In the present work some results of polarization measurements in the planetary boundary layer are reported. The observations described were performed by a ground-based lidar over the region of the Sofia city (where various industrial enterprises exist along with a heavy transport and aerial communications). The data obtained when sounding in different meteorological situations (`clear' atmosphere, rain, fog, snowfall) are considered. Certain differences are established in the depolarization properties of `clear' atmosphere in dependence of the meteorological condition. The developments of fog and snowfall are traced; the effects of the underlying surface are sought when sounding in fog; various types of situations during a snowfall are pointed as well. An attempt is made to follow the development of the processes which proceed during an interaction between air masses with different properties (e.g. a front advection); as a result the formation of the cloud system over the region is traced. Definite differences between the microphysical properties of the aerosol near the clouds (Fmb, Ns) base and top are established as well.

The Environmental Research Institute of Michigan (ERIM) has developed a simple elastic backscatter lidar system using multiple aperture incoherent detection to study radiative properties of clouds and aerosols. The capabilities of the system are elastic backscatter measurement of cloud height, aerosol spatial density distribution, atmospheric mixed layer height and optical thickness of thin aerosol and cloud layers. The ERIM lidar system utilizes an Nd:YAG laser and collects return signal using a several aperture telescope array to demonstrate the concept of multiple aperture incoherent detection. Initial tropospheric measurements of clouds and aerosols made with the system are presented.

The main purpose of this paper is to include analytically the angular backscattering dependence of phase function characteristic of rather large cloud droplets. We are doing so for two reasons, at least. First, the widely used approximation for the phase function, e.g. assuming it to be essentially constant near backscattering direction or using some its average value over the whole angular range of backscattering, can be shown to lead to overestimated or underestimated, respectively, light power recorded by a lidar. Second, the glory region bears the information on some microphysical parameters of clouds. So, the analytical description of backscattered light power would provide the simple prediction of lidar opportunities to measure, e.g., mean sizes or halfwidth of size distributions of cloud aerosols. To this purpose, the small-angle diffusion approximation of the radiative transfer theory (RTT) is used here to derive a lidar signal from intermediate optical thicknesses of clouds, where neither the asymptotic formulas of the RTT nor the single scattering approximation are working well. The analytical integration of radiative transfer with aerosol size distribution has enabled us to derive the explicit form of lidar signal power as a function of the microphysical parameters.

Lidar observations of stratospheric ozone, aerosol and temperature have been carried out at Toronto (43.8 N, 79.5 W) since 1989 and during winter months at the Arctic Stratospheric Observatory (AStrO) at Eureka (80 N, 86 W) since 1992. The Raman DIAL (Differential Absorption Lidar) systems utilized at both observatories are briefly described and the measurements are discussed. The measurements of AStrO are discussed in relation to the dynamics of stratospheric polar vortex and the presence of polar stratospheric clouds (PSC). Results from the winters of 1994/95 and 1995/96 indicate very low polar stratospheric temperatures, capable of inducing PSCs and exhibit an appreciable ozone depletion.

Simultaneous measurement of aerosol extinction coefficient and atmospheric temperature by a Rayleigh-Mie lidar using an ultranarrow band-stop filter was proposed in 1983. An atomic or molecular vapor is used to separate the Rayleigh and Mie scattering signals which allows the lidar to measure atmospheric state variables (atmospheric temperature and density) as well as optical aerosol properties (aerosol extinction coefficient and backscatter phase function) as a stand alone device. This feasibility was demonstrated by a lidar operated at 537 nm using a barium atomic filter. The achieved temperature measurement uncertainty was about +/- 10 K. The temperature accuracy was limited by the stability of the barium filter oven. To improve measurement accuracy, a lidar based on an iodine vapor filter has been developed. Since iodine vapor pressure is much lower than that of barium, the oven of an iodine filter may be operated at a much lower temperature and therefore controlled to higher precision. The estimated temperature uncertainty of the current lidar system is 2.1 K at present. The temperature uncertainty due to photon noise for a measurement time of 80 minutes is 3 K at 1 km, giving rise to a resulting measurement uncertainty of 3.7 K.

