Yotsumi Yoshii

Long-path measurement of atmospheric NO2 with an obstruction flashlight and a charge-coupled-device spectrometer

A novel method of differential optical absorption spectroscopy (DOAS) is proposed and demonstrated to monitor the concentration of atmospheric pollutant gas. In contrast to conventional DOAS measurements with continuous light sources, the present method relies on white flashlights such as aviation obstruction lights that are generally installed on tall constructions. A simple detection system is devised by means of a telescope and a compact CCD spectrometer. A path length of 5.5 km allows us to measure atmospheric NO2 concentration with a detection limit of approximately 1 part per billion. We also discuss the possibility of deriving the aerosol optical thickness through the horizontal atmosphere from this pulsed DOAS measurement.

Enhanced Detection of Gas Absorption Using an Erbium-Doped Fiber Ring Laser

We demonstrate gas sensing using a fiber ring laser composed of an erbium-doped fiber as the laser medium and a compact gas cell module. Acetylene absorption at 1.5 µm is enhanced by an intracavity configuration, in which multiple band-pass filters are used to control the wavelength of laser oscillation. A maximum enhancement factor of about 80 is achieved with an 18.3 mm cell under a pressure range below 100 Pa. Experimental results can reasonably be explained by a ring laser theory that describes the influence of acetylene pressure on the threshold and slope efficiency of the laser output characteristics.

Monitoring of atmospheric nitrogen dioxide by long-path pulsed differential optical absorption spectroscopy using two different light paths

Measurements of the local distribution of atmospheric nitrogen dioxide (NO(2)) by long-path pulsed differential optical absorption spectroscopy (LP-PDOAS) in Tokyo during August 2008 are presented. Two LP-PDOAS systems simultaneously measured average NO(2) temporal mixing ratios along two different paths from a single observation point. Two flashing aviation obstruction lights, located 7.0 km north and 6.3 km east from the observation point, were used as light sources, allowing spatiotemporal variations of NO(2) in Tokyo to be inferred. The LP-PDOAS data were compared with ground-based data measured using chemiluminescence. Surface wind data indicated that large inhomogeneities were present in the spatial NO(2) distributions under southerly wind conditions, while northerly wind conditions displayed greater homogeneity between the two systems. The higher correlation in the NO(2) mixing ratio between the two LP-PDOAS systems was observed under northerly wind conditions with a correlation factor R(2) = 0.88. We demonstrated that the combined deployment of two LP-PDOAS systems oriented in different directions provides detailed information on the spatial distribution of NO(2).

Cavity-Enhanced Detection of Molecular Absorption under the Scheme of Wavelength Modulation Spectroscopy

We propose a new scheme of trace gas detection by the use of a Fabry?Perot cavity (FPC) and the wavelength modulation technique. Optical propagation length is enhanced using a 25 cm FPC with a finesse of ~100. By applying the same modulation to both the laser and cavity frequency, we observe the first derivative signal of acetylene absorption at 1.5 ?m. It is found that a sample-and-hold circuit serves well for restoring the signal level, which otherwise is degraded because of the pulsation of the detected laser intensity due to a feedback loop required for scanning the laser and cavity frequency simultaneously. Compared with the signal intensity observed with a 20 cm Brewster cell, this experiment yields the improvement of signal intensity by a factor of 81.2 with an acetylene pressure of 2.6 Pa.

Differential optical absorption spectroscopy of atmospheric NO/sub 2/ with a pulsed, white flashlight

We propose and demonstrate a novel method of differential optical absorption spectroscopy (DOAS), in which an obstruction flashlight equipped on a tall stack is exploited for monitoring concentration of atmospheric NO/sub 2/. This pulsed DOAS measurement also brings about information on the aerosol optical thickness through the horizontal atmospheres.

Automatic correction of optical alignment deterioration of a continuously operated Mie lidar

Alignment of the laser beam and telescope axes is crucial to the acquisition of high quality lidar data. For a continuously operated Mie lidar, a routine alignment correction is needed to ensure that its data are not influenced by the misalignment and thus truly represent the aerosol and cloud behavior. This paper describes the operation of such a lidar system with automatic correction of optical alignment.

Wavelength modulation detection of trace gas using a Fabry-Perot cavity

We report on wavelength modulation detection of trace amounts of acetylene at 1.5 /spl mu/m. A Fabry-Perot cavity (FPC) is employed to enhance the effective absorption length. The wavelength modulation is applied by synchronously modulating the laser cavity and the FPC cell.