SINGAPORE - The high concentration of ozone in the northern parts of Singapore on Sunday was likely the result of a combination of factors such as high ambient temperatures, light winds and elevated ultraviolet (UV) levels during the day, the National Environment Agency (NEA) said on Monday (March 29).
The 24-hour Pollutant Standards Index (PSI), which measures air quality, entered the unhealthy range from 6pm to 8pm on Sunday.
Air quality is considered unhealthy when the PSI is in the range of 101 to 200. Under these conditions, people are urged to reduce prolonged or strenuous outdoor physical exertion, as ozone can irritate the linings of the respiratory passage.
Ozone is formed when components of the air, nitrogen oxides and volatile organic compounds react under specific environmental conditions, including the presence of sunlight.
However, NEA noted that the nitrogen dioxide and volatile organic compounds were within normal levels on Sunday.
Instead, weather conditions such as wind speed and UV levels, likely contributed to the high concentration of ozone, NEA said in response to queries from The Straits Times.
Dr Erik Velasco, an air pollution expert, said it is probable that Singapore is facing a local episode of phytochemical pollution, formed by chemical reactions in the air.
He noted that these often come from emissions that are anthropogenic in nature, such as traffic or the use of solvents in households.
"Many cities have schemes to reduce some industrial activities and vehicular traffic to stop pollution episodes caused by ozone," he said.
Associate Professor Koh Tieh Yong, a weather and climate scientist at the Singapore University of Social Sciences, pointed out that the ozone sub-index contributed to the peak in the PSI measured in Woodlands from 5pm to 10pm on Sunday, and the smaller peak measured at MacRitchie Reservoir one to two hours later.
"This index is an eight-hour trailing average, meaning that the peak hourly concentration of ozone probably occurred around 2pm to 4pm in Woodlands, and was blown by southward winds to MacRitchie Reservoir later on," he told ST.
He noted that weather stations in Admiralty and Ang Mo Kio had indicated winds blowing from the north during those times on Sunday.
Prof Koh also noted that ozone tends to form rapidly at high temperatures.
Between 2pm and 4pm temperatures of 32 to 34 deg C were recorded at the Admiralty and Ang Mo Kio weather stations.
Meanwhile, the low humidity levels meant the air's natural cleaning agents, known as hydroxyl radicals, were in low supply.
"Thus, the ozone that was formed rapidly was not removed fast enough, causing it to accumulate," he said.
But it is "not unusual" for intense heat and low humidity to develop in northern Singapore in March as the area is furthest from the expanse of sea to the south, said Prof Koh.
However, he added that the concern over ozone exposure is limited, because once night falls, its concentration is depleted with no more light to aid its production.
Similar conditions were also reported on Feb 27, when PSI levels had gone into the unhealthy range due to high ozone concentrations.