The process is not complicated: He puts microscope slides on his windowsill(窗台) and collects the samples covered by fallen dust after 10 hours.
Zhang's apartment is on the eighth floor, which he says is perfect for capturing the densest levels of smog.
As a Beijing native, Zhang explains his original purpose is to "deconstruct" Beijing's smog and unveil its shroud of mystery.
"People have higher and higher standards for the environment," he says. "When I was a child, I was annoyed by the frequent sandstorms, so I feel Beijing's air seems much better now all in all because there are hardly any sandstorms in recent years. Now, we've noticed PM2.5, which has long been neglected."
Although he graduated as a science major from Beijing Normal University, Zhang says it is difficult for him to give an exact definition of what smog is in Beijing. "It's too complicated, and its ingredients keep changing."
Under his scope, which magnifies the sample 1,000 times, the smog is found to contain minerals, fragments of animal fur and insects, seeds and burnt dust. But many are compounds, he says, which are more difficult to identify.
"I wanted to know if there are many pollen grains in the early spring's smog, and my guess turns out to be right," Zhang says, noting there are on average 10 pollen grains found on each slide.
"Though the number is low compared with other ingredients of smog, it may partially explain why smog makes us feel uncomfortable because many people are allergic to pollen."
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