Some of the hazardous PM2.5 plaguing Bangkok may be the result of crop burning in Cambodia, but officials stress vehicles are still the main cause.
"Most of the PM2.5 in Bangkok comes directly come from vehicles in the city and our activities are to be blamed, yet according to research, some of the haze is from neighbouring countries," Siwat Phongpiachan, chief of the Centre Research and Development of Disaster Prevention and Management at National Institute of Development Administration (Nida), told the Bangkok Post in a phone interview.
"The finding means that we need to conduct more research and monitoring on trans-boundary haze in the Asean region," he added.
The finding of what experts call the "transboundary mobility" of PM2.5 pollutants means that the government must not only keep a close watch on vehicle emissions in the city but also have a bird's eye view across countries.
"It's time to push a clean air law in the Asean region," said Mr Siwat
A test on whether it was possible for PM2.5 to be carried by wind from elsewhere was conducted Jan 11 to 15 when city residents were encouraged to wear masks against the dust, which is so small it can easily lodge in the lungs and enter blood vessels.
At a height of between 500 and 1,000 metres from the ground, experts found that "air mass drifted from the east and north east of Bangkok", Mr Siwat said.
During the same period, a satellite detected multiple hotspots in neighbouring countries, referring to crop burning for harvest or land clearance.
Pollution Control Department (PCD) director-general Pralong Damrongthai told the Bangkok Post that the department acknowledged there was transboundary haze from Cambodia and that he had urged the government to monitor the burning.
However, PCD air quality monitoring stations along the Thai-Cambodian border in Trat and Chachaoengsao have not detected severe air pollution in Cambodia recently.
Prof Serm Janchai, a lecturer on physics at Faculty of Science at Silpakorn University also shared the same view that "part" of the PM2.5 comes from open burning in Cambodia.
Prof Serm has conducted tests and found a correlation between open burning in Cambodia, wind direction, and the level of PM2.5 in Bangkok recently.
He said his study of atmospheric dust had found a high level of fine PM2.5, suggesting the source could be opening burning.
"It is not blame shifting. Of course, Thailand has a lot of work to do, but if we want to talk about a solution, we need to understand the nature of air pollution and that includes transboundary haze," he told the Bangkok Post.