In order to increase the detected range and reduce the effect of signal-induced noise in light detection and ranging (LIDAR) measurements of the high atmospheric layers, we have developed a normally off-gated photomultiplier tube module for use in the photon-counting mode. The module allows the detection of the weak backscattering signal, while at the same time rejecting the intense backscattering signal from the near range with an adjustable rejection range. The gating of our module is based on the application of positive voltage pulses on all the even dynodes of a photomultiplier head. We describe the electronic circuit design and discuss the implementation of the module. The capability of the module in rejecting intense backscattering signal from the near range was tested on our LIDAR system. The agreement between the range-corrected LIDAR signals and the standard atmospheric molecular number density up to a height of 60 km indicates a good performance by the module. Comparison between Raman and elastic backscattering signals under cloudy sky conditions indicates that the intense backscattering from near range is sufficiently suppressed and has a negligible effect on the performance of our module.

Cirrus cloud is one of the most commonly occurring cloud types globally. The clouds are found mainly to consist of floating ice crystals, which can reflect strongly incoming radiation. Lidar measurements provide an opportunity to study the microphysics and ice compositions of cirrus clouds. We have developed a polarization lidar system to measure the depolarization characteristics of the lower atmosphere over Hanoi. From our measurements of cirrus clouds over the period from 2010 to 2012, the depolarization ratio of cirrus clouds is found to be very high ranging from 20% up to 80{\%}, indicating large fraction of backscattering from ice crystals. We find a trend of increasing depolarization ratio with height and decreasing temperature. We also compare our lidar measurements with other results obtained by lidar technique in other regions of the world.

Monitoring the concentration and distribution of nitrogen dioxide NO2 in urban environment is of great interest because of the importance of this gaseous pollutant in affecting air quality. In this paper we present the development of a multi-axis differential optical absorption spectroscopy instrument capable of sensitively detecting NO2. The passive instrument collects the sun light scattered by the air molecules and aerosols in the atmosphere and measures the spectrum using a highly sensitive portable spectrometer. The viewing direction of the instrument is controlled through a motor and can be changed continuously. Data analysis of the measured spectra allows us to simultaneously determine the differential scant column density of NO2 and oxygen dimer O4. From the accurately known concentration of O4, the effective optical path length of scattered sun light near the horizontal direction could be derived, which in turn provides an estimate the concentration of NO2. The measured data show that the concentration of NO2 in Hanoi is in the range ~1.5 ppb. We also present the detection of formaldehyde HCHO and possible detection of glyoxal CHOCHO. Our sensitive instrument opens up the possibility to monitor the concentration of other molecular species of interest in urban environment of Hanoi.

We conducted a study to develop a transmitter consisting of two Distributed Feedback Dye Lasers (DFDL) using Rhodamine 6G as an active medium, pumped by a frequency-doubled Nd:YAG laser (5ns, 10 Hz, 532 nm). The BBO crystals are used for frequency-doubling of DFDLs to generate UV missions. This system has been set up and it generates selected wavelenghts at 282.9 nm and 286.4 nm with the energy of 30 \(\mu\)J/pulse and 60 \(\mu\)J/pulse respectively. A DIAL system using this UV transmitter is designed to measure the vertical distribution of ozone with high temporal and spatial resolution. Our simulations of received DIAL signal indicate that the altitude of ozone distribution measurement can reach to over 5000 m along with range resolution of 100 m.