Slight haze on Wednesday morning due to build-up of particles; air quality unhealthy again in the evening

SINGAPORE - The slight haze on Wednesday morning (Nov 13) was due to a build-up of particles in the air, said the National Environment Agency (NEA).

This was even as air quality entered the unhealthy range in parts of Singapore in the morning for the first time since late September. It improved later in the afternoon and was in the moderate band. But re-entered the unhealthy range in the early evening.

The source of the particles could be from various sources, from vehicle traffic emissions to smoke from fires in the region, said an air pollution expert.

NEA said in a post on Facebook in the late morning that the haziness earlier in the day was taking time to clear as winds have remained light since Tuesday evening.

“However, thunderstorms are expected in the afternoon, which should help to improve the situation,” the agency added then.

NEA said it was also monitoring the situation closely and would provide updates when necessary.

At 7pm, the 24-hour Pollutant Standards Index (PSI) reading ranged from 88 to 105. A reading of 51 to 100 is in the moderate range, while a reading from 101 to 200 is in the unhealthy band.

The 24-hour PSI first entered the unhealthy range at 10am in the east with a PSI reading of 101.

The 1-hour PM2.5 concentration reading ranged between 34 and 71 micrograms per cubic m at 7pm.

Earlier at 10am, it was between 36 and 66 micrograms per cubic m.

A PM2.5 reading of 0 to 55 is in the normal band, while a reading of 56 to 150 falls in the elevated band.

PM2.5 readings measure the concentration of tiny particles less than 2.5 micrometres in diameter - or about one-thirtieth the diameter of a human hair - in the air.

Air pollution researcher Erik Velasco told The Straits Times that several factors could have caused air quality to tip over into the unhealthy band.

He noted there were a few hot spots in Lampung, South Sumatra, on Tuesday and that some smoke from fires there may reach Singapore due to where the winds have been blowing from.

However, he said that during conditions of calm winds and a lack of turbulence in the air, “pollutants tend to be trapped close to the surface, increasing their ambient concentrations”.

“In such conditions, our own emissions – such as those from traffic, industrial activities, power generation and households – can also contribute (to worsening air quality). So the (air) pollution we saw could be a mix of trans-boundary pollutants and locally emitted pollutants,” he said.

While Dr Velasco added that no extreme pollutant levels like those experienced in September can be expected, he said: “If the levels remain high, the authorities may consider reducing or limiting some industrial activities to control our local emissions while the meteorology changes.”

Wednesday’s slight haze comes even as the inter-monsoon conditions over the island since last month are expected to continue in November.

Low-level winds are forecast to be light and may temporarily blow from the south-west or west on a few days in the first half of the month.

Air quality in Singapore became unhealthy for the first time since 2016 on Sept 14. This persisted for days and the last time the 24-hour PSI was in the unhealthy range was on Sept 24.

Haze had shrouded Singapore on many days in September because of smoke haze from forests burning in central and southern Sumatra being blown in by winds.

Rain in late September eased the haze situation and improved air quality.

NEA said on Nov 1 that a few mornings in the first half of November could be slightly hazy as particulate matter accumulates in the atmosphere under light wind conditions.

Such haziness usually improves in the later part of the morning as winds strengthen, which helps to disperse pollutants.