This winter may turn out to be Beijing's cleanest in five years. Looking beyond official data, one Beijinger had the foresight to visually document changes to the city's skyline during that time. Here's what he came up with. Xu Xinchen reports.

One single spot, 365 -- or 366, days, for five years, these collages of photos document daily changes to the skyline in Beijing's central business district. From time to time, the skyscrapers show up sharply. Other times, they vanish in thick smog.

ZOU YI "The heavy smog, which affects me too much. I didn't know what it happened and how we can react."

Zou Yi in his 50s started to add one more thing to his daily routine in 2013 -- taking photos through his apartment window every day. Zou shares these photos on the internet for millions to see. And for the past five years, people from different walks of life have joined him. Zou sits down with them in every January -- showing his yearly discoveries.

YAN HUANG "Zou has turned from a complainer into an actual contributor who can help change our environment."

TANG XINYING "Environmental activists can help give more awareness to the society, but government efforts are still needed to finally resolve the problems."

Zou is trying to expand his mission and he has received help from other volunteers. Now, Zou's photos have been digitized and analysed to create a model that can tell the presence of smog by simply taking a picture."

ZOU YI "I quit everything and concentrate and focus on this monitoring the air quality by big data. But we should remember that the reason behind is still not very clear and we should do more to testify what we have done is good and right."

According to the latest data from the Beijing Municipal Environmental Protection Bureau, the city was able to reduce the number of heavily polluted days by more than half from 2013 to 2017. The average PM2.5 particle concentration dropped by 20% in that period. Experts still caution, however, that the city will need to maintain this control in the long term. Environmental protection is more than just bringing the blue sky back. XXC, CGTN, Beijing.