacramento valley of northern California during July 1993. The second field experiment involving the ozone lidar was the Free Radical Study, carried out in the Los Angeles basin during September 1993, where the highest ozone episode of the year was observed during the experiment. The system will be described and examples of ozone profiles during the high- ozone episode in inland Los Angeles will be shown.

A very compact, transportable differential absorption lidar (DIAL) for ozone and aerosol profiling in the lower troposphere (from near surface to about 3 km) has been developed at the National Oceanic and Atmospheric Administration's Environmental Technology Laboratory. The ozone lidar has been employed in two field experiments in California. The first was an intercomparison experiment of the lidar and an airborne ozone analyzer carried out in the Sacramento valley of northern California during July 1993. The second field experiment involving the ozone lidar was the Free Radical Study, carried out in the Los Angeles basin during September 1993, where the highest ozone episode of the year was observed during the experiment. The system will be described and examples of ozone profiles taken during the high ozone episode in inland Los Angeles will be shown.

In a series of laser sounding experiments the registered shapes of retroreflected light pulses demonstrate an anomalously high levels of signals from isolated seawater layers. We propose here an interpretation for such peculiarities of echo-signal shape with the use of developed theory of backscattered light pulse kinetics in turbid medium. Our approach is based on the accounting of the angular anisotropy of the scattering in the back hemisphere and the geometrical conditions of typical lidar experiment, which determine the small angle divergence of collected light beam from the retro-reflection direction. According to the analysis presented below, the enhanced backscattering from isolated depth levels may be caused by high concentration of large biological particles (alga cells - coccolitophorids, diatoms, etc.) in the corresponding water layers. The explanation is shown to be consistent with the known unique measurements of volume scattering phase function in the vicinity of backward direction.

For the past 20 years, the Department of Defense has sponsored investigations and studies on the use of laser remote sensing techniques and light detection and ranging (lidar) methods for the detection, identification, and tracking of toxic and hazardous battlefield materials. The same lidar methods used by NASA, EPA, and several industry research groups to detect and measure the movement and concentration of air pollution near urban centers have been applied to the national security problem of detecting chemical and biological warfare agents that might be used on the modern battlefield. Significant government investment in the technology base and laser technology has resulted in advanced hardware configurations that are now available for demonstration and evaluation for industrial and environmental monitoring.

This brief review deals with various types of new multipass system developed for urgent high resolution spectroscopic applications at the Institute of Chemical Physics of the Russian Academy of Sciences. Some of them have been widely acknowledged and independently applied in different fields of modern science and technology, i.e. laser technology, metrology, spectral instrument engineering and environment.