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CHIANG MAI BURNING SEASON
This webpage is a short summary of the composition of air pollution in Chiang Mai and Northern Thailand. Further information and references can be found in 'Comprehensive Review of the Annual Haze Episode in Northern Thailand (Pirard & Charoenpanwutikul, 2023)'.
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CAUSES OF AIR POLLUTION: ATMOSPHERIC EFFECTS
Air pollution is enhanced by dry and hot weather and low winds
Colder surface temperature and warmer in altitude (thermal inversion)
prevents polluted air to escape from valleys
Shrunk layer where air is mixed prevents dilution over the whole atmosphere
El Nino creates weather conditions for bad burning season
La Nina is more humid with favorable weather conditions
Climate Conditions
Biomass burning alone would not be able to create the high levels of air pollution seen over the whole region if it wasn't for a series of other factors that help accumulation of pollution to create thick haze. Many areas of the world suffers much larger annual burning events yet do not reach excessively high level of haze.
The typical seasonal pattern starts to show above background PM concentration in December followed by a more drastic increase around mid-January to early February, reaching its peak in March with a gradual to sharp drop by the end of April or early May. The initial rise is linked to agricultural fires followed by forest fires in February-March (see below). This time of the year coincides with the increasing dominance of thermal lows over Northern Thailand, producing conditions of hot, dry, stagnant air with clear skies, light winds and a low dew point. These conditions are ideal for wildfires but more importantly, they also help mobilizing the smoke produced by these fires and accumulating it in valleys and basins below 1500 meters. So while regional pollution levels are strongly correlated with the number of fires (themselves correlated with seasonal temperature and humidity), it appears that medium ground temperature, low humidity, low wind speed, stable atmosphere with a strong temperature inversion are the factors that explain concentration of smoke to very high levels at the bottom of valleys.
Inter-annual variations
The intensity of these meteorological parameters varies from year to year and are strongly related to the state of the climate pattern El Niño - Southern Oscillation (ENSO). During El Niño or Neutral ENSO, the dry season is prolonged and sometimes extreme, creating conditions ideal for forest fires. These conditions also produce a dominant air flow with a clear NW-SW direction that accentuate the haze situation in Northern Thailand by bringing air pollution from regions experiencing heavy biomass burning. The worse years of the last two decades (2007, 2010, 2019, 2023) are all associated with this situation. La Niña produces a higher amount of precipitation and atmospheric humidity, limiting fires but also induces more erratic air flow with no specific source for long-distance aerosols. The best years in recent times (2003, 2011, 2022, 2025) had a clear La Niña weather system during the burning season.
Figure 9: A rather complicated plot of monthly PM10 values between January 2010 and December 2023. The colour code (red-orange for El Nino, yellow for neutral, greenish to green for La Nina) shows that the average haze during the burning season can be at least twice higher during El Nino and Neutral ENSO than during La Nina.
Weather Conditions
The mixing layer of the atmosphere is a zone where air is carried by vertical flow, in effect, mixing the whole lower atmosphere. While night-time mixing layers are often near the ground, the daytime thickness of the mixing layer is usually 3±0.5 km thick (more or less where the top of good weather clouds are). During the burning season, this layer is reduced to almost nothing, well below 1000 meters during daytime. It means that pollutants, instead of being diluted over several thousands meters of air, are concentrated in a collapsed layer. With the exception of some high mountains such as Doi Inthanon or Doi Ang Khang that penetrates out of this layer, the whole region lies within it and the situation is particularly problematic in valleys and basins where the high air pressure and topography produce a thermal inversion layer trapping pollutants close to the ground.
Figure 10: Atmospheric profile for temperature during the months of the burning season with a qualitative curve of air pollution distribution and the maximum height of mixing layer (horizontal lines). The daytime mixing layer is at its minimum in March and the highest thermal inversion is also observed for that month, up to 10ºC increase with altitude.
The temperature inversion is an atmospheric situation where the air near the ground is colder than in altitude. Since cold air means more dense, that air stays near the ground and the layer becomes a very stable setting with minimal air movement. The thermal inversion (where temperature increases with altitude) reaches its maximum during early mornings of