Charles Robert Quick

Development of a scanning, solar-blind, water Raman lidar

The need for an instrument capable of measuring water-vapor fluxes over mixed canopy and large areas has long been recognized. Such a device would greatly enhance the study of evapotranspiration processes and has great practical value for water management. To address this problem, a scanning water Raman lidar has been designed and constructed. Analytical methods have also been developed to take advantage of the type of information that this lidar can generate. The lidar is able to measure the absolute water content and calculate the evaporative flux quickly over relatively large areas. This capability provides new opportunities for the study of microscale atmospheric processes. The variogram data indicate that the spatial sampling size must be of the order of 10 m if fluxes and scalars are to be properly represented. Examples of data are presented.

Spatial variability of water vapor turbulent transfer within the boundary layer

Although the physics of evaporation within the inner region of the boundary layer is believed to be well understood, observations of mass-energy exchange processes have been hindered by the limitations of point sensors. A combination of point sensors and active remote sensing, namely, water-Raman Lidar measurements, offers new opportunities to study relatively large areas at temporal and spatial scales previously unattainable. Results from experiments over uniform canopies both confirm some traditional theories and challenge some of the underlying assumptions concerning the homogeneity of the surface-atmosphere interface and the use of point sensors to characterize large areas.

Wave optics simulation of atmospheric turbulence and reflective speckle effects in CO 2 lidar

Laser speckle can influence lidar measurements from a diffuse hard target. Atmospheric optical turbulence will also affect the lidar return signal. We present a numerical simulation that models the propagation of a lidar beam and accounts for both reflective speckle and atmospheric turbulence effects. Our simulation is based on implementing a Huygens-Fresnel approximation to laser propagation. A series of phase screens, with the appropriate atmospheric statistical characteristics, are used to simulate the effect of atmospheric turbulence. A single random phase screen is used to simulate scattering of the entire beam from a rough surface. We compare the output of our numerical model with separate CO(2) lidar measurements of atmospheric turbulence and reflective speckle. We also compare the output of our model with separate analytical predictions for atmospheric turbulence and reflective speckle. Good agreement was found between the model and the experimental data. Good agreement was also found with analytical predictions. Finally, we present results of a simulation of the combined effects on a finite-aperture lidar system that are qualitatively consistent with previous experimental observations of increasing rms noise with increasing turbulence level.

Derivation of water vapor fluxes from Lidar measurements

Two techniques are described by which the flux of water vapor can be derived from concentration measurements made by a Raman-Lidar. Monin-Obukhov similarity theory and dissipation techniques are used as the basis for these methods. The resulting fluxes are compared to fluxes from standard point instruments. The techniques described are appropriate for measuring the flux of any scalar quantity using Lidar measurements in the inner region of the boundary layer.

Quantitative chemical identification of four gases in remote infrared (9–11 µm) differential absorption lidar experiments

A combined experimental and computational approach utilizing tunable CO(2) lasers and chemometric analysis was employed to detect chemicals and their concentrations in the field under controlled release conditions. We collected absorption spectra for four organic gases in the laboratory by lasing 40 lines of the laser in the 9.3-10.8-mum range. The ability to predict properly the chemicals and their respective concentrations depends on the nature of the target, the atmospheric conditions, and the round-trip distance. In 39 of the 45 field experiments, the identities of the released chemicals were identified correctly without predictions of false positives or false negatives.

Quenching of C2H emission produced by vacuum ultraviolet photolysis of acetylene

Excited C2 H* is produced by vacuum ultraviolet photolysis of acetylene using a frequency tripled laser. Time‐dependent emission is measured from 400–940 nm. The use of a coherent photolysis source produces an excitation spectrum in which the rotational band contour is resolved. The absorption spectrum of C2 H2 taken in the same apparatus closely resembles the excitation spectrum indicating a homogeneous predissociation. Time‐dependent quenching of the C2 H* emission by Xe, Kr, Ar, He, N2, H2, D2, and C2 H2 is measured. The rapid quenching rates and lack of strong dependence on atomic weight suggest a spin‐allowed process is involved in this channel of C2 H2 photolysis. Quench rates are compared with several theoretical models.