A novel ultra-narrow Na Faraday vapor filter was developed and is in use with our Na temperature lidar system for daytime measurements of the atmospheric temperature near the mesopause. The first 34 hour continuous observation was carried out from 1 pm, Oct. 10 to 11 pm Oct. 12, 1995, terminated by clouds. Signal to background ratio at high noon was about 15 at Na D2a peak frequency.

In this paper we present the work carried out at the University of Calabria regarding a prototype of a dial system. This has been realized for remote pollution monitoring. Most of the effort have been done to perform several measurements on an horizontal path in order to scan the wide surrounding area. The concentrations of ozone and water vapor have been carried out using two different methods both related with the dial technique. With the integrated one average concentrations have been evaluated up to 5 km using topographical targets. In the range resolution technique profile of ozone and water vapor have been performed up to 700 m with a spatial resolution of about 30 m. Although the system needs a revision in several subsystem of its set up, the experimentation has pointed out the performances available and the necessary improvements.

A differential absorption lidar (DIAL) system, based on a tunable solid-state Ti:sapphire laser, was developed for nitric oxide monitoring in the UV region. The UV light was generated through second-harmonic generation (SHG) of sum- frequency mixing between a Ti:sapphire laser, pumped by the SH of a Nd:YAG laser, and the fundamental of a second Nd:YAG laser. The output pulse energy of 4 mJ at 226.8 nm with 2 micrometers linewidth was achieved. As a demonstration of DIAL capability, the spatial distribution of nitric oxide emission from a diesel engine was investigated. The detection error achieved was +/- 0.05 ppm up to a distance of 180 m, with the spatial resolution of 20 m.

In response to the increasing interest in formaldehyde production, emission and photochemistry in polluted and clean atmospheres, we investigate the possibility to detect formaldehyde (CH₂O) by the use of differential absorption Lidar (DIAL) techniques. In the first part of our investigations we review chemical and spectroscopic characteristics of formaldehyde with special respect to obtain the absorption and the differential absorption cross sections of the A¹A₂ - X¹A₁ vibrational band extended over the range of 250 nm - 360 nm. Four pairs of wavelengths, 321.2 nm/323.2 nm, 351.2 nm/352.9 nm, 307.8 nm/309.3 nm and 287.4 nm/288.8 nm, turn out to be adequate for remote mapping of atmospheric formaldehyde by differential absorption Lidar systems because of their large differential absorption cross sections and their absorption line profiles. In addition no interferences of the wavelengths of interest with the absorption of other atmospheric constituents is found. In a simulation study, where we use different set of DIAL-system- and spectroscopic input parameters, a formaldehyde detection limit of 1 - 5 ppb in clean air is predicted.

We are exploring a new approach to remote chemical identification that promises higher precision than can be achieved by conventional DIAL approaches. This technique also addresses and potentially solves the problem of detecting a target gas in the presence of an interfering gas or gases. This new approach utilizes an eye-safe infrared optical pumping pulse to deplete the population of a specific rotational level(s) and then sends probe pulses at the same or different wavelengths to interrogate the bleaching of the absorption. We have experimentally measured optical saturation fluence level at atmospheric pressure for HCl, and find this level to all be approximately 1 mJ/cm², significantly below eye-safe limits in agreement with calculations. Calculations have been performed on other molecules of interest with similar results. In the laboratory, using time delay replicated pulses at a single frequency we have made absorption measurements with precision levels routinely approaching 0.1% on averaging 200 laser pulses. These results as well as those of two other pulse experiments will be presented.

Combined spectral topographical surface albedo and atmospheric transmission for a rapidly tuned CO₂ airborne lidar are presented for 19 wavelengths, separated by 5 ms. The measurements were gathered over a large topographical area and show high signal-to-noise ratio (greater than 5) with single pulse acquisition for ranges up to 8 km. The temporal cross-correlation for different wavelengths is also presented and shows a relatively high correlation between wavelengths for about 20 - 50 ms. The implication of the measurements to the DIAL method is discussed.

