China has been troubled by severe haze pollution under accelerating industrialization. On June 11, 2015, the Chinese social media were flooded by photos of Beijing landmarks against rare blue and clear sky. As a result, the phrase "Beijing blue” was listed as one of the ten clean air keywords published by the Beijing Environmental Protection Publicity Center in 2016, and a Blue-Sky Protection Campaign was rolled out in 2018.

To examine the effect of the campaign, a direct indicator needs to be developed from studying the blue days based on a long-term record. However, there is neither a clear definition for Chinese blue days, nor a quantitative attribution of its long-term trend.

In a study published in Environ. Res. Lett., scientists from Institute of Atmospheric Physics (IAP) of the Chinese Academy of Sciences and Peking University defined a novel Chinese blue days index, which contains a double meanings of sunny days and clean air.

The results revealed a steady incline in Chinese blue days (CBD) during 1980-2014. Three low-value centers can be found in the southeast Sichuan Basin, southern Hebei, and northeast Guangxi, with annual mean average CBD of 74, 86 and 92, respectively. The minimum/maximum trends (-7.5/9.5 d/10y) appeared in Yangtze-Huai River Valley (YHRV) / southwestern China (SWC). Approximately 42% of stations showed a prominent, increasing trend and approximately 23.4% of stations showed a decreasing trend.

Besides, scientists found out the important relationship between blue days and wind speed, relative humidity and rain-free days.

Moreover, they found that considerable parts of the CBD trend are due to the change of atmospheric circulation in the two regions.

"Our work is innovative in that it studies the reverse side of air pollution. It would benefit the determination of livable cities and national energy strategy, and the formulation of a better energy-saving emission reduction inventory,” said HUANG Gang from IAP, the corresponding author of the paper.

"Beijing Blue" in four seasons (Image by HUANG Gang)