Introduction Although carbon dioxide (CO 2 ) is found in the air at a level of 0-0.03% (0-300 ppm), it is a vital gas in terms of amount and variability. Of the carbon dioxide that is mixed into the atmosphere, approximately 80-85% is generated by the use fossil fuels. The respiration processes of living things and the decomposition of organic substances regen- erate the remaining 15-20% by microorganisms [1]. The amount of carbon dioxide increases due to human activities, especially in densely populated regions, and reaches health- threatening levels in certain cities. The CO 2 cycle on earth in the last 400,000 years shows that CO 2 in the atmosphere increased every 80,000 years and 100,000 years on average and then dropped back later. The highest CO 2 amount throughout the past 400,000 years was 320 ppm. However, CO 2 in the atmosphere is current- ly around 385 ppm. This increase is not caused by the nat- ural cycle of nature; it is a human-induced increase [2]. After health problems related to air quality gained prominence, air quality has been an important factor in peo- ple’s preferences about the city they will live. In cities with clean air, large forest areas that do not have sources of con- tamination began to be considered and preferred as more habitable [3]. The carbon storage values are slightly lower than the US cities average because of differences in forest structure. Urban trees can affect carbon emissions in urban areas. Planting trees around buildings can reduce building energy use, besides reducing emissions for plants. [4-6]. All trees, Pol. J. Environ. Stud. Vol. 24, No. 1 (2015), 253-256 Original Research Effects of Forests on Amounts of CO 2 : Case Study of Kastamonu and Ilgaz Mountain National Parks Hakan Sevik 1 , Mehmet Cetin 2 *, Nur Belkayali 2 1 Department of Environmental Engineering, Kastamonu University, Kuzeykent, 37100, Kastamonu, Turkey 2 Department of Landscape Architecture, Faculty of Engineering and Architecture, Kastamonu University, Kuzeykent, 37100, Kastamonu, Turkey Received: 12 February 2014 Accepted: 3 August 2014 Abstract The CO 2 cycle on earth in the last 400,000 years shows that CO 2 in the atmosphere increased every 80,000 years and 100,000 years on average and then dropped back later. Although there is 0-0.03% (0-300 ppm) carbon dioxide (CO 2 ) in the air, it has a vital importance because of its amount and variety. Plants reduce the amount of CO 2 in the atmosphere with photosynthesis. But plants cannot do photosynthesis in winter or night. As a result, they might have a negative impact on the amount of CO 2 . In this study, the amounts of air carbon dioxide are measured in forests and urban areas and evaluated depending on season and day or night. Results of our study show that, despite the amount of carbon dioxide decreases in the summer depending on the sunlight, it can double its level at night. In addition to day and night, there is a big difference between the amount of carbon dioxide in terms of summer and winter seasons. Keywords: air pollution, air quality, carbon dioxide (CO 2 ) amount, environmental problem, impacts on forest, seasonal change, urban cities *e-mail: mehmet.cetin@temple.edu DOI: 10.15244/pjoes/28691