Because the trees are photosynthesis process (photosynthesis) in the daytime so it can reduce voc. During the day, because at night the trees will have to emit toxic gas released does not reduce in the night.
Because the tree is photosynthesis process (photosynthesis. ) During the day, so processes can reduce voc. In the daytime, at night, because the trees are spitting out poison gas, the reduction is not at night.