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Remote Sensing

Lidar probes air pollution

Lidar effectively detects and characterizes air contaminants with spatial and temporal resolution.
21 February 2008, SPIE Newsroom. DOI: 10.1117/2.1200702.1096

Our research group has one of the most useful systems for studying the presence of atmospheric pollutants (ozone, SO2, NO2, benzene, toluene, and p-xylene, among others). Researchers dream of developing models to explain complex phenomena. However, even the best models require real data to validate the predictions. For atmospheric dispersion models, a light detection and ranging (lidar) system is the only instrument capable of providing the type of data needed to validate those models. (Along with sodar and radar, lidar is a member of the detection and ranging instrumentation family.)

Our system uses differential absorption lidar, meaning that the concentration of a given pollutant with a known absorption spectrum is calculated based on the differential absorption of two wavelengths, λon and λoff. With this technique, we use the power of the backscattered wavelengths to perform the calculations, so no estimates are required for the lidar equation.1

Figure 1. Screen from lidar system. The image represents the backscatter signal of a pair of wavelengths used to measure toluene. In this experiment, a known amount of toluene was released in a chamber and measured 275m away from the lidar. The blue line represents the λon. The red line is λoff, and, it is unaffected by the presence of toluene.

Lidar is well suited for the remote measurement of pollutants, with numerous applications depending on the purpose. The most obvious use is to track the evolution of a pollutant over time. If the lidar's laser beam is oriented vertically, the lidar acts as a profiler. If we change the vertical angle of the laser beam, we have a succession of alignments that, with the proper interpolation, defines a concentration plane. The profiler is the usual configuration of the lidars, providing very valuable information, such as the depth of the planetary boundary layer2 and the evolution of the concentration of ozone during a summer episode3 at elevations ranging from 250m to 2km. The mobile telescope configuration, in which the angle is varied, is more complex and thus less frequently used. In our opinion, the information given by a series of concentration planes is more valuable than that from a series of vertical alignments. For example, the use of concentration planes and wind fields enables us to identify the geographical position of the source of the pollutants. This is of particular interest for ozone pollution at night.4,5

Figure 2. The graph displays the propagation of sulfur dioxide downwind of the power plant in the photograph. The lidar was located beside the chimneys. The lidar data and the power plant picture were taken at the same time. Concentrations are in μg m−3.

One useful application is detecting leakage of organic pollutants in storage facilities and industrial plants, such as oil refineries.6 Using conventional sampling techniques to detect such leakages entails costly field work to complete a survey of the site. By contrast, with a lidar located offsite, it is possible to perform a horizontal scan to obtain a concentration plane in less than 30 minutes. This data can be introduced into a geographic information system to provide immediate recognition of a leak and its location. Once the problem has been identified, closer inspection is needed to establish the exact source. Figure 1 shows how the lidar signal is affected by the presence of toluene. Notice how λon, the wavelength where absorption from the pollutant is at a maximum, changes its shape when the pollutant is present.

Another interesting application of the technology is plume tracking.7 Where do pollutants go when exhaust gas is released from a stack? Though it may appear easy to determine the answer by looking at the smoke trajectory, costly monitoring stations are needed to rigorously define the way pollutants disperse in the atmosphere and their concentrations at different locations. These stations provide information on the presence and concentration of pollutants at ground level, but little or no information on the plume behavior. In contrast, lidar enables live monitoring of the plume, with observation in minute detail of the dispersion process. Figure 2 shows the propagation of a sulfur dioxide plume from a power plant, and how this phenomenon is monitored by lidar data.

Unfortunately, lidar is expensive to buy, has a high operating cost, and requires a skilled operator. However, these drawbacks are more than compensated for by its capabilities. Our research group is interested to explore new applications that may be suggested by SPIE members.

Jose Moreno, Stella Moreno-Grau, Antonio García-Sánchez 
Chemical and Environmental Engineering
Technical University of Cartagena
Cartagena, Spain

José‚ Moreno is an associate professor, and the current head of the Atmospheric Chemistry research group. He obtained his PhD from the University of Murcia in 1998. His research interests focus on photochemical pollution and atmospheric aerosol characterization.

Stella Moreno is a professor and leads the Aerobiology and Environmental Toxicology research group. She is responsible for the postgraduate program in the department.

Antonio García-Sánchez is a professor with extensive experience in signal processing. He has been working on speech recognition and now is in charge of handling all the data obtained with the lidar.