Today marks China's National Low-Carbon Day. As a vital part of ecological protection and low-carbon development, air pollution control profoundly shapes people's daily lives. From 2013 to 2014, Beijing was often shrouded in smog, and blue skies were rare. In preparation for the APEC Summit in November 2014, Beijing implemented a series of temporary measures that brought several days of rare clear skies, popularly known as "APEC Blue." At the summit, Chinese President Xi Jinping remarked, "I hope and believe that with persistent efforts, 'APEC Blue' can be sustained."

Thanks to Xi's strong emphasis on ecological progress, Beijing's air quality has undergone a fundamental transformation since 2013. Since then, Beijing resident Zou Yi has taken a photo of the sky every day, building a collection of more than 4,000 images. Through his lens, he has faithfully documented the steady improvement in Beijing's air quality and witnessed the return of blue skies. Click the video to see how Beijing's skies have changed over time.