Basic Principles of Surface-Based Remote Sensing - Surface Based Remote Sensing of the Atmospheric Boundary Layer - page 51

disturbances. Eye safety has to be ensured when operating LIDARs with strong
light sources (laser) following international guidelines such as IEC 60825-1.

3.5.1 Backscatter (Aerosol) LIDAR

A backscatter LIDAR is employed to determine aerosol and trace gas profiles in the
atmosphere. The first such application is described in Fiocco and Smullin ( 1963 ).
The intensity of the backscattered signal P R is expressed by the LIDAR equation
(Menut et al. 1999 ):

r − 2 ( c , r )] e − 2 σ r P R ( λ , r ) = τ A ε/ 2) P 0 [ β m ( λ , r ) + β p ( λ + P bg , (3.17)

with the distance r between the LIDAR and the backscattering object, the speed of
light c , the pulse duration , the antenna area A , a correction term for the detec-

tor efficiency and losses due to the lenses

τ

, the emitted energy P 0 , the backscatter

coefficient for molecules β m and for particles β p , the absorption of light in the

atmosphere , and the background noise P bg . The background noise also com-

prises scattering of light from other sources (e.g. the sun) into the optical axis of the
receiving telescope. The ratio of both terms on the right-hand side of (3.17) is the
signal-to-noise ratio (often abbreviated as SNR). The noise- and distance-corrected
signal P RSC can be calculated from P R :

σ

P bg ) r 2 , r )] T 2 ( P RSC ( λ , r ) = ( P R − = ( c τ A ε/ 2) P 0 [ β m ( λ , r ) + β p ( λ λ , r ), (3.18)

e − σ r . Normalizing with the emitting power P 0 yields with the transmission T =

P 0 ) r 2 , r )] T 2 ( P RC ( λ , r ) = (( P R − P bg ) / = ( c τ A ε/ 2) [ β m ( λ , r ) + β p ( λ λ , r ). (3.19)

Due to the varying aerosol content of the atmosphere the measurement of a trace
gas concentration with a simple backscatter LIDAR is not possible, because a sepa-
ration of the two influences of

β p on P R is not possible without additional
assumptions. Furthermore, also assumptions on the transmission T are necessary,
the most fundamental problem of LIDAR trace substance measurements. Usually,
this problem is circumvented by assuming an aerosol type-dependent fixed ratio
between backscatter and absorption in spectral ranges free of molecule absorption.

β m and

3.5.1.1 Ceilometer

Ceilometers are simple backscatter LIDARs that merely record the optical backscat-
ter intensity in the near infrared at about 0.9

m (Fig. 3.1 ). The light emission into
the vertical direction is pulsed (10 kHz) with a pulse duration of 110 ns. Due to the
chosen wave length and the restricted emission power (11 W peak), these instru-
ments are eye-safe (Münkel et al. 2003 , Weitkamp 2005 ) and can be run mostly
unattended. Originally, ceilometers have been developed for the measurement of

μ