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On Monday, Beijing's gray skies made residents miss last week's clean air and pure blue skies that prompted millions posted beautiful pictures online.

"It's just a comparison, I mean, psychologically. It reflects the real effects of the emissions-reduction efforts by Beijing, neighboring Tianjin municipality and Hebei province which are sacrificing their economic benefits."

Wu Fangjie, real estate agent in Qingdao, Shandong province