SINGAPORE: The air quality in Singapore could deteriorate further into the Very Unhealthy range through Tuesday (Sep 29), said the National Environment Agency (NEA).

Hazy conditions worsened on Monday due to a shift in prevailing winds, bringing denser haze from Sumatra towards Singapore, said NEA in an advisory on Monday. The 24-hour Pollutant Standards Index (PSI) entered the Unhealthy range late Sunday.

NEA said that the 24-hour PSI for the next 24 hours is expected to be in the "mid to high sections of the Very Unhealthy range", with some improvement in air quality expected on Tuesday morning. According to data from NEA, the 24-hour PSI has been mostly trending upwards since 9pm on Sunday.

NEA added that 127 hotspots were detected in Sumatra on Monday and most of them were located in the southern region.

"Moderate to dense smoke haze continues to persist in central and southern Sumatra, and some haze has also spread into the sea areas south of Singapore," said NEA.

TYPHOON DUJUAN CAUSING HAZE TO WORSEN, SAYS EXPERT

Meanwhile, one expert has said that Typhoon Dujuan, which made landfall on the northeastern tip of Taiwan on Monday evening, is one of the reasons for the worsening haze in Singapore. Associate Professor Matthias Roth from the National University of Singapore's Department of Geography said this is because the typhoon caused winds to change direction.

Winds travel from high-pressure to low-pressure places. A typhoon causes a low-pressure environment, such that winds from high-pressure zones are sucked towards the low-pressure zone. These winds can carry with them haze to Singapore, said Assoc Prof Roth.

The occurrence of typhoons north of Singapore is one factor that makes it difficult to predict the haze levels.

Assoc Prof Roth explained: “In terms of the number of hotspots, it is relatively difficult to determine the number of hotspots we have if we do not have total satellite coverage. Maybe we have clouds in the area, in which case satellites cannot tell us how many hotspots there are.

“The prediction of the wind direction is also not always very accurate, especially in this time of the year. Winds are relatively light. That means that winds can veer from one direction to the other, unpredictably."

He said this is why haze levels last Friday did not turn out to be as high as anticipated. Authorities had decided the day before to shut schools in view of the worsening haze situation.

Apart from favourable wind directions, heavy rain can also bring reprieve from the haze. "If we have very heavy rain, obviously the larger - and probably some of the smaller - haze particles will be rained out and be deposited on the surface," said Assoc Prof Roth.

However, NEA earlier said that rain may not occur as often due to the El Nino effect. NEA said Singapore is set to be drier and warmer this season, due to the El Nino condition in the equatorial Pacific Ocean.

Experts said cloud seeding may not be an option for Singapore to take to lighten the haze situation, because for cloud seeding to take place, there must be enough clouds. There are not enough clouds at this time of the year, with Singapore in the southwest monsoon season.

Experts also said that there is not enough scientific evidence to suggest that cloud seeding can work in the tropics, although it has been proven to work in the mid-latitude areas like central US.