The absolute values of the Raman scattering cross-section of the 1347 cm^-1 NO₂ symmetric stretch mode of nitrobenzene vapor were measured at some discrete excitation wavelengths in the range 240 nm to 420 nm. An analytical expression for their wavelength dependence was obtained from the numerical `best fit' of the data to Albrecht `A' term. The Raman excitation profile, thus created, exhibits a strong resonance effect, attributed to the charge-transfer state in C₆H₅NO₂ corresponding to the absorption peak at 240 nm. At near resonance excitation (250 nm) the Raman cross-section was found to be more than two orders of magnitude higher than that expected at the same wavelength from (lambda) ^-4 dependence. Although the excitation profile remained the same, the vapor phase cross-section values were almost an order of magnitude lower than those measured in liquid phase. The implication of resonance enhancement on the Raman lidar detection of NB vapor was analyzed for a standard atmospheric model with different ozone loadings using LOWTRAN7 computer code.

The first Multi center Airborne Coherent Atmospheric Wind Sensor (MACAWS) field experiment demonstrated an airborne high energy TEA CO₂ Doppler lidar system for measurement of atmospheric wind fields and aerosol structure. The system was deployed on the NASA DC-8 during September 1995 in a series of checkout flights to observe several important atmospheric phenomena, including upper level winds in a Pacific hurricane, marine boundary layer winds, cirrus cloud properties, and land-sea breeze structure. The instrument, with its capability to measure 3D winds and backscatter fields, promises to be a valuable tool for climate and global change, severe weather, and air quality research. In this paper, we describe the airborne instrument, assess its performance, discuss future improvements, and show some preliminary results from the September experiments.

A new incoherent lidar for measuring atmospheric wind using iodine molecular filter is presented. A unique feature of the proposed lidar lies in its capability for simultaneous measurement of aerosol mixing ratio, with which the radial wind can be determined uniquely from lidar return. A preliminary laboratory experiment using a dye laser at 589 nm and a rotating wheel has been performed demonstrating the feasibility of the proposed wind measurement.

With early and ongoing climatological surveys established, continuing observations of the mesopause region over Fort Collins Colorado are discussed, emphasizing the modification of Colorado State University's two frequency lidar for simultaneous measurement of temperature via Doppler Broadening and radial wind via Doppler shift. The application of a frequency-agile, tandem acousto-optical modulator system for three-frequency temperature/wind measurements free of contamination from atmospheric Na density fluctuations, as well as frequency characterization of this new apparatus, and future applications are considered.

We describe a first generation mid-infrared transmitter with pulse to pulse frequency agility and both wide and narrow band capability. This transmitter was used to make multicomponent DIAl measurements in the field.

We discuss the optical and laser properties of two new tunable laser crystals, Ce:LiSAF and Cr:ZnSe. These crystals are unique in that they provide a practical alternative to optical parametric oscillators as a means of generating tunable radiation in the near ultraviolet and mid-infrared regions (their tuning ranges are at least 285 - 315 nm and 2.2 - 2.8 microns, respectively). While these crystals are relatively untested in field deployment, they are promising and likely to be useful in the near future.

This paper discusses the atmospheric dynamics of signal propagation associated with lidar operation through a realistic turbulent atmosphere in the vicinity of chemical plumes. Traditional phase screen models represent the propagation through artificial turbulence without dynamic compatibility. In contrast, three new models, AIRFLOS, CHAFFSIM, and SIMLIGHT have been developed to provide a first principles physics approach to modeling lidar signal propagation through a dynamically consistent atmosphere. Solving the Navier Stokes equations the AIRFLOS model computes the turbulence and index of refraction over complex terrain and structures as a function of space and time, CHAFFSIM computes the chemical concentration motion, and SIMLIGHT models the electromagnetic energy propagation. Together these models allow simulation of lidar propagation through realistic atmospheric environments.

We introduce an adaptive mixing algorithm for estimating the relative ratios of chemicals in a mixture spectrum. This procedure is particularly well suited to mixtures with a large dynamic range of mixture weights. It has the advantage of being able to be used in conjunction with a band-pass (difference-to-Gaussian or DOG) filter, and a correction of baseline off-set and tilting of the spectrum. Output of these filtering techniques is a cleaner signal retaining most of the relevant Raman spectral signature while minimizing artifacts due primarily to Rayleigh, dust, and atmospheric aerosols. We will describe the results of applying these algorithm to mixture spectra with both real and simulated additive noise.

