Geoscience Reference
In-Depth Information
Oxygen (O 2 ) and nitrogen (N 2 ) molecules also have absorption bands in the UV
range. O 2 ,H 2 O, NO 2 ,ClO 2 , and BrO have absorption bands in the visible range.
All these absorption bands originate from electronic transitions.
In the infrared range, combined rotational and vibrational energy changes lead to
absorption bands. The absorption frequency depends on the vibration frequency of
the molecules. All atmospheric gases except diatomic gases have absorption bands
in the infrared range and thus also contribute to the greenhouse effect, which keeps
the Earth warm.
In the far infrared (> 40
μ
m), molecules with a permanent dipole moment have
rotational absorption bands. These gases include H 2 O, O 3 , OH, CO, HCl, HF, HBr,
N 2 O, HCN, HOCl HO 2 , HNO 3 , and ClO.
In the microwave range (>1 mm), rotational bands of H 2 O, O 3 ,H 2 O 2 ,ClO,
and HNO 3 can be utilized for the detection of these trace gases. The following
subsections give a few examples.
4.3.4.1 Vertical Profiling
Ozone profiles in the lower troposphere can be measured with a Raman LIDAR
using a DIAL analysis of the ratio of the vibrational Raman signals for nitrogen
(284 nm) and oxygen (278 nm), which are on the steep side of the Hartley band of
ozone (Philbrick and Mulik 2001 ). Figure 4.27 shows an example which reveals a
threefold vertical structure of the ozone concentration in that night. There is a shal-
low surface layer with very low concentrations due to ongoing titration of ozone
with freshly emitted nitric oxides from car traffic. A second layer with a uniform
concentration level extends from 200 to 1700 m. This is most probably a residual
layer which was left from the CBL of the preceding day. Above this residual layer,
even higher ozone concentrations can be found in the free troposphere. In the free
troposphere, ozone patches can exist for relatively long period of time and are fre-
quently observed to drift in the background wind, as observed in this case (Philbrick
and Mulik 2001 ).
The left frame in Fig. 4.3 displays an example for a time-height cross section of
the optical backscatter intensity sampled by a ceilometer with one optical axis (the
emitted light beam is sent through a hole in the receiving mirror). The backscatter
is similarly caused by aerosol particles and water droplets floating in the air. The
aerosol information from such a backscatter LIDAR is qualitatively only, because
the particle size distribution is not known. Furthermore, the backscatter intensity
may be modified by the atmospheric humidity as well, if the particles are hygro-
scopic. The figure shows high backscatter intensities due to mist or weak fog in
the surface layer in the night and early morning hours (yellow to red colours in the
lower left of Fig. 4.3 ). Above this foggy layer a second layer with enhanced aerosol
concentrations is visible. This layer is topped at 1200- 1400 m. At about 8 a.m., the
formation of a convective boundary layer (CBL) starts which rapidly gains height.
In the afternoon the maximum vertical extension of this CBL is reached with about
1400 m. The air within the CBL seems to be nearly perfectly mixed, because the
backscatter intensity is very even throughout the whole CBL. The shown backscatter
Search WWH ::




Custom Search