Plants uptake Carbon dioxide and release oxygen during the day (photosynthesis) and uptake oxygen and release Carbon dioxide during the night (respiration). Some plants such as Peepal tree/Arali Mara can uptake Carbon dioxide during the night as well because of their ability to perform a type of photosynthesis called Crassulacean Acid Metabolism (CAM). 

Most plants largely uptake Carbon dioxide (CO{-2})and release oxygen during the day (photosynthesis) and uptake oxygen and release CO{-2} during the night (respiration). Some plants such as Peepal tree can uptake CO{-2} during the night as well because of their ability to perform a type of photosynthesis called Crassulacean Acid Metabolism (CAM).

Which purifies the Air/Environment around us. Plant trees every where in the city, find a vacant land, start growing Trees. Only way to purify the Air other wise, We may have to buy oxygen masks in next couple of years.