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SINGAPORE – Singapore’s air quality reached unhealthy levels in the north over the weekend due to higher levels of a pollutant known as ozone, and not because of transboundary haze.
“At 7pm on Saturday (February 27), the Pollutant Standards Index (PSI) in the northern region entered the unhealthy range due to high ozone levels,” said the National Environment Agency (NEA) in response to inquiries from The Straits Times.
At the time, the PSI reading – a measure of air quality here – in the north was 102. It rose to a high of 108 at 8 p.m., before dropping to 90 at 10 p.m.
Air quality is considered unhealthy when the PSI is in the range of 101 to 200.
During such periods, healthy people are urged to reduce prolonged or strenuous physical exertion outdoors, while vulnerable groups, including pregnant women, children and the elderly, should minimize such exertion.
When air quality is “moderate”, with PSI readings between 51 and 100, normal activities can be performed for all groups.
PSI is calculated based on six pollutants: particulate matter, fine particulate matter (PM2.5), sulfur dioxide, carbon monoxide, ozone, and nitrogen dioxide.
This means that an increase in concentrations of any of the six can lead to deterioration in air quality.
During foggy periods caused by forest fires in the region, the dominant pollutant is PM2.5. A NEA spokesperson said the PSI last entered the unhealthy range for eight hours on November 13 and 14, 2019, in the southern region, due to elevated levels of PM2.5.
What is ozone?
Ozone is a gas that is made up of three oxygen atoms.
Whether it is beneficial or harmful to humans depends on where you are.
“Good ozone” occurs naturally in the upper parts of the atmosphere, where it forms a protective layer that protects life on earth from the sun’s harmful ultraviolet rays, according to the website of the US Environmental Protection Agency. That is why a hole in the ozone layer is worrisome.
But when ozone is at ground level, it is considered a pollutant. It is one of the main components of smog.
Unlike other forms of pollution that are caused by direct emissions, such as carbon monoxide from the tailpipe of a vehicle, ozone is not emitted directly into the air.
Instead, it forms when other components of the air, nitrogen oxides and volatile organic compounds, react under specific environmental conditions, including the presence of sunlight.
“The pollutants that contribute to the formation of ozone – nitrogen dioxide and volatile organic compounds – were within normal levels,” explained the NEA spokesman. “But weather conditions such as ambient temperature, ultraviolet levels, wind speed, wind direction and rainfall can also influence ozone formation at ground level.”
For example, the maximum temperature on February 27 was 35.3 degrees Celsius. This was the highest recorded in 2021 for the northern region, the spokesperson said.
“This, along with the high levels of ultraviolet, could have contributed to the elevated ozone levels, reaching an unhealthy range,” he said.
Associate Professor Koh Tieh Yong, a meteorological and climate scientist at the Singapore University of Social Sciences, told ST that ground-level ozone can also be destroyed by hydroxyl radicals, a “cleaning agent” in the atmosphere that forms naturally from water vapor.
But since the northern regions of Singapore are farther from the large expanse of sea to the south, the concentration of hydroxyl radicals would have been lower, Professor Koh explained.
Additionally, ongoing dry conditions due to the current phase of the northeast monsoon season are bringing drier winds.
“This makes it more difficult to remove ozone once it forms,” he said.
Update on cross-border haze
In February, Indonesian President Joko Widodo urged local authorities to prepare for potential wildfires later this year, as hot spots had been detected in Sumatra, southwest of Singapore.
His comments had led to speculation on social media whether Saturday’s poor air quality had been caused by cross-border haze from the fires there.
But it’s unlikely that smoke from hot spots in Sumatra contributed to poor air quality in Singapore this time.
This is because during the current northeast monsoon season, the winds are blowing mainly from the north or northeast.
A spokesperson for the Singapore NEA Meteorological Service told ST that as the wind is likely to continue to blow from the north for the next few days, “it is not likely that any smoke haze from the Sumatra fires is heading our way.”
He added: “However, the haze could affect us if more hot spots develop in Sumatra and the winds change to blow from the west or northwest.”
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