The Great Green Wall

Wei Longzhao, a sophomore student at the Beijing University of Science and Technology, has been organizing activities for students there to get together and plant trees around Beijing for seven years.

"Beijing's green coverage is getting larger by the year, but tree-planting still needs lots of work," he said. "Judging from the sandstorm that just took place Saturday, it's obvious that tree-planting is not yet having enough of an effect."

Every year on Tree Planting Day, which falls on March 12, Beijing gets serious about protecting its trees. With the recent PM 2.5 particle pollution strike, a new plan has been proposed.

In 2012, the Beijing government released a plan to plant 66,667 hectares of trees in the next three years in order to combat PM 2.5.

Trees have a positive effect in blocking PM 2.5, but it is unknown exactly how big that effect is at this stage. At the same time, experts say the real problem in combating PM 2.5 using forestation is the way trees are planted, not what type or how many.

Current efforts

Volunteers in Beijing have been planting trees since the fourth session of the 5th National People's Congress on December 13, 1981 made a decision to harness national efforts in tree-planting.

However, last year was the first time the prevention of PM 2.5 was mentioned as a goal.

"PM 2.5 is a relatively new phenomenon in society, so starting last year, it was the first time the concept of reducing these particles through planting trees was brought up," said Tong Yonghong, spokesperson for the Beijing Municipal Bureau of Landscape and Forestry.

At last year's 5th session of the 13th Beijing Municipal People's Congress on January 12, Beijing announced the release of eight specific policies to combat PM 2.5, one of which was to plant 66,667 hectares of trees in the next three years between the 5th and 6th ring roads after some rural villages have been renovated and land has been freed, and also around the edges of rural fields and deserted lands.

In 2012, 16.71 million trees were planted on 16,973 hectares of land. Tong said the trees were mostly deciduous, which are local trees that save water and have a strong ability to attract dust.

Controlling PM 2.5

Even though this phenomenon is new, Yuan Shibao, the division chief for tree-planting at the bureau, believes there's a positive connection between tree-planting and controlling PM 2.5.

"Trees can prevent high wind and sand from invading Beijing, because they are tall and prevent the dust from spreading," he said.

He provided data that showed one deciduous tree can attract and hold about 3.16 kilograms of dust per year. In a forest of Chinese Scholar trees, the amount of dust emitted is 40 percent less than in regular forests.

Feng Yongfeng, founder of the environmental NGO Nature University, said even though PM 2.5 particles are miniscule, trees can pick them up and trap them.

"The small particles are mostly nitrogen oxide or sulfite dioxide. To the trees, it's nutrition, so they can absorb them," he said.

Green plants can attract particles through transpiration, Yuan said. Through steam released by leaves, moisture is increased around the edges of the leaves and the crown of the tree, which reduces the activeness of particles in the air. The attached particles can then be washed away by the rain into drains.

More studies are being carried out on trees' ability to reduce PM 2.5 particles, he said. In 2012, a research project was launched by Chinese forestry scientists to study the effect of trees on PM 2.5 particles.