All mentioned above try to convey the non-synergy correlation between air pollution and blue days when considering the unfavorable meteorological conditions dominated by large-scale synoptic circulation (Bei et al. 2016) can be seen even during the COVID-19 lockdown period (Huang et al. The inconsistency between dropping PM and unchanged low visibility events (Ding et al. On the other hand, fewer air pollutions do not directly bring in more blue skies, considering days with higher cloud cover or precipitation. However, with increasing anthropogenic emissions, the sky might be less or not blue even when the weather condition is “sunny (defined by cloud cover)” (Fast et al. Traditionally, a sunny day with few clouds means blue sky day. With the improvement in living standards, there is a growing desire for a better atmospheric environment and more clear blue skies. Specifically, the decrease in surface wind speed hinders the cleaning of air by winds, the increase in surface air temperature, and decrease in relative humidity is favorable for low cloud increase, and the increasing emission of pollution reduces atmospheric visibility. Both air pollution and the change in meteorological conditions contribute to the decrease of wintertime deep blue days in China. The most prominent decreases of deep blue days are seen in winter. The maximum decreasing trend is observed in eastern China. From 1980 to 2018, annual deep blue days show a prominent decreasing trend in most of China, with area-mean annual deep blue days decreasing by −0.48 days per year (d/y) in China, and the variation becomes more obvious after 2013. Climatologically, annual deep blue days increase from southeast China to northwest China, with the maximum number in Xinjiang and eastern Inner Mongolia and the minimum number in western Qinghai and southern Hebei. Due to a lack of direct measurements, we use atmospheric visibility and low cloud cover to classify blue sky days into three grades: light blue day, medium blue day, and deep blue day. This study investigates the long-term changes of the deep blue sky in China from 1980 to 2018. With increasing human emissions, a trend towards days with fewer deep blue skies might indicate a decline in a good living environment for humans. The deep blue sky is an indicator of a lower concentration of aerosols and a cloudless sky.
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