Thomas D. Wilkerson

Water vapor differential absorption lidar development and evaluation

A ground-based differential absorption lidar (DIAL) system is described which has been developed for vertical range-resolved measurements of water vapor. The laser transmitter consists of a ruby-pumped dye laser, which is operated on a water vapor absorption line at 724.372 nm. Part of the ruby laser output is transmitted simultaneously with the dye laser output to determine atmospheric scattering and attenuation characteristics. The dye and ruby laser backscattered light is collected by a 0.5-m diam telescope, optically separated in the receiver package, and independently detected using photomultiplier tubes. Measurements of vertical water vapor concentration profiles using the DIAL system at night are discussed, and comparisons are made between the water vapor DIAL measurements and data obtained from locally launched rawinsondes. Agreement between these measurements was found to be within the uncertainty of the rawinsonde data to an altitude of 3 km. Theoretical simulations of this measurement were found to give reasonably accurate predictions of the random error of the DIAL measurements. Confidence in these calculations will permit the design of aircraft and Shuttle DIAL systems and experiments using simulation results as the basis for defining lidar system performance requirements.

Observed coupling of the mesosphere inversion layer to the thermal tidal structure

Rayleigh lidar observations of mesosphere temperature profiles obtained from 40 to ∼100 km from Logan, Utah (41.7, 111.8 W, altitude, 1.9 km) over 10 nights in late February, 1995, revealed an interesting development between 60 to 75 km of a winter mesosphere inversion layer with an amplitude of ∼20–30 K and a downward phase progression of ∼1 km/hr. The data also showed two altitude regions exhibiting significant cooling of 10–30 K in extent. These were located below and above the peak of the inversion layer, respectively, at altitudes of ∼50–55 km and ∼70–80 km. When these results were compared with the predictions of a global wave scale model (GSWM), the observed thermal mesosphere structure is similar to the computed composite tidal structure based upon the semi-diurnal and diurnal tides with the exception that observed amplitudes of heating and cooling are ∼10x larger than predicted GSWM values. We suggest that these events over Utah are caused through a localized mechanism involving the coupling of gravity waves to the mesopause tidal structure.

Intensities and N2 collision-broadening coefficients measured for selected H2O absorption lines between 715 and 732 nm☆

Abstract Intensities and N2 collision-broadening coefficients were measured for 62 lines of H2O vapor between 715 and 732 nm. The lines selected are potentially useful for remote laser measurements of H2O vapor in the earths atmosphere. The spectra were obtained with several different H2O vapor abundances and N2 broadening gas pressures; the spectral resolution was 0.05 cm-1. Measured H2O line strengths range from 4 × 10-25 to 4 × 10-23 cm-1/(molec./cm2), and N2 collision broadening coefficients are approx. 0.1 cm-1/atm.

Multiple Stokes wavelength generation in H 2 , D 2 , and CH 4 for lidar aerosol measurements

We report experimental results of multiple Stokes generation of a frequency-doubled Nd:YAG laser in H(2), D(2), and CH(4) in a focusing geometry. The energies at four Stokes orders were measured as functions of pump energy and gas pressure. The characteristics of the Stokes radiation generated in these gases are compared for optical production of multiple wavelengths. The competition between Raman components is analyzed in terms of cascade Raman scattering and four-wave mixing. The results indicate the possibility of using these generation processes for atmospheric aerosol measurements by means of multiwavelength lidar systems. Also this study distinguishes between the gases, as regards the tendency to produce several wavelengths (H(2), D(2)) versus the preference to produce mainly first Stokes radiation (CH(4)).

Water absorption line, 931–961 nm: selected intensities, N2-collision-broadening coefficients, self-broadening coefficients, and pressure shifts in air☆

Intensities were measured for 97 lines of H2O vapor between 932 and 961 nm. The lines were selected for their potential usefulness for remote laser measurements of H2O vapor in the earths atmosphere. The spectra were obtained with several different H2O vapor abundances and N2 broadening gas pressure; the spectral resolution was 0.046 cm−1 FWHM. Measured H2O line intensities range from 7 × 10−25 to 7 × 10−22 cm−1/molecules/cm2. H2O self-broadening coefficients were measured for 13 of these strongest lines; the mean value was 0.5 cm−1/atm. N2-collision-broadening coefficients were measured for 73 lines, and the average was 0.11 cm−1/atm HWHM. Pressure shifts in air were determined for a sample of six lines between 948 and 950 nm; these lines shift to lower frequency by an amount comparable to 0.1 of the collision-broadened widths measured in air or N2. The measured intensities of mainly lines of the 300-000 band are much larger than expected from prior computations, in some cases by over ab order if magnitude. Coriolis interactions with the stronger 201-000 band appear to be the primary cause of the enhancement of these line intensities.

