Haze, hotspots, and night-time spikes: What’s behind the return of haze in S’pore
The National Environment Agency (NEA) reported higher haze levels in a Facebook post on March 24. Here are three things you should know about the recent haze.
1. Where is the haze coming from?
NEA said hotspots have been observed in eastern Johor, with smoke plumes drifting towards Singapore.
The 1-hour PM2.5 level in the east reached 69µg/m³ at 2am on March 24, entering the Elevated range. Levels below 55µg/m³ are considered Normal, while readings above 150µg/m³ fall in the High range.
At 6pm that day, the 24-hour Pollutant Standard Index (PSI) ranged between 42 and 68, within the Good to Moderate range.
“Dry conditions are expected over Singapore and the surrounding region till the end of the week, and there continues to be a risk of haze,” the agency added.
Netizens have also reported higher levels of air pollutants in the eastern region of Singapore, with one commenting on Reddit that “it’s almost like the haze is being generated” there.
“East side haze is super strong. Especially when the day darkens,” said a netizen on NEA’s Facebook post.
According to NEA data, PSI levels in the east rose to 68 at 2pm on March 25, well into the Moderate range, while the western parts of the island recorded readings of about 40.
2. Why is it worse at night?
Some residents have said the haze feels more noticeable at night.
As one netizen asked in a Reddit post: “Is it only at night ah, or is my nose tricking me.”
NEA’s air quality indicator showed PM2.5 levels spiking to 30µg/m³ between 2am and 6am on March 25, while remaining largely stable at 10µg/m³ for the rest of the day.
Wind speeds typically drop at night, which means air pollutants are not dispersed as quickly as in the day. Cooler temperatures also limit air pollutants from moving upwards, resulting in a more potent smell at night.
An air quality monitoring platform, Airly, also suggested that air quality dips between 10pm and 6am due to fewer atmospheric changes that can disperse harmful particles.
3. Why does it smell worse than what is indicated?
Even when PSI readings remain in the green or Good range, a strong burning smell may still be noticeable.
Smell is not always accompanied by an increase in PSI readings.
This is because vegetation and peat fires also release a mix of gases that produce an acrid smell. These gases are different and not always captured by standard air quality indicators, but can resemble the smell of transboundary haze.
The same principle applies to visibility. Poor visibility does not always mean that air is more polluted, according to NEA.
Instead, water vapour plays a significant role. High humidity or rain can reduce visibility when combined with haze, even if pollution levels are not significantly elevated.
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