Time-resolved fluorescence measurements of KrF emission produced by vacuum ultraviolet photolysis of KrF2 and Kr/F2 mixtures

Vacuum ultraviolet (VUV) light radiation was used to produce electronically excited KrF excimers (in D-, B- and C-states) by the photolysis of KrF2 and F2/Kr mixtures at various excitation wavelengths. The excited KrF photoproduct quantum yield was measured over the excitation wavelength range of 120 to 200 nm, and a quantum efficiency of 0.11 was estimated at the peak absorption wavelength of 159 nm for KrF2. The collision-free fluorescence lifetime of the B-X transition near 248 nm was determined to be 9.5 ± 0.6 ns when the KrF2 was excited with the 159 nm light. Near gas kinetic rate constants were measured for the quenching of KrF B-X emission by KrF2 and CO2. Using the threshold wavelength needed for the production of excited KrF photofragments, an upper bound for the bond dissociation energy of KrF2 was determined to be 1.03 ± 0.05 eV.

Atmospheric effects on CO2 differential absorption lidar sensitivity

The ambient atmosphere between the laser transmitter and the target can affect CO2 differential absorption lidar (DIAL) measurement sensitivity through a number of different processes. In this work, we will address two of the sources of atmospheric interference with CO2 DIAL measurements: effects due to beam propagation through atmospheric turbulence and extinction due to absorption by atmospheric gases. Measurements of atmospheric extinction under different atmospheric conditions are presented and compared to a standard atmospheric transmission model (FASCODE). We have also investigated the effects of atmospheric turbulence on system performance. Measurements of the effective beam size after propagation are compared to model predictions using simultaneous measurements of atmospheric turbulence as input to the model. These results are also discussed in the context of the overall effect of beam propagation through atmospheric turbulence on the sensitivity of DIAL measurements.

Spectroscopic analysis of infrared DIAL measurements

A combined experimental and computational approach utilizing CO2 infrared gas lasers and chemometric multivariate analysis was employed to detect chemicals and their concentrations in the open atmosphere under controlled release conditions. Absorption spectra of four organic gases were collected in the laboratory by lasing 40 lines of a Synrad 15 W CO2 laser in the 9.3 to 10.8 micron range. Several chemometric calibration models were constructed based on this IR data using the Partial Least Squares computational technique. The chemometric models were used to analyze in near real time the field DIAL data acquired over this exact wavelength range at round trip distances of 7 and 13 km. It will be shown that the ability to predict the chemicals and their respective concentrations depends on a variety of factors. In 39 of the 45 experiments, the identities of the released chemicals were correctly identified without predictions of false positives or false negatives. Under the best field conditions, we achieved predictions of absolute concentrations within 30% of the actual values.

Using active TIR imaging for ground cover identification and mapping

The authors report examples of the use of a scanning tunable CO/sub 2/ laser lidar system in the 9-11 ym region to construct images of vegetation and rocks at ranges of up to 5 km from the instrument. Range information is combined with horizontal and vertical distances to yield an image with three spatial dimensions simultaneous with the classification of target type. Reflectance spectra in this region are sufficiently distinct to discriminate between several tree species, between trees and scrub vegetation, and between natural and artificial targets. Lidar sensing of vegetation offers some complementary characteristics to passive remote sensing. In the thermal infrared (8-12 /spl mu/m), lidar interrogation of natural targets is essentially a pure reflectance measurement, unaffected by topographical shading and spatial variations in temperature. Differential measurements using CO/sub 2/ lasers have been known for some time to be useful in discriminating between vegetation types, tree species, and rock types. The authors have investigated the utility of a scanning CO/sub 2/ DIAL system in constructing vegetation maps for broad areas.