Ambient formaldehyde detection with a laser spectrometer based on difference-frequency generation in PPLN

A laser spectrometer based on difference-frequency generation in periodically poled LiNbO₃ (PPLN) has been used to quantify atmospheric formaldehyde with a detection limit of 0.32 parts per billion in a given volume (ppbV) using specifically developed data-processing techniques. With state-of-the-art fiber-coupled diode-laser pump sources at 1083 nm and 1561 nm, difference-frequency radiation has been generated in the 3.53-μm (2832-cm^-1) spectral region. Formaldehyde in ambient air in the 1- to 10-ppb V range has been detected continuously for nine and five days at two separate field sites in the Greater Houston area operated by the Texas Natural Resource Conservation Commission (TNRCC) and the Houston Regional Monitoring Corporation (HRM). The acquired spectroscopic data are compared with results obtained by a well-established wet-chemical o-(2,3,4,5,6-pentafluorobenzyl) hydroxylamine (PFBHA) technique. Received: 8 November 2000 / Revised version: 30 January 2001 / Published online: 21 March 2001     

Long-range spectrally and spatially resolved radiation from filaments in air

The distance-resolved spectral intensity distribution of the backscattered light from long filaments generated in air using ultra-short and intense laser pulses is presented. A clean fluorescence spectrum from N₂ molecules and ions, which is produced by the high peak intensity inside the plasma filament of the fundamental pulse, was clearly resolved from the backscattered supercontinuum. The supercontinuum generated by both the fundamental and the third-harmonic pulses developed progressively and became fully developed only at the end of the filamentation.     

Multiply scattered component of lidar returns

The results of statistical simulation of the spatiotemporal structure of the multiply scattered component of lidar returns by the Monte Carlo method are discussed for the case of monostatic sensing geometry. The spatial characteristics of the region of the medium where occurs the last scattering of photon before arriving at the reciever. This region of the medium is called the instantaneous brightness body of multiply scattered radiation. It is demonstrated that the instantaneous brightness body of multiply scattered radiation that propagates toward the receiver may occupy a large volume that does not necessarily coincide with the region of formation of the singly scattered component. The main factors influencing the spatial and brightness characteristics of this volume source are established. The effect of scattering order on the spatiotemporal structure of lidar returns is analyzed for the case of sensing of aerosol haze and advective and radiative fogs with optical thickness 2<τ<8. Received: 2 August 2001 / Revised version: 7 January 2002 / Published online: 25 September 2002 RID="*" ID="*"Corresponding author. Fax: +7-38/2225-8026, E-mail: belov@iao.ru     

Statistical properties of correlated radial stochastic electromagnetic array beams on propagation

A kind of array beam named the correlated radial stochastic electromagnetic array beam that is generated by an electromagnetic Gaussian Schell-model source is introduced by use of tensor method. The analytical expression for the cross-spectral density matrix of this array beam propagating through the turbulent atmosphere and in free space is obtained after performing vector integration. Some typical numerical calculations are illustrated for the changes in the spectral density, spectral degree of polarization, and spectral degree of coherence of the beam on propagation. We find that the atmospheric turbulence can destroy the correlated effect among the beamlets.     

Experimental determination of the multiple-scattering effect on the lidar-signal polarization characteristics during liquid- and solid-phase precipitation

We present results of experimental investigations of the signal-polarization characteristics in the case of lidar sounding during precipitation. We show and discuss the lidar signals and the depolarization profiles along the sounding path for liquid- and solid-phase precipitation. In the former case we compare the signal characteristics at different degrees of precipitation rate. In the latter situation, we consider snowfall with particle shape close to that of Chebyshev particles. We also follow the lidar-signal changes depending on the field-of-view of the receiving optics. The experimental data are compared with results of theoretical estimates and models concerning the optical and microphysical characteristics of the rain and snow particles. In the case of liquid-phase precipitation – rain – the observed dependence of the lidar’s signal-polarization structure on the precipitation intensity has two aspects: on the one hand, the change of the raindrops’ shape, and, on the other, the multiple-scattering effects. The lidar data demonstrate that the signal depolarization, and, more specifically, its behavior along the sounding path, can be used as a criterion for the presence of multiple scattering. In the case of a snowfall consisting of Chebyshev particles, the simultaneous role is evident of two factors influencing the lidar-signal depolarization, namely, the non-spherical shape of the particles and the multiple-scattering effects. When the scattering takes place off particles with a large size and a shape strongly differing from spherical, we observed the predominant role of the non-sphericity of the scattering centers in the signal depolarization. Received: 6 December 2000 / Revised version: 11 July 2001 / Published online: 19 September 2001     

