SINGAPORE: Air quality in Singapore entered the unhealthy range on Saturday (Feb 27) as the Pollutant Standards Index (PSI) went beyond the 100 mark.
At 7pm on Saturday, the 24-hour PSI reading in the northern part of Singapore breached the 100 mark to hit 102.
The reading edged up to 108 at 8pm before dropping to 104 at 9pm and 90 at 10pm.
According to the National Environment Agency (NEA), PSI readings of 50 and below denote “good” air quality, “moderate” for 51-100 and “unhealthy” for 101-200.
As of 10pm, the rest of the readings were:
- 61 in the south
- 70 in the east
- 58 in the west
- 65 in the central region
The 24-hour Pollutant Standards Index (PSI) is computed based on six air pollutants - PM2.5, PM10, ozone, sulphur dioxide, nitrogen dioxide and carbon monoxide.
According to NEA's website, the ozone sub-index was in the unhealthy range between 7pm and 9pm.
The sub-indices for the PM2.5 and PM10 readings were in the moderate and good range, respectively.
In response to CNA's queries, the NEA said on Sunday that the PSI in the northern region had entered the unhealthy range at 7pm on Saturday due to heightened levels of ozone. The ozone sub-index of the PSI is based on an average of ozone levels of the past eight hours.
"The pollutants which contribute to the formation of ozone, (such as) nitrogen dioxide (NO2) and volatile organic compounds (VOCs), were within the normal levels," NEA said.
"However, weather conditions such as ambient temperature, UV levels, wind speed, wind direction and rainfall can also influence the formation of ground-level ozone.
"The maximum ambient temperature on Feb 27 of 35.3 degrees Celsius was the highest recorded in 2021 for the north region. This coupled with the high UV levels, could have contributed to the elevated ozone levels, reaching the unhealthy range.
"The PSI level returned to the moderate range at 10pm."
The PSI in Singapore last entered the unhealthy range in November 2019. It was at the unhealthy range for eight hours from Nov 13 to Nov 14 in the south region, due to elevated PM2.5 levels.
READ: Understand the haze: What do Singapore's air quality readings mean and how do they differ from others?
Separately, according to the latest weather and haze situation update on NEA's website at about 6pm on Saturday, isolated to scattered hotspots were detected over much of the sub-region on Saturday.
"Thin to moderate smoke haze was observed over much of the sub-region, with dense smoke haze observed over Myanmar, Thailand and Cambodia. However, in areas with cloud cover, the full extent of the smoke haze could not be fully discerned," said NEA on the website.
"Most of the air quality stations in the central parts of the Mekong sub-region reported 'Unhealthy' air quality values, with a few in the northeastern Thailand and its central highland regions reporting 'Very Unhealthy' air quality," it added.
Isolated hotspots were also detected in Peninsular Malaysia, northern Sumatra and western Kalimantan, said NEA.
"Thin to moderate smoke haze was observed over parts of southwestern Kalimantan, with dense smoke haze observed to emanate from a cluster of hotspots in western Kalimantan," said NEA.
"However, the full extent of the smoke haze situation over Sumatra and Peninsular Malaysia could not be discerned due to cloud cover."
Looking ahead, NEA said on its website that dry weather is expected to persist over the Mekong sub-region over the next few days. As such, the hotspot and smoke haze situations are likely to remain elevated, it said.
It added that dry conditions are also expected to persist over Peninsular Malaysia, Singapore, the northern and central parts of Sumatra, as well as the western and southern parts of Borneo Island, increasing the risk of hotspot activities in these areas.