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Bangkok Metropolitan is a rapidly growing city with both industrial and urban area expansion resulting in the generation of a significant air pollutant; Nitrogen Oxide (NOx = NO + NO2). This research studied the variation of Nitrogen Oxide concentration in an urban area at 30 m and 110 m above ground by applying the HYSPLIT model to simulate the backward trajectories movement of air mass using the past 3 days of data from an air quality and microclimate monitoring station at Kasetsart University, Bangkok from January 2016 to February 2017. The results showed that the concentration of Nitrogen Oxide at 30 m above ground is higher than the concentration at 110 m above ground. The concentration trended to increase in winter (October 2016 to February 2017). According to the backward trajectories modeling, the major direction of air mass movement at 30 m above ground, category 1 (N-NE) and category 2 (NE-E), occupying 86% of total movement direction (concentration of NOx ranged from 4.02 to 96.35 ppb) meanwhile the major direction of air mass movement at 110 m above ground, category 1 (N-NE) and category 2 (NE-E), occupying 79% of total movement direction (concentration of NOx ranged from 3.93 to 51.50 ppb). The air mass moved through the different land use types, human activities and industrial areas. This study can be applied as a guideline for microclimate surveillance and monitoring of NOx concentration influenced by air mass movement.
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