Singapore experienced a sudden worsening of the haze in the evening hours of Monday Oct 19. The 3-hour PSI reading throughout the day was between 75 to 136 between 1am to 9pm for the day before spiking from 152 at 10pm to 224 at midnight on Oct 20.
The most surprising development, however, was the 1-hour PM2.5 readings, which remained below 100 for most of the day before suddenly jumping to 442 in the west side of Singapore at 10pm, and to 299 in the southern part of the island. At the same time, though, PM2.5 readings in the north and east side of Singapore was only 35 and 66 respectively.
PM2.5 readings increased even further in the west side of Singapore at 11pm to 471, and in the south to 301. But by midnight, the readings fell almost by half to 262. Here's a screenshot of the readings:
The PM2.5 levels fell in the west, central and south of Singapore. The north and the east had marginal increases but still remain below 100.