Geoscience Reference
In-Depth Information
nuclei (CCN). Radiative and microphysical properties of aerosols are
variable in time and space. The vertical distribution of aerosols is an
important parameter to be measured, in order to evaluate their radiative
effect.
1
,
2
In addition, the relationship between the aerosol properties and
ambient relative humidity (
) is also important for atmospheric studies.
3
In particular, there is little information about the vertical distribution of
aerosol properties in the atmosphere due to the technical diculties and
analytical limitations. In addition, there are few observational data on
the relation between the aerosol properties and ambient
RH
.Sincethe
aerosol shape, size, and chemical composition critically depend on
RH
,the
properties of atmospheric aerosol particles can be expressed as function of
the
RH
at thermodynamic equilibrium with the surrounding atmosphere.
4
The remote sensing depolarization techniques are developed for
the above-mentioned requirements. A depolarization lidar can provide
information on particle shape and size by employing the short pulses
of polarized light of laser. The radiation backscattered by homogeneous
spherical particles will maintain the original (parallel) polarization, whereas
nonspherical particles will induce some degree of depolarization. Thus,
the degree of depolarization will provide the distribution of particle and
water in different phases. Lidar polarization measurements for many cloud
states existing below 3 km have nonzero depolarization ratios (
RH
), but
these clouds should be liquid clouds consisting of spherical droplets. These
liquid clouds consisting of aspherical particles appeared mainly in the
spring season during which Asian dust mainly burst. In this chapter, we
have studied and discussed the optical properties of aerosols and clouds in
the altitude range of 0.7-3 km by using depolarization lidar during 2002-
2004. The depolarization ratios for different sources of clouds are traced
by HYSPLIT (Hybrid Single-Particle Lagrangian Integrated Trajectory)
calculations. The statistics of the optical properties of aerosols and clouds
with respect to relative humidity have also been discussed.
DR
2. Lidar Methodology and Related Definitions
2.1
. Lidar system
The lidar system used in the present study consisted of a Nd:YAG laser
at a wavelength of 532 nm. The receiver consists of a 20 cm Schmidt-
Cassegrain telescope. Signals are measured by photomultiplier tubes (PMT)
and analyzed by multi-channel analyzers (MCA) with a vertical resolution
