Audrey Tan, Straits Times AsiaOne 17 Sep 15;
A tropical storm brewing over the South China Sea has been blamed for blowing the current haze towards Singapore.
The storm acts like a "magnet", which pulls the wind in a direction different to that of the south-southeasterly winds usually expected during this period, said Assistant Professor Winston Chow of the National University of Singapore's (NUS) geography department.
He was responding yesterday to a weather and haze situation briefing by the National Environment Agency (NEA) on Tuesday.
During the briefing, the NEA said the presence of the tropical storm in the South China Sea has brought about a shift in winds to blow from the south-south-west direction. As a result, haze was blown in from Sumatra, Indonesia, causing air quality in Singapore to deteriorate.
The Republic is in the midst of the south-west monsoon season, during which winds are expected to blow from the south and south- east.
But the tropical storm "very subtly alters the monsoonal pattern", said Prof Chow. "It shifts the wind direction in Singapore from mostly the south or south-east to south- west instead."
However, the NEA also gave good news during Tuesday's briefing. It said that the wind direction could change for the better tomorrow, when the storm reaches land and prevailing winds are forecast to blow from the south-east.
Prof Chow said tropical storms are usually fuelled by the evaporation of sea water in the open sea. But when they reach land, their source of fuel is cut, and they start to dissipate.
Yesterday, air quality in Singapore improved slightly. The 24-hour Pollutant Standards Index (PSI) hovered in the low end of the unhealthy range for most of yesterday, compared with earlier this week. On Monday, the 24-hour PSI was in the range of 146-173 at 3am.
Air quality is considered unhealthy when 24-hour PSI readings are in the range of 101-200, and very unhealthy when 24-hour PSI readings are between 201 and 300. When they cross 300, air quality is deemed hazardous.
The NEA attributed the improvement to "a shift in the prevailing winds to blow from the south-east".
Yesterday afternoon's showers also brought a temporary respite from the haze, the NEA said.
"Rainfall removes particles from the air, leading to a temporary reduction," said Assistant Professor Jason Cohen from the NUS civil and environmental engineering department. "But if the fires are still burning, the fresh particles will continue to move in after the rain clears."
Moderate to dense haze is still persisting in central and southern Sumatra.
The NEA said that hazy conditions are expected to persist, and thundery showers are forecast over some areas of Singapore in the late morning and early afternoon today.
The 24-hour PSI for today is expected to be in the low end of the unhealthy range, and could cross the mid-section of the unhealthy range "if denser haze from Sumatra is blown in by unfavourable winds".
Why the haze looks denser at night
The sun and the Earth's atmosphere height are the factors that cause haze to look denser in the evening, according to Dr Erik Velasco, a research scientist from Singapore-MIT Alliance for Research and Technology.
Liyana Othman Channel NewsAsia 17 Sep 15;
SINGAPORE: The sun and the Earth's atmosphere height are the factors that cause haze to look denser in the evening, according to Dr Erik Velasco, a research scientist from Singapore-MIT Alliance for Research and Technology.
Before the sun rises, the height of the atmosphere over the city is at around 50m to 100m. This means that particles from pollution sources like vehicles, and now the haze, are all concentrated in a shallow layer.
As the day progresses, the atmosphere height gradually increases to about a height of 2km as the sun gives off heat, which hits surfaces like roads and buildings.
That energy generates turbulence in the air, which results in the higher atmosphere height. It also promotes the formation of new particles. And because there is more room, these particles are more dispersed and diluted.
After sunset, however, the atmosphere height collapses, compressing all the pollutants back into a shallow layer. The air is also usually more polluted during this time, thus causing the haze to be denser at night.
The best way to measure this, said Dr Velasco, is through PM2.5 - particulate matter smaller than 2.5 micrometres. The reading gives real time measurement of air quality, unlike the Pollutants Standards Index (PSI)
"The Pollutant Standards Index is a good parameter to understand the air quality conditions in the city. However, this index tells us about the average conditions during the last 24 hours or during the last 3 hours. It does not tell us anything about the real conditions. Maybe 5 hours ago, the air was very clean. But this index is not reporting the real condition. It's taking the average,” said Dr Velasco.
Dr Velasco added that these fine particles are harmful for health.
"They are not retained by our nose or our throat. They will go directly to our lungs. And once in the lungs, they can trespass through our bloodstream, and from there they can distribute through our body and even to our brains. That's the concern with the PM2.5 particles,” he said.
“On a normal day, during daytime, the levels of these particles go from 20-40 microns per cubic metres. That's typical in Singapore. During hazy winds, during the last two weeks, the levels have been around 70-100 microns, and even the maximum have passed 200 microns per cubic metres. On Monday night, the concentration went up to 340 microns per cubic meter. Just to give you an idea, international guidelines recommend concentrations no larger than 25 microns per cubic metre.”
Dr Velasco recommended that the public stay indoors during this period. This is in line with the advisory issued by the National Environment Agency, which has also warned that hazy conditions are expected to persist.
- CNA/dl
