Research Projects

Project at a Glance

Project Status: complete

Title: Utilization of remote sensing data in the evaluation of air pollution characteristics in the south coast.

Principal Investigator / Author(s): Smith, Ted B

Contractor: Ted B. Smith and Associates, Inc.

Contract Number: A2-106-32


Research Program Area: Atmospheric Processes


Abstract:

Lidar aircraft data were collected in July 1981 by EPA (McElroy et al, 1982) in conjunction with the Southeast Desert Transport Study (Smith et al, 1983). The lidar aircraft was flown on 10 days during the study period. The flight schedule was coordinated with the tracer releases for the Transport Study. On eight of the flight days, one or two flights were made on the day of the tracer release, followed by a morning flight on the following day to evaluate the pollutant carry-over in the desert.

The present report represents the first opportunity for analysis of the lidar data in conjunction with the data obtained during the Transport Study. The analyses and data used in the previous study were of very great value in interpretation of the lidar data.

The EPA aircraft lidar system in 1981 was relatively new and the system operation improved substantially during the 10-day program, particularly with respect to noise. The lidar system operates on two different wave lengths, the present analyses were performed using the green portion of the system.

The aircraft flew at about 3000 m-msl during the flight program. The lidar is mounted to direct its beam vertically toward the ground. Back-scatter returns from aerosols within the 3000 m layer are recorded as a function of distance below the aircraft. The data were subsequently processed by EPA into a gray-scale representation (McElroy et al, 1982) and a digital format using bin averages 150 m deep and one km along the flight path.

In contrast to radar operations, techniques for quantitative calibration of the lidar system have not been developed. The size distribution of the aerosols is also not known for realistic atmospheric targets. Attenuation of the lidar signals due to heavy aerosol loading is an additional limitation on the quantitative analysis of the data. In spite of these limitations, semi-quantitative comparisons between lidar back-scatter measurements and visibility on bscat observations were reasonably satisfactory.

The principal value of the lidar data for the present study was to provide information on relative aerosol concentrations aloft where measurements from the surface were either not available or not possible. Unique data on the presence of upslope flow along the San Gabriel Mts., of convergence zones and of layers aloft were provided by the lidar observations. Of particular interest were cross sections through the San Fernando Valley and Elsinore Convergence Zones which graphically illustrate these two important mechanisms for removal of pollutants from the Los Angeles basin.

A number of lidar aircraft flights were made through the passes which form additional exit routes from the basin into the Southeast Desert Air Basin. Estimates of the aerosol loadings were calculated from the lidar data by summing the bin-averaged back-scatter values within the mixed layer for each traverse through the pass. These data are presented as a function of time of day. Results when grouped according to the individual pass clearly show that San Gorgonio Pass carried by far the heaviest aerosol loading during the flight measurement period.

Data taken on the desert side of San Gorgonio and Cajon Passes show the top of the aerosol layer descending rapidly with distance traveled into the desert. This results in a shallow layer at the entrance points into the desert which frequently breaks up into a deeper convective layer because of the strong surface heating.

The most interesting case of carry-over into the desert occurred on the morning of July 31. Substantial transpost into the desert had occurred during the previous afternoon. Heavy aerosol loadings were observed by the lidar on the 31st in the Coachella Valley. Reported visibilities were reduced to 5-7 miles during the forenoon. It is speculated that high humidity in the desert may have contributed to these unusual values.

Within the South Coast Air Basin lidar aircraft observations would be particularly useful in studying upslope flow, convergence zones, recirculation of pollutants, pollutant transport through passes and total aerosol burden. All of these problems involve pollutant concentrations aloft which are not easily observed by other means. The lidar observations may therefore be extremely valuable both as a data source and an analytic tool when applied to complex photochemical models. Interpretation of the lidar data requires sufficient upper wind information to reconstruct possible source regions of the aerosols. It is also suggested that repeated flight paths during the day over the same geography would help greatly in defining the time evolution of such processes as the convergence zones and upslope flows.


 

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