In the study of the atmosphere, aerosols are the least-known geophysical parameters. The reason for studying them is their major influence on the terrestrial radative balance, as they have a direct effect on the ground temperature. Aerosols are natural or man-produced and usually place themselves in the troposphere’s lower part. This area of aerosol concentration is called Atmospheric Boundary Layer (ABL).

There are several systems for the remote sensing of the atmosphere. They are divided into two groups: active and passive. The lidars are active measurement systems, because in order to obtain information it is necessary to excite the atmosphere’s molecules with laser pulses and then, with the obtention of the measure of the backscattered photons, their characteristics are also obtained.

The lidars are very powerful systems: they allow to obtain discriminated  information according to the altitude, with a high spatial resolution, thus providing a more detailed information than that offered by other remote sensing instruments, such as the sun photometers or the radiometers. The atmospheric parameters to be studied are: the volumetric backscatter coefficient (β) , the extinction coefficient (α), the aerosol optical thickness  (A.O.T.) and the evolution of the atmosphere’s boundary layer. It is possible to distinguish three main blocks within a lidar system:

1)     Laser emission system

2)     Reception system

3)     Data acquisition system

There are two different ways of acquiring the signal from the photodetectors: a) photocurrent and b) photocounting. The selection of the method depends on the photons’ arrival speed. Depending on their efficiency, the photodetectors generate current pulses derived from the detected photons. If such pulses can be discriminated by the data acquisition system so that it is possible to count them, the photocounting technique is used. This method is used for ozone measurements due to the low rate of photon arrival, since they come from the stratosphere.

When the counting of the pulses proceeding from the photodetectors can not be performed due to a high rate of photon arrival into the detector, the photocurrent technique is used. It consists in digitalizing the signal and storing its values. This is the technique used for aerosol lidars, since the photon arrival rate is high and produces a “piling up” in the pulses provided by the photodetectors. This makes them impossible to discriminate and virtually prevents the possibility of performing the counting. The usual resolution of each of the samples of these systems is of 12 bits. The sampling speed will provide us with the spatial resolution of the LIDAR system, which we can obtain through the following formula:

where ΔΖ is the spatial resolution, Tm the sampling period and c the speed of light.

The typical sampling frequencies are around 25 MHz. For this frequency, a 6 meter spatial resolution is obtained. Thus, the rough information obtained by the LIDAR system is a measure of the photons backscattered over the atmosphere, discriminated according to the altitude. This information must be processed to obtain the wanted atmosphere parameters, which are the coefficient of volumetric backscattering (β) and the coefficient of extinction (α).