Edward E. Uthe

An automatic method for determining the mixing depth from lidar observations

Abstract Lidar observations of the atmosphere reveal layers having different concentrations of aerosols. A computer method has been developed to analyze digital records of lidar backscatter in a manner that reveals the presence and heights of significant aerosol layers and the depth of the mixed layer. Boundaries between layers are represented by vertical gradients in the backscatter data. To compute the mixing depth, the backscatter gradients for a given time period (e.g. 10 min) are treated in a two-dimensional height-time array. Positive gradients and negative gradients are organized into groups that are contiguous in space and time. Then an algorithm selects one group as representing the mixing depth, on the basis of the height of each group and the strength of the gradients. Mixing depths determined automatically from the lidar records agreed with subjective estimates of mixing depth (based on data from lidar, radiosondes, and helicopter temperature soundings) 91% of the time for data collected in St. Louis, Missouri during the RAPS program of July and August, 1976. A slightly altered version of the automatic method, applied to lidar observations from the October 1977 AMBIENS experiment in Indiana, was 97% accurate. Real-time application of the method is believed to be feasible.

Airborne lidar measurements of smoke plume distribution, vertical transmission, and particle size.

Observations were made of a dense smoke plume downwind from a forest fire using the ALPHA-1 two-wavelength downward-looking airborne lidar system. Facsimile displays derived from lidar signatures depict plume dimensions, boundary layer height, and underlying terrain elevation. Surface returns are interpreted in terms of vertical transmission as function of cross-plume distance. Results show significantly greater plume attenuation at 0.53-microm wavelength than at 1.06-microm, indicating ~0.1-microm mean particle diameters or the presence of gaseous constituents that absorb the visible radiation. These results demonstrate the potential of multiple-wavelength airborne lidar for quantitative analysis of atmospheric particulate and gaseous constituents.

Regional Patterns of Mixing Depth and Stability: Sodar Network Measurements for Input to Air Quality Models

Abstract A network of 13 sodars (acoustic radars) was operated in the San Francisco Bay Area for 2.5 months of the 1976 smog season. The goal was to produce a data base on time-dependent mixing depth and stability patterns for input to air quality models. The large set (∼1000 site-days) of sodar facsimile records was filmed to provide a more compact, convenient, and accessible data product. A manual digitization scheme was devised and used to convert the continuous sodar data to hourly values describing mixing depth and a near-surface stability indicator. These values can easily be input to computerized air quality models and can readily be understood by a user with no sodas experience. Numerous sodar-inferred mixing depths were compared to those inferred from 1) simultaneous measurements of temperature and humidity profiles and 2) lidar-measured haze and cloud layering. These and previous tests show good overall agreement, demonstrating that in the San Francisco Bay Area, sodar measurements compare very ...

Lidar study of the keystone stack plume

A lidar study of the dispersion of the effluent from the tall (245-m) stacks of Keystone Generating Station in western Pennsylvania was conducted with the aid of two field experiments in May and October 1968. The observations, which were mostly obtained during stable conditions in the morning hours, reveal in detail the important plume characteristics that must be included in a realistic diffusion model for plumes from tall stacks: 1. (1) fanning and tilting due to wind veering with height. 2. (2) fumigation, which brings high concentrations to ground level in a pattern which, because of the tilting, progresses with time from the left to the right of the plume looking downwind. 3. (3) plume trapping by elevated stable layers. A comparison between the predictions of the Briggs/ASME plume rise formula and the lidar observations for 17 cases when temperature profiles were available gave a mean absolute difference of 30 m. A secondary experiment involving two sets of lidar plume measurements when the electrostatic precipitators at the power plant were all and half in operation, furnished estimates of 86 and 97 per cent for the precipitator efficiency on two separate days, compared with the rated value of 99 per cent. To investigate the potential of lidar for making quantitative measurements, a sample attenuation-corrected cross section of absolute mass concentration was computed on the basis of Mie scattering theory and independent particle size measurements. The integrated mass per unit plume length represented by this cross section was 680 gm−1, compared to 875 g m−1 calculated from the power-plant data and wind speed.