Dependence of the lidar signal depolarization on the receiver’s field of view in the sounding of fog and clouds

We report an experimental study of the lidar signal depolarization as a function of the relative contribution of the multiple scattering in case of optically dense objects in the atmospheric planetary boundary layer. Results of the observation of fog and stratus clouds are presented, as well as those obtained by sounding of stratocumulus clouds during a snowfall. The lidar data point to a rise of the depolarization coefficient as the influence of the multiple scattering increases in consequence of both viewing angle enlargement and penetration into the object sounded. The variations of the depolarization coefficient are studied as a function of the field of view. In the case of fog, this dependence is approximated by a three-parameter exponential law; it is found that the depolarization increases steeply when the viewing angle is increased from 9 mrad to 12.5 mrad. The relationships between the approximation parameters and the microphysical characteristics of the scattering medium are considered. The experimentally determined size of the area where multiple scattering occurs is in good agreement with that calculated according to the diffusion model. The results obtained on the multiple scattering effect on the depolarization can also be employed in determining the extinction coefficient profiles in optically dense objects, as well as in evaluating the characteristic size of the scattering particles. Received: 6 September 1999 / Revised version: 7 February 2000 / Published online: 6 September 2000     

Pulsed cavity ring-down spectroscopy of NO and NO2 in the exhaust of a diesel engine

The application of pulsed cavity ring-down spectroscopy has been demonstrated for the in situ quantitative determination of NO and NO₂ in the exhaust of a diesel engine. NO absorption has been monitored at the transition from the Χ²Π ground state to the A²Σ⁺ state at 226 nm. For NO₂, absorption bands in the spectral region from 438 nm to 450 nm were used. At the selected engine conditions, concentrations of 212±22 ppm and 29±4 ppm have been measured for NO and NO₂, respectively, in good agreement with separate chemical exhaust gas analysis. The method is sensitive enough to meet the European Euro V standard directive on NO_x emissions. This communication discusses the relatively simple setup needed for this type of measurement, the problems encountered, as well as the prospects for single-stroke, simultaneous measurements of both NO and NO₂ at the sub-ppm level. Received: 30 November 2001 / Revised version: 18 February 2002 / Published online: 14 March 2002     

Change in degree of coherence of partially coherent electromagnetic beams propagating through atmospheric turbulence

We study the change in the degree of coherence of partially coherent electromagnetic beam (so called electromagnetic Gaussian Schell-model beam). It is shown analytically that with a fixed set of source parameters and under a particular atmospheric turbulence model, an electromagnetic Gaussian Schell-model beam propagating through atmospheric turbulence reaches its maximum value of coherence after the beam propagates a particular distance, and the effective width of the spectral degree of coherence also has its maximum value. This phenomenon is independent of the used turbulence model. The results are illustrated by numerical curves.     

Propagation of fs TW laser filaments in adverse atmospheric conditions

The propagation of femtosecond terawatt laser pulses at reduced pressure (0.7 atm) is investigated experimentally. In such conditions, the non-linear refractive index n ₂ is reduced by 30%, resulting in a slightly farther filamentation onset and a reduction of the filament number. However, the filamentation process, especially the filament length, is not qualitatively affected. We also show that drizzle does not prevent the filaments from forming and propagating.     

