SINGAPORE: Scores of people have taken to social media complaining of a burning smell in parts of Singapore in recent days and pondering if it is haze season again. The 3-hour Pollutant Standards Index also hit a new high this year, registering a reading of 91 at 9pm on Thursday (Mar 31), while the 24-hour PSI stayed in the Moderate range at 64 - 79.
While authorities have yet to pinpoint the exact cause of the smell or concretely linked it to haze, atmospheric chemists said it could be caused by volatile organic compounds (VOCs) blown in from hotspots in the northern ASEAN region.
These VOCs are by-products of combustion, and take the form of gas molecules which cannot be detected by equipment used to measure the Pollutants Standard Index (PSI). And this could be one reason why even as PSI levels remain in the Moderate range, many Singaporeans have noticed a lingering smell of burning.
According to Professor Mikinori Kuwata, of the Asian School of the Environment at Nanyang Technological University (NTU): "Haze is caused by liquid or solid particles suspending in the air. These particles in haze scatter light. So they are responsible for the degradation of visibility. What we are smelling is gas. That's the reason why the current smell is not reflected in the PM2.5 concentration.”
WATCH: What could be causing that burning smell, and why is it not reflected in PSI readings? Nanyang Technological University, Singapore's Professor Mikinori Kuwata explains.Posted by 938LIVE on Thursday, 31 March 2016
Dr Rajasekhar Balasubramanian, an atmospheric chemist from the National University of Singapore, said the VOCs could contain gases such as sulphur dioxide.
"They could also come along with organic gases. So we have different gases mixed with each other in different concentration, so the air will be different in terms of its smell."
The Associate Professor added that the health impacts depend on how long people are exposed to the gases and its concentration.
Earlier this week, the National Environment Agency (NEA) said there could be a few possible causes for the smell, including transboundary smoke haze from forest and peat fires in the region, the occurrence of local fires or other localised sources of burning.
On Wednesday (Mar 30), the Meteorological Service Singapore (MSS) said it observed "isolated hotspots" in Cambodia, Laos and Vietnam. "The full extent of the hotspot activities in Myanmar and Thailand could not be determined due to cloud cover," MSS said, adding that in southern ASEAN, wet weather conditions continued to subdue hotspot activities.
A look at hotspots in the region on Mar 30. (Map: MSS)
STRONGER AT NIGHT
The burning smell has been particularly strong for some residents in central and northern parts of Singapore.
"It happened about two weeks ago. Usually it's rather bad when it's about 2am to 3am and it's quite choking," said 23-year-old Eunice Fun, who lives in Bishan.
Dr Rajasekhar said the smell is also stronger at night because the air blown in from hotpots in the northern ASEAN region remains concentrated at a shallow altitude.
He explained that during the day, with warmer ground temperatures, particles and gases move upwards, to a higher altitude, which results in polluted air being mixed with a larger volume of air in the atmosphere. This could explain why burning smells are not as intense during the day.
"But on the other hand, during the evening, you are still bringing the air from an external source but little upward movement. So you are concentrating the polluted air in a smaller volume. The clean air and polluted air are not so well mixed. The smell is more intense in the late evening or late night and early morning," explained Dr Rajasekhar.