Particle size evaluations using multiwavelength extinction measurements

As the first phase of a program to develop a lidar method for remote evaluation of mean particle size of stationary source emissions, a data base was experimentally collected consisting of multiple-wavelength extinction coefficients and mean particle sizes of generated aerosols. Extinction data were collected using multi-wavelength (14) transmissometers and a 10-m long aerosol tunnel facility. Generated aerosols consisted of five size fractions of fly ash, three size fractions of silica, and single-size fractions of six other types of particulate material. Particle size evaluations were made by multistage impactor and by air permeability (Fisher) analysis of packed powder. The data base indicates that mean particle size smaller than l-microm diam could be estimated usefully from aerosol extinction measurements using a single-laser lidar system operating at 1.06- and 0.53-microm wavelengths. For larger mean particle sizes the extinction ratio is near unity, and longer wavelength systems are required. The data indicate that a two-laser lidar operating at 10.6 and 0.53 microm could provide estimates of mean particle size to diameters of at least 6 microm.

Lidar evaluation of smoke and dust clouds

Lidar provides the means to evaluate quantitatively the spatial and temporal variability of smoke and dust clouds as they are transported downwind from particulate sources. Quantitative evaluation of cloud optical and physical densities from cloud backscatter is complicated by effects from particle size, shape, and composition and by attenuation and multiple scattering for dense clouds. Examples are presented that review use of the lidar technique to provide useful evaluations of smoke and dust clouds.

Acoustic and direct measurements of atmospheric mixing at three sites during an air pollution incident

Abstract Simultaneous measurements at three sites in the San Francisco Bay Area were used to document the depth and vigor of atmospheric mixing before, during, and after an air pollution incident. The measurements included sodar (acoustic radar) sensing and direct temperature profile measurements. The sodar records graphically documented the transition in atmospheric behavior from days with long periods of near-surface instability and relatively large mixing depth, to days with long periods of near-surface stability and relatively small mixing depth, and back again. The sodar records were in turn corroborated by the direct temperature profile measurements (i.e. time-average differences between acoustically inferred and directly measured inversion base heights were small compared to the absolute heights). The development of poor mixing conditions was accompanied by increased pollution near the surface, which later dissipated with the return of good mixing conditions. We conclude that sodar measurements can provide very useful inputs to Bay Area air quality simulation models, but caution against indiscriminately extrapolating the present results to areas or seasons with significantly different meteorology.

Airborne CO2 DIAL measurement of atmospheric tracer gas concentration distributions.

An airborne differential absorption lidar system employing high-energy line-tunable CO2 lasers has been used to map cross-plume vertical distributions resulting from a near-surface SF6 tracer gas release. The remote SF6 tracer measurement technique may be suitable to evaluate distributions of toxic and hazardous materials accidentally released into the atmosphere providing tracer gas is also released during the accident. The technique also may provide transport and diffusion data needed for development and validation of atmospheric computational models that predict downwind distribution of materials released into the atmosphere.

Airborne Lidar Plume and Haze Analyzer (ALPHA-1)

A new two-wavelength airborne lidar system has been constructed and field-tested. The system was designed to observe the distribution of particle concentrations over large regional areas. During a ...

Airborne Lidar Tracking of Fluorescent Tracers for Atmospheric Transport and Diffusion Studies

Development and validation of transport models for the study of regional acid deposition require improved observations of pollutant transport and dispersion processes. No suitable method for air-parcel tracking along nonconstant density surfaces is available. The feasibility of using an airborne lidar system to observe atmospheric transport and dispersion of fluorescent-dye-particle (FDP) tracers was demonstrated for various meteorological conditions and FDP-release scenarios in the general area of the Cross-Appalachian Tracer Experiment (CAPTEX) during October 1983. This paper presents some of the results obtained on six case studies, each of which illustrates a unique application of the technique.