Proceedings Article | 29 August 2009
Proc. SPIE. 7382, International Symposium on Photoelectronic Detection and Imaging 2009: Laser Sensing and Imaging
KEYWORDS: Modulation, Semiconductor lasers, Molecular spectroscopy, Atmospheric sensing, Spectroscopes, Atmospheric monitoring, Atmospheric particles, Single molecule spectroscopy, Atmospheric optics, Absorption
Gaseous ammonia is the most abundant alkaline trace gas in the atmosphere. As ammonia plays an
important role in acid deposition and aerosol formation, it influences the regional air quality and
atmospheric visibility. TDLAS (Tunable Diode Laser Absorption Spectroscopy)is a method to obtain the
spectroscopy of single molecule absorption line in the characteristic absorption spectrum region as the
characteristic of the distributed feed back (DFB) diode laser with narrow linewidth and tunability, which
makes it possible to detect trace-gas qualitatively or quantificationally. The NH<sub>3</sub> in-situ monitoring
instrument based on TDLAS and long open path technology have been developed combining with
wavelength modulation and harmonic detection techniques to obtain the necessary detection sensitivity.
This instrument has been used to measure atmospheric NH<sub>3</sub> concentration at an urban site near Beijing
National Stadium from July to October in 2008, especially in the period of Beijing Olympics and
Paralympics. The continuously monitoring results show that the atmospheric NH<sub>3</sub> concentration variation
has an obvious diurnal periodicity in the urban of Beijing. First of all, the general diurnal variation rule is
the concentration decreased to the minimum in the daytime, and then increased to the maximum at
night. Moreover, the NH<sub>3</sub> peak concentration decreased obviously at the beginning of the Beijing Olympics
then it kept descending during the Paralympics. The obtained maximum of NH<sub>3</sub> is between 20.31μg/m<sup>3</sup>～
48.54μg/m<sup>3</sup> with the daily average concentration between 12.6μg/m<sup>3</sup>～27.5μg/m<sup>3</sup>. During these three
months, Air Quality Assurance Scheme for the Olympics (AQASO) was implemented through the joint
actions of Beijing Municipal Government and the five neighboring provinces/municipalities in north China.
The measures such as auto restriction and plant ejection-decreasing are carried out in Beijing. In conclusion,
the open-path TDLAS instrument is suitable for atmospheric trace gases detection on-line with its high
resolution, high sensitivity, rapid response time without gas sampling. Also correlation studies have
verified the positive effects of measures taken for improving the air quality including restricting ammonia
formation.
