After months of clear skies and fresh air, air quality in Singapore deteriorated over the past two days, with a strong burning smell hanging over many areas.
Although the 24-hour Pollutant Standards Index, a measure of air quality here, continued to hover in the moderate range, it reached a high of 84 in northern Singapore at 8pm yesterday.
This is the highest 24-hour PSI reading registered this year.
The culprit this time may not be Sumatra in Indonesia, where most of the haze-causing fires that affected Singapore in September and October last year were located.
This time, the pollution is likely to have come from Peninsular Malaysia, Borneo or even Singapore's own backyard, experts say.
Winds blowing from the north-east and east could be carrying the haze to Singapore.
Local vegetation fires reported on Tuesday could also be a contributing factor, the National Environment Agency (NEA) said.
The Singapore Civil Defence Force said the largest fire it responded to on Tuesday was near the junction of Tampines Avenues 1 and 10, and it, too, was relatively minor.
Associate Professor Koh Tieh Yong, a weather researcher from SIM University, said there was a spike in the number of hot spots in Peninsular Malaysia, particularly in Pahang, over the weekend.
"In the late afternoon on Tuesday, moderate hazy conditions reached Singapore. The time lapse is consistent with our distance from the hot spot sources," he said.
Mr Chris Cheng, strategic development and research director at volunteer group People's Movement to Stop Haze (PM.Haze), suspects the haze may be coming from peat fires around an oil palm plantation located at eastern Sedili Kechil in Johor, Malaysia.
"We checked the wind direction, hot spot, peat and plantation data, but we need an image of a local fire, and on-the-ground investigations there to verify the data," he said.
The current monsoon season, when winds blow mainly from the north-east, is transitioning to the inter-monsoon season, when winds are more variable.
Over the past few days, they have been blowing from the north-east and east, said Dr Erik Velasco, a research scientist from the Singapore-MIT Alliance for Research and Technology. "It is probable that winds blowing from the east brought plumes from fires in Borneo to Singapore," he said.
While only a small number of fires were reported there over the past week, cloudy conditions could be preventing satellites from picking up more hot spots, he added.
Singapore could still be badly hit when the usual haze season rolls in around June.
Dr Velasco said: "We are still experiencing the effects of a monster El Nino that started last year. In the region, El Nino enhances dryness, and therefore fires.
"Because of the magnitude of El Nino this year, we must be prepared for a new period of intense haze similar to last year's, once the winds start blowing from the south and south-west, bringing plumes from Sumatra and Kalimantan."
Today, however, the NEA says the air quality is expected to stay in the moderate range, and normal activities can be carried out.