Hazy conditions in parts of Singapore this week might have prompted fears that fires were once again burning in the region, but experts point to culprits closer to home: factories and cars.
Air quality scientist Erik Velasco told The Straits Times it was not transboundary haze: "The haze-like condition could have been due to the concentration of particle vapour and local emissions - from cars and factories."
Usually these particles would disperse on their own, but Dr Velasco said that cloudy conditions and "not too much wind" could have caused the vicinity to appear hazy.
Residents in some areas, including Pasir Ris, had reported hazy conditions on Tuesday even though the 24-hour Pollutant Standards Index (PSI) level was in the moderate range at 6pm that day.
The hourly concentration readings for PM2.5 - tiny pollutant particles associated with haze - were also in the normal band, said the National Environment Agency (NEA) in a daily haze update.
In response to queries, NEA said that the "slightly hazy conditions" on Tuesday morning were due to an accumulation of particulate matter under "light wind conditions".
This was coupled with some convergence of winds around the south-west areas of Singapore. "The accumulation of particulate matter had dispersed around noon and PM2.5 concentration readings have remained in Band I (Normal)."
Dr Velasco added that rainy conditions meant there might not have been much solar radiation as well. Solar radiation releases heat, which produces turbulence that helps to disperse particles in the air.
"All these contributed to the concentration of particles and what looked like hazy conditions. People have nothing to worry about."
NEA said that due to cloud cover, it did not detect any hot spots in Sumatra on Tuesday, but added that there were showers over most parts of the area that day.
The incident comes after the events of last Monday, which saw a chemical stench cloaking parts of the island, in areas like Sengkang, Punggol and Bishan. Experts had ruled out volcanic action in Bali and the haze as its cause.
The source of the stench was later traced to a chemical plant in the Johor Baru industrial town of Pasir Gudang.
The daily haze update for yesterday showed that the 24-hour PSI was in the good-to-low end of the moderate range, and the hourly PM2.5 concentration was in the normal band.
There were showers over most parts of Sumatra and no hot spots were detected due to cloud cover yesterday.