BNL has been developing a remote sensing technique for the detection of atmospheric pollutants using resonance Raman LIDAR that has also incorporated a number of new techniques/technologies designed to extend it performance envelope. Chief among these new techniques is the use of pattern recognition to take advantage of the spectral fingerprint and a new laser frequency modulation technique, referred to as Frequency Modulated Excitation Raman Spectroscopy, designed to suppress broadband fluorescence. In the laboratory, broadband fluorescence suppression approaching 3 orders-of-magnitude has been achieved. In addition, the application of a BNL designed knife-edge Rayleigh filter has also bee demonstrated using our LIDAR system where spectral features as close as 200 cm^-1 from the excitation line were observed. How all these features help increase the overall performance of Raman LIDAR will be discussed.

A criterion of spatial-angular efficiency (SAE) of remote electro-optical systems for atmosphere monitoring is formulated. The dependencies of the SAE from normalized range and minimal operating range for different optical receiving schemes of ground-based biaxial LIDAR are analyzed. It is shown that low SAE of traditional VIS & NIR systems is a main cause of a low signal-to-background-noise ratio at the photodetector input, the considerably measurements errors, and the following low accuracy of atmospheric optical parameters reconstruction. The most effective protection against sky background radiation in such systems consists in forming an angular field according to the introduced SAE criterion. Some approaches to achieve high value of the SAE-parameter for receiving system optimization are discussed.

Concern exists over the impact of the emissions of large rockets powered by solid motors upon the ozone layer. A scanning lidar, operating at 532, 355 and 308 nm, has been developed to address this issue. We report the first lidar observations of the rocket exhaust plume in the stratosphere. Over 500 sets of lidar profiles were collected in three campaigns covering Titan IV K-21 (Nov 6, '95), Space Shuttle STS-76 (Mar 22, '96) and Titan IV K-16 (Apr 24, '96). Thin particle layers were observed in the 18 - 43 km altitude region for up to three hours after launch. Plume dimensions and dispersion rate were inferred from the observation. The plume thickness was measured to be less than 250 meters. Backscattering signals dependence on wavelengths were analyzed showing a weakening trend with time.

This model describes variations of aerosol size distribution function, aerosol fluxes and their residence times as a function of two different formulae for roughness length coefficient including developing roughness and fully developed roughness, diverse sea bottom types with various slopes and different weather conditions with changing wind velocity, direction and duration. This model has been verified experimentally on two types of Baltic Sea bottoms and it allows for the good estimation of aerosol dynamics in the coastal zone provided that wind conditions and the sea bottom type are known.

This paper reports the results of a system analysis of the performance of a helicopter-based backscatter lidar system for long-range stand-off detection of clouds of biological warfare (BW) agents. With models we developed, we calculated the dispersion, transport, and detectability of a BW agent line-source (`crop-duster'-like) cloud as a function of elapsed time in an employment scenario. For a given BW attack, we calculated the time of first detection, determined the warning time, and from that, estimated the number of troops saved from exposure.

The performance of the Lidar Atmospheric Profile Sensor (LAPS) instrument for measurements of water vapor in the lower troposphere has been investigated. LAPS is an automated lidar system that measures water vapor from the vibrational Raman backscatter in the visible and in the ultraviolet wavelength range. We present a comparison of water vapor profiles measured with the lidar and balloon sondes as well as measured with the two lidar channels. With the UV channels it is possible to infer ozone profiles in the boundary layer. Data are presented that reveal the high variability of the water vapor in the boundary layer.

We report progress in developing a high spectral resolution lidar system that measures vertical profiles of atmospheric temperature and aerosol optical properties using an iodine molecular vapor filter at a transmission wavelength of 532 nm.

A new lidar-inversion technique is presented for the determination of the extinction-coefficient profile within a spatially restricted zone of atmospheric aerosol inhomogeneity such as a plume, thin cloud, etc. The return lidar signal is measured through the aerosol plume under investigation and also in a direction close to but outside the plume. By using the ratio of these signals, the constituent produced by the aerosol plume is separated from the aerosol background constituent. An iterative lidar- inversion technique is applied to the ratio of these signals rather than to the original signal. This technique is shown to be relatively insensitive to the assumed value of parameters used for the extinction-profile retrieval, and yields an acceptable measurement results even when the accuracy of the assumed parameters is poor.