Figure 4.

Time series of 1/PBLH (planetary boundary layer height, red lines) and PM_2.5 (blue lines) over (a) Beijing and (b) northeast China, with a 3‐day smoothing window. Seasonal cycles, trends, and climatology have been removed in the time series. Pink areas represent the Lunar New Year (LNY), and gray areas represent the COVID lockdown (CLD) period. The green areas indicate the periods with daily rainfall above 1 mm. A notable haze event that occurred in Beijing during the CLD is pointed out in (a).