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Atmospheric ammonia monitoring near Beijing National Stadium from July to October in 2008 by open-path TDLAS system

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 NH3 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 NH3 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 NH3 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 NH3 peak concentration decreased obviously at the beginning of the Beijing Olympics then it kept descending during the Paralympics. The obtained maximum of NH3 is between 20.31μg/m3～ 48.54μg/m3 with the daily average concentration between 12.6μg/m3～27.5μg/m3. 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.