Aglite lidar: a portable elastic lidar system for investigating aerosol and wind motions at or around agricultural production facilities

The Aglite Lidar is a portable scanning lidar that can be quickly deployed at agricultural and other air quality study sites. The purpose of Aglite is to map the concentration of PM 10 and PM 2.5 in aerosol plumes from agricultural and other sources. Aglite uses a high-repetition rate low-pulse energy 3-wavelength YAG laser with photon-counting detection together with a steerable pointing mirror to measure aerosol concentration with high spatial and temporal resolution. Aglite has been used in field campaigns in Iowa, Utah and California. The instrument is described, and performance and lidar sensitivity data are presented. The value of the lidar in aerosol plume mapping is demonstrated, as is the ability to extract wind-speed information from the lidar data.

Aglite lidar: calibration and retrievals of well characterized aerosols from agricultural operations using a three-wavelength elastic lidar

Lidar (LIght Detection And Ranging) provides the means to quantitatively evaluate the spatial and temporal variability of particulate emissions from agricultural activities. AGLITE is a three-wavelength portable scanning lidar system built at the Space Dynamic Laboratory (SDL) to measure the spatial and temporal distribution of particulate concentrations around an agricultural facility. The retrieval algorithm takes advantage of measurements taken simultaneously at three laser wavelengths (355, 532, and 1064 nm) to extract particulate optical parameters, convert these parameters to volume concentration, and estimate the particulate mass concentration of a particulate plume. The quantitative evaluation of particulate optical and physical properties from the lidar signal is complicated by the complexity of particle composition, particle size distribution, and environmental conditions such as heterogeneity of the ambient air conditions and atmospheric aerosol loading. Additional independent measurements of particulate physical and chemical properties are needed to unambiguously calibrate and validate the particulate physical properties retrieved from the lidar measurements. The calibration procedure utilizes point measurements of the particle size distribution and mass concentration to characterize the aerosol and calculate the aerosol parameters. Once calibrated, the Aglite system is able to map the spatial distribution and temporal variation of the particulate mass concentrations of aerosol fractions such as TSP, PM 10, PM 2.5, and PM 1. This ability is of particular importance in the characterization of agricultural operations being evaluated to minimize emissions and improve efficiency, especially for mobile source activities.

A self-seeded SRS system for the generation of 1.54 μm eye-safe radiation

Abstract A light source is described for the efficient generation of 1.54 μm for eye-safe aerosol lidar operation. The system is based upon a Nd:YAG laser at 1.06 μm which is then Raman-shifted in methane to produce light at the first Stokes wavelength of 1.54 μm. First Stokes light generated in the backward direction was retroreflected back into the Raman cell for amplification in the tail of the 10 ns pump beam. The energy conversion efficiency and the spatial beam quality of the amplified first Stokes were found to be adversely affected when operated at higher repetition rates due to a thermal gradient produced in the generation region. A Stokes energy of 25 mJ was obtained for a pumping energy of 140 mJ at a repetition rate of 10 Hz. The beam divergence of the amplified Stokes radiation was measured to be less than 1 mrad. The optimized results demonstrate the applicability of this radiation for eye-safe lidar measurements.

Lidar Based Emissions Measurement at the Whole Facility Scale: Method and Error Analysis

Particulate emissions from agricultural sources vary from dust created by operations and animal movement to the fine secondary particulates generated from ammonia and other emitted gases. The development of reliable facility emission data using point sampling methods designed to characterize regional, well-mixed aerosols are challenged by changing wind directions, disrupted flow fields caused by structures, varied surface temperatures, and the episodic nature of the sources found at these facilities. We describe a three-wavelength lidar-based method, which, when added to a standard point sampler array, provides unambiguous measurement and characterization of the particulate emissions from agricultural production operations in near real time. Point-sampled data are used to provide the aerosol characterization needed for the particle concentration and size fraction calibration, while the lidar provides 3D mapping of particulate concentrations entering, around, and leaving the facility. Differences between downwind and upwind measurements provide an integrated aerosol concentration profile, which, when multiplied by the wind speed profile, produces the facility source flux. This approach assumes only conservation of mass, eliminating reliance on boundary layer theory. We describe the method, examine measurement error, and demonstrate the approach using data collected over a range of agricultural operations, including a swine grow-finish operation, an almond harvest, and a cotton gin emission study.

Two-photon lidar technique for remote sensing of atomic oxygen.

A technique is proposed for remote sensing of atomic oxygen by laser excitation of a two-photon transition. Excitation at 2256 A and subsequent fluorescence at 8447 A allow probing of the mesosphere and lower thermosphere without interference by absorption. The two-photon excitation rate is calculated, and its dependence on laser power, duration, and linewidth is discussed. Values are given for remote sensing from a spacecraft at 200 km ranging down to 80 km with 1-km range resolution. Accuracies of better than 10% are expected with a 1 J, a 20-psec excitation pulse, and a 1-m(2) collecting telescope.