Scientists have called on the government to study the causes of the North's smog crisis so that it can adopt a strategy to prevent a repeat.

Jariya Boonjawat, of the Southeast Asia Regional Centre for Global Change System Analysis, Research and Training, said she was upset with news reports putting the blame only on man-made forest fires and people burning household garbage and farm waste.

Chiang Mai, Chiang Rai, Mae Hong Son, Lampang and other upper northern provinces have been covered with thick smoke for more than a month.

Chiang Mai and Mae Hong Son have had the highest levels of air pollution.

As a scientist who has monitored the climate and air conditions for years, Jariya is certain the crisis is the result of a combination of factors that contribute to generating and trapping heat and smoke.

"This year was a jackpot," she said, referring to the way all factors had come together to create the phenomenon.

The burning of biomass (vegetable and animal waste) often takes place in the dry season, she said.

Other factors this year include the increasing number of hot spots created by biomass burning, new buildings and roads contributing to a rise in temperature, and higher rates of toxic emissions from vehicles and industrial plants.

Nature has also played its part with high temperatures, a high-pressure air mass from China, less humidity in the air and the region's geography.

Jiraya said all these factors, which started happening many years ago, were linked to each other like a chain.

Pakpong Pochanart, a scientist at the Frontier Research Centre for Global Change at Japan Agency for Marine-Earth Science and Technology, came up with a similar opinion.

In an e-mail interview with The Nation, he said Thailand's dry season featured high temperatures and less humidity. When biomass burning takes place, it lasts longer and is harder to extinguish. The longer the fires, the more smoke is emitted, generating more particles and soot, he said.

Moreover, high temperatures generally mean clear skies and more solar radiation, which eventually accelerates the photochemical reaction that creates brown haze, he added. For Pakpong, the key factor in creating the smog was specific weather conditions.

His idea was supported by data from the Meteorological Department which showed that a high-pressure air mass from China had covered the region for longer than in previous years.

Normally, Chiang Mai would have cold air only until early February, but this year it stayed until the end of March.

The air mass had blocked vertical wind and did not allow smoke and particles to blow to a greater height. In a city surrounded by mountains, the smog could be trapped for a long time, said Pakpong.

Data from the Forest Fire Control Division showed there were 3,957 forest fires in the North from last October to March 27. In the whole year from October 2005 to September 2006, there were only 3,129 forest fires.

Moreover, the area covered by roads with dark materials also increased. Data from the Rural Roads Department showed it had built 2,300 square kilometres of roads in Chiang Mai as of last October - more than 10 per cent of the total area of the province.

The department is just one of many state agencies involved in constructing roads. The area taken up by roads is increasing year by year.

Jariya called on the government to tackle the problem as a whole, not just focus on biomass burning.

She said the government should not overlook air pollution generated by urban development.

"Increasing air humidity by spraying water will only reduce the number of small particles generated by biomass burning, not aerosol generated by fuel combustion, which contains volatile organic compounds that will not mix with water," she said.

She also wants the government to give more support to research on changing climate patterns.

Pennapa Hongthong

The Nation