Remote Sensing Study on Blue-sky Days in Beijing, Tianjin, and Hebei During the Period of 2000-2023

Blue sky often represents better air quality and lower air pollution. Using satellite aerosol optical depth (AOD) data of Beijing, Tianjin and Hebei Province from 2000 to 2023, combined with the blue-sky data observed at noon time in 2023, a blue-sky grade index is established based on satellite AOD, in which the monitoring index of blue-sky grade is the AOD at 550 nm less than 0.36, and that of deep blue-sky grade is the AOD at 550 nm less than 0.2.Spatial and temporal characteristics of blue-sky grade days in Beijing-Tianjin-Hebei Region from 2000 to 2023 are investigated. Results show that the multi-year average blue-sky days in Beijing, Tianjin, and Hebei are 144.2 d·a-1, 96.3 d·a-1, and 119.6 d·a-1, respectively, with the highest number of blue-sky days for Beijing, followed by Hebei and the lowest number is recorded in Tianjin. In terms of spatial distribution, the northern part of Hebei has the highest annual average of blue-sky days, while the southern part of Hebei has the lowest number of blue-sky days. The number of blue-sky days in Beijing-Tianjin-Hebei exhibits noticeable seasonal changes, with the highest number of blue-sky days in winter and autumn, followed by spring, and the lowest in summer.From 2001 to 2023, the average annual number of clear-sky days in Beijing, Tianjin, and Hebei takes on an increasing trend, with an increase of 18.1 d, 22.3 d and 16.3 d per decade, respectively. There is no significant trend change from 2001 to 2013. However, the annual average blue-sky days in Beijing-Tianjin-Hebei from 2013 to 2023 all show increasing trends, with increments of 26.9 d, 46.5 d, and 36.4 d per decade, respectively. The annual average blue-sky days and deep blue-sky days in Beijing-Tianjin-Hebei from 2013 to 2023 are higher than those in 2001-2013, with the annual average blue-sky days in Beijing, Tianjin, and Hebei of 153.5 d, 107.5 d, and 128.5 d, respectively, which are 17.5 d, 21.8 d, and 16.9 d higher than those in 2001-2013. It may be largely due to the implementation of regional air pollution prevention and control measures, which have led to a reduction in atmospheric particulate matter concentration since 2013.