Basic Principles of Surface-Based Remote Sensing - Surface Based Remote Sensing of the Atmospheric Boundary Layer - page 64

Fig. 3.15 DOAS instrumentation for open-path trace gas concentration measurements. Left : light
source and receiving telescope, right : mirror. In operation the two parts are installed several
hundreds of metres apart

method (Platt and Perner 1984 ). In contrast to the FTIR spectroscopy, the spec-
tral resolution is lower ( f / f approximately some 100 up to some 1000, Hase and

Fischer ( 2005 )).
The DOAS light source should ideally emit white light. For this purpose xenon
or halogen lamps are used in the focus of a mirror with 0.3 m diameter and 0.25
m focal length. In special cases, e.g. for the detection of OH radicals, dye lasers
nay be employed as light source. The receiving telescope (focal length 0.25-0.85
m) focuses the light on a grate with 600-2160 lines per millimetre (Platt and Perner
1984 ). DOAS detects trace gases, which absorb in the UV or visible spectral range.

3.9 Scintillometry

Scintillometers use a bistatic method that analyses refractive index fluctuations
influencing the propagation of optical (e.g. in the near infrared at 940 nm) or electro-
magnetic waves (e.g. microwaves at 94 GHz having a wave length of 3.2 mm) along
the path from the emitter to the receiver (Fig. 3.16 ). This fluctuation phenomenon,
which is called scintillation, is the same which makes stars twinkling at night. Two
types of optical scintillometers are available: a small aperture or laser scintillome-
ter (SAS, see, e.g. de Bruin 2002 ) and a large aperture scintillometer (LAS, see