Coniferous trees are believed to have a better effect on air quality control, especially in decreasing PM 2.5, particulate matter smaller than 2.5 micrometers in diameter, according to an expert.

Yin Shan from the School of Agriculture and Biology at Shanghai Jiao Tong University, said coniferous trees, including cedar, metasequoia and dragon juniper, have a more positive effect in this respect than deciduous trees, such as the ginkgo and camphor, as they are better able to absorb pollutant particles.

"The rough surfaces of these coniferous trees' leaves can produce sap that has a detention function — some of them can keep the atmospheric particulates on their surface, while some can even absorb PM 2.5," said Yin.

Bao Manzhu, dean of Huazhong Agricultural University's College of Horticulture and Forestry Sciences, said, "Those trees with relatively rough surfaces have a better dust retention effect compared with those with smooth surfaces."

Meng Shujie, spokeswoman for the Shanghai Forestry Bureau, said the city picks its tree types according to local climatic conditions and their ornamental functions.