Airborne atmospheric measurement of nitrogen dioxide (NO<sub>2</sub>) column density was performed using the solar spectroscopic method. The measurement was carried out at different altitudes, from 460 to 700 meters above sea level during a flight in Hong Kong (22.2°N, 114.1°E), the People's Republic of China (PRC). In the territory, the boundary layer height in a sunny day is about 1000 meters. Below the boundary layer, most of the NO<sub>2</sub> exists. The airborne solar spectroscopic measurement gives the NO<sub>2</sub> vertical profile below the boundary layer. In particular, the solar spectroscopic measurement requires a solar tracking system to collect the direct sunlight. However, in an acceptable small percentage of error in tracking the sun position, it is possible to collect the direct sunlight manually. In this paper, to reduce the complexity of the experimental setup, the sunlight is collected by a portable miniature CCD spectrometer. In the retrieval of NO<sub>2</sub> column density, the airborne solar spectrum is normalized to a reference solar spectrum, which is taken at a high altitude (11,230 meters) during another flight in Xinjiang (42.208°N, 83.949°E) province, PRC. The column density retrieval is achieved from the normalized solar spectrum using the differential optical absorption spectroscopy. A ground-based off-axis control experiment is also performed to estimate the error in the slant column density from the airborne measurement.
Atmospheric concentration measurement of nitrogen dioxide (NO<sub>2</sub>) pollutant was demonstrated by a new type of differential optical absorption spectroscopy (DOAS) using a spectral scanning device of a visible acousto-optic tunable filter. The measurement requires a stable artificial light source such as a xenon lamp, and the light beam is directed into the environment where the concentration of NO<sub>2</sub> is to be monitored. The retrieval of NO<sub>2</sub> concentration is then achieved by analyzing the residual light using the DOAS signal processing. In this paper, we present results obtained from this new DOAS system during continuous measurement of atmospheric NO<sub>2</sub> concentration in the campus of the City University of Hong Kong. Another DOAS system, using a miniature CCD grating spectrometer, was established as a control experiment. The CCD spectrometer acts as a traditional DOAS system for performance evaluation. Instead of using the NO<sub>2</sub> absorption cross section for concentration retrieval, differential absorption area provides an alternative quantity for concentration retrieval. The monitoring results from both of the DOAS systems are compared with the pollutant concentrations reported in a nearby pollutant monitoring station, operated by the Hong Kong Environmental Protection Department.
In studies of the internal boundary layer, it is important to make the lidar both eye-safe and capable of measuring near range extinction to high accuracy. The inversion of data is much more challenging due to the weaker signal from an eye-safe lidar. The method of inversion employed in this paper is the Fernald's method. Because the digitization system is capable of obtaining returns in the time interval of a few seconds, formal statistical analysis and error propagation are introduced in the inversion. Different weighting schemes are used during the averaging of the inversion and the results from scattered cloud data as well as pure aerosol profiles will be discussed. Results from several sites and under different meteorological conditions will be reported.