A four-wavelength depolarization backscattering LIDAR for polar stratospheric cloud monitoring

A four wavelength backscattering depolarization LIDAR designed for polar stratospheric cloud and stratospheric aerosol measurement is described. The system uses the following wavelengths: 355 nm, 532 nm, 750 nm, and 850 nm. These wavelengths, obtained by means of the third- and second-harmonic of a Nd: YAG laser and by means of a tunable Ti: Sapphire laser, are chosen in a way to better characterize the particel size of such stratospheric aerosols. They are not emitted simultaneously as the LIDAR system is designed with only two detection channels permitting to detect, in the analog and in the photon counting mode, both the direct and the depolarized backscattered signal. The system has been operational in northern Finland since the end of November 1991.     

Study of atmospheric water in gaseous and. liquid state by using combined elastic–Raman. depolarization lidar

A combined elastic–Raman lidar system based on a tripled Nd:YAG laser is used for the separate detection of elastic backscatter and Raman signals from atmospheric nitrogen, water vapor and liquid water and for their depolarization measurement. Vertical profiles of water-vapor and liquid-water content measured under clear-sky conditions behave differently: inside the boundary layer the ratio of liquid-water to water-vapor Raman backscatters rises with altitude. The depolarization measurements bring additional information about atmospheric scattering. The observed depolarization ratio of the water-vapor Raman signal is about 14%, while for liquid water this ratio varies in the 30–75% range, which exceeds the depolarization of bulk water and is attributed to the water-aerosol effects. Raman contours of water vapor and liquid water are partially overlapped, and bleed-through of liquid-water Raman backscatter leads to enhancement of depolarization of the water-vapor Raman signal. This parameter may be used as a convenient indicator of liquid-water interference in water-vapor measurements. Received: 12 December 2000 / Revised version: 27 September 2001 / Published online: 7 November 2001     

Remote sensing of mass fluxes of trace gases in the boundary layer

The combination of remote sensing methods like Doppler lidar and FTIR offers the possibility to determine mass fluxes of gases remotely. Doppler lidar measures the three-dimensional wind vector in the vicinity of diffuse sources or the velocity of air in a chimney plume if an industrial complex is monitored. FTIR is a multi-component remote sensing method for gas concentrations. The Fourier transformation of an interferogram of a Michelson interferometer within a FTIR system converts the recorded intensity (function of optical path length) to a spectral signal (function of wavenumber). Both information, velocity and concentration, give the mass fluxes of the tracer (gas). A first test was performed at Munich-North power station with FTIR and cw-Doppler lidar. Fluxes of CO₂, CO, NO, and HC1 were determined. The results are in good agreement with the fluxes measured by in-situ instruments of the power station. The method can be used to control industrial complexes from an outside observation site.     

Extension of the van Cittert-Zernike theorem for completely polarized incoherent electromagnetic beam propagating through non-Kolmogorov turbulence

Based on the 2 × 2 cross-spectral density matrix, the van Cittert-Zernike extended theorem is developed for the completely polarized incoherent beams propagation through the paraxial non-Kolmogorov turbulence. On the consequence of the extended theorem and the definition of general spectral degree of cross-polarization of a beam, we found that the spectral degree of cross-polarization of the resultant field is independent of the refractive index structure constant of atmospheric turbulence. We investigated the influences of the propagation distance and the distance of two detection points on the degree of coherence and the spectral degree of cross-polarization.     

Measurement of filament length generated by an intense femtosecond laser pulse using electromagnetic radiation detection

We present a new method to measure the length of a filament induced by the propagation of intense femtosecond laser pulses in air. We used an antenna to detect electromagnetic pulses radiated from multipole moments inside the filament oscillating at the plasma frequency. The results are compared with the values detected from the backscattered fluorescence induced by multiphoton ionization of nitrogen molecules excited inside the filament. The values are found to be in good agreement. Received: 6 November 2002 / Revised version: 27 January 2003 / Published online: 24 April 2003 RID="*" ID="*"Corresponding author. Fax: +1-418/656-2623, E-mail: shosseini@phy.ulaval.ca     

Evaluation of dual differential absorption lidar based on Raman-shifted Nd:YAG or KrF laser for tropospheric ozone measurements

Based on Raman-shifted Nd:YAG or KrF laser, a method of three-wavelength Dual DIfferential Absorption Lidar (DIAL) for tropospheric ozone measurements is proposed. A theoretical analysis and numerical simulations of the measurement error have been performed. The results show that this method can reduce the error in ozone measurements caused by the aerosol layer in the troposphere by a factor of ten. The proposed method is also shown to be insensitive to aerosol optical properties, and therefore, one does not need to know the wavelength dependence of aerosol scattering. The dual-DIAL with 277.1, 291.8, 313.2 nm radiation based on a Raman-shifted KrF laser can be used both during day- and night-time. The dual-DIAL with 289.0, 299.1, 316.1 nm radiation based on Raman-shifted Nd:YAG laser can only be used during night-time.     

Initial spring temperature profiles of the mesopause region over Fort Collins,CO, measured by the Colorado State Na temperature lidar

After its first measurement in late August, 1989, the new Na temperature lidar has been in operation during springs 1990 and 1991 at Fort Collins, CO. A total of nine nights (over 2600 profiles) of mesopause temperature measurements, each for a period longer than 4 hours, have been taken. We present these high quality initial profiles which demonstrate the effectiveness of the new two-frequency narrowband lidar technique for mesopause temperature measurements. The average temperature profiles suggest that the spring mesopause temperatures range from 168 K to 205 K. The mesopause heights are around 88 km before the midnight and around 99 km after the midnight. The nightly averaged temperature profiles in the mesopause region display considerable variability. The richness in new geophysical information obtainable with a Na temperature lidar is made evident by noting strong perturbations in a short time scale (15 min) in the data taken on March 11, 1990 and March 18, 1991.     

Arrival of Pinatubo disturbances in the stratospheric aerosol layer over Fort Collins,CO, observed by a lidar at 589 nm

Using the Colorado State Na Lidar, surges of stratospheric aerosols caused by the eruption of Mt. Pinatubo have been observed from August to December 1991 over Fort Collins, CO (40.6° N, 105° W), showing a clear increasing trend in stratospheric aerosol activities. These aerosol layers are characterized by the backscatter-ratio profile at 589 nm for altitudes from 12 km to 40 km.On leave from: Department of Radioelectronics, Peking University, Beijing, P.R. China     

Current state-of-the-art of LIDAR inversion methods for atmospheres of arbitrary optical density

It is the intention of this paper to report on the currently used methods to solve the different LIDAR signal inversion problems for molecular atmospheres, aerosols and clouds. Apart from more traditional approaches, we shall present a recent one using multiple scattering effects rather than avoiding them, which is useful especially for dense clouds.     

Off-axis-Gaussian beam intensity in random medium

The receiver intensity profile of an off-axis-Gaussian beam travelling in random medium is formulated. By examining the related exponential terms of this intensity expression, the rules governing the receiver plane displacements are deduced. Off-axis-Gaussian beam is characterized by introducing into a Gaussian beam, complex displacement parameters that exhibit transverse source coordinate dependent attenuation and phase shifts. Our results are applied to turbulent horizontal links. Intensity plots describing the dependence on the source and propagation parameters both on the source and the receiver planes are provided. Even though the normalized intensities of the off-axis-Gaussian beam having the same source sizes but differing displacements in x- and y-directions may look the same on the source plane, they will differentiate after propagation. The views from the progress of an off-axis-Gaussian beam along the turbulent link length show that a source displaced beam will act according to rules set by related exponential terms. An asymmetrical (ellipsoidal) off-axis-Gaussian beam will initially be converted into symmetric (circular) shape at the intermediate link lengths, then it will start to expand in the other direction, thus reverting to an ellipsoid shape whose major axis is now along the transverse coordinate opposite to that of the source plane.