This paper studies the relevance between carbon dioxide (CO 2 ) absorption rates of common trees in Chulalongkorn University (Thailand), and environmental factors --light intensity, air temperature, leaf temperature, and CO 2 concentration in air --by forming non-linear models. The common tree species are Pterocarpus indicus, Samanea saman, Peltophorum pterocarpum, and Terminalia catappa. Measuring CO 2 absorption was done by chamber analysis approach. The experiment was carried out by gauging

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... 0 leaves, 7 hours per day, and 2 days per species. According to the models, it is obvious that light intensity is the most influential factor to CO 2 absorption for all studied species. Peltophorum pterocarpum and Samanea saman reach their maximum CO 2 uptake rates of 24.5 and 20.9 CO 2 µmol·m -2 s -1 , when photosynthetically active radiation is 1100 and 1500 µmol·m -2 s -1 respectively. The other two do not reach their maximum rate within model data range. The regressions were best fitted with Gaussian function and Sigmoidal function. It is also suggested that Peltophorum pterocarpum and Samanea saman are good carbon sink and they should be planted more in the city for optimal CO 2 absorption.