Lidar remote sensing

The basis for lidar remote sensing lies in the interaction of light with gas molecules and particulate matter in suspension in the atmosphere (aerosols). More particularly, a lidar uses a laser (emitter) to send a pulse of light into the atmosphere and a telescope (receiver) to measure the intensity scattered back (backscattered) to the lidar. By measuring the scattering and attenuation experienced by the incident pulse of light, one can investigate the properties of the scatterers (concentration of gaseous species, aerosol distribution and optical properties, cloud height) located in the atmosphere. The light scattered back to the detector comes from various distances, or ranges, with respect to the lidar. Because the light takes longer to return to the receiver from targets located farther away, the time delay of the return is converted into a distance (range) between the scatterers and the lidar, since the speed of light is a well-known quantity. By pointing the laser beam in various directions and at various angles with respect to the ground surface (scanning), a ground-based lidar system can gather information about the three-dimensional distribution of aerosols in the atmosphere.

from: www.nsf.gov/geo/egch/solar/gc_solar_cedar.html

The backscattered radiation detected by a lidar is described by the lidar equation. In general terms, the received power is expressed as a function of range R. For a simple backscatter lidar (measuring backscattered light at the same wavelength as the laser wavelength), the lidar equation is written as:

                            (1)

where P_r is the power returned to the lidar at the laser wavelength ( ), C is the lidar constant, R is the range, h=c·t_p, where t_p is the pulse duration and c the speed of light. The term O(R) describes the overlap between the laser beam and the receiver field of view. The term is equal to 1 for ranges where there is complete overlap of the laser beam and the receiver’s field of view. Here,  and  are the combined aerosol and molecular backscatter and extinction coefficients respectively, at the laser wavelength. The combined backscattering coefficient can be re-written as the sum of molecular and aerosol backscattering ( ). For an elastic backscatter (one wavelength) lidar, this combined backscattering can be obtained by solving the lidar equation following the method suggested by Fernald (1984).

With a Raman lidar, more information is available. Independent retrievals of aerosol backscatter and extinction can be obtained (Ansmann et al., 1990; Ansmann et al., 1992). A Raman lidar is able to detect specific gaseous species (O₂, N₂ or H₂O) by measuring the wavelength-shifted radiation returned to the lidar due to inelastic scattering by the gas molecules. The lidar equation describing the return at the Raman-shifted wavelength ( ) is written as:

          .      (2)

The first term in the exponential term describes the extinction of the laser beam (at laser wavelength) going up toward the target while the second term describes the extinction of the return signal back toward the lidar (at Raman-shifted wavelength). The inelastic Raman backscattering coefficient ( ) is only associated to the inelastic molecular scattering and is not affected by aerosol scattering. Returns at the laser wavelength and at the Raman-shifted wavelengths can be combined in various ways to obtain information about the aerosols and the water vapor content of the atmosphere. More details about the products available from the CART Raman lidar, as they pertain to this study, and how parameters are derived are provided hereafter.  

A Raman lidar designed for 24-hour automated operations has been making measurements at the Atmospheric Radiation Measurement (ARM) Southern Great Plains (SGP) Clouds and Radiation Testbed (CART) near Lamont Oklahoma for a few years now. It is a vertically pointing (non-scanning) lidar so it provides vertical profiles of various aerosol optical properties over the site, as well as profiles of water vapor mixing ratio (Turner et al., 2002). The CART Raman lidar uses a frequency tripled Nd:YAG (neodymium:yttrium/aluminium/garnet) laser transmitting 350 mJ pulses of 355 nm light at 30Hz. The backscattered light is collected with a 61-cm telescope. The system measures backscattered light at the laser wavelength (355 nm), as well as at 387 and 408 nm wavelengths. These correspond to the Raman-shifted nitrogen (N2) and water vapor (H2O) wavelengths respectively.

from: www.arm.gov/general/photolibrary/ ramanlidar.html

Products available from the CART lidar are listed in Table I. Various algorithms are used to obtain these products from the return signals measured at the three wavelengths. A typical vertical resolution of the products is of the order of 40m.

Table I. Automated data products from the CART Raman lidar.

The inversion algorithms are based on Ansmann et al. (1992). A brief overview of how some of these products are derived is given here. More detailed information is provided in the Appendix.

The aerosol scattering ratio (ASR) is defined as the ratio of the total (aerosol + molecular) backscattering to the molecular backscattering:

                                                 ,  (3)

where  is the aerosol backscattering coefficient and is the molecular (Rayleigh) backscattering coefficient. The elastic return at the laser wavelength depends on both the Rayleigh and Mie (molecular and aerosol) scattering, while the Raman-shifted return is only a function of molecular scattering. Therefore, the ratio of these return signals is proportional to the ASR. Profiles of the ASR are thus derived from the ratio of the signal detected at the laser wavelength to the signal of the N₂ Raman channel (Ferrare et al., 2001). Corrections are applied to account for the difference between the atmospheric transmission of the return signal at the laser wavelength and the return signal at the Raman N₂ wavelength (Ferrare et al.,1998, Ferrare et al. 2001). Another correction is applied to take into account that the laser beam is not fully within the detector field of view until a height of about 800m (Ferrare et al., 1998).

The molecular backscattering ( ) can be estimated using air density profiles obtained from radiosondes or from a co-located ground-based Atmospheric Emitted Radiance Interferometer (AERI). Then, the ASR profile is used in conjunction with the molecular (Rayleigh) backscattering profile to obtain the corresponding aerosol backscattering ( ) profile.

Raman lidars have the distinct advantage of providing profiles of water vapor in the same atmospheric volume as the aerosol measurements. The mass of water vapor to the mass of dry air (mixing ratio) is proportional to the ratio of the H2O Raman signal and the N2 Raman signal (Turner et al., 2002). The same type of corrections for the wavelength dependence of transmission and the overlap function below 800m are applied here as in the case for aerosol retrieval. The relative humidity is then derived by combining the water vapor mixing ratio and temperature profiles measured either from radiosondes or the Atmospheric Emitted Radiance Interferometer (AERI).

The capabilities of the CART Raman lidar makes it an interesting tool to study the effect of hygroscopic aerosols on lidar backscatter. Data from the Raman lidar are used here to assess the relationship between hygroscopic aerosol backscattering coefficient and relative humidity for continental aerosols over the ARM-SGP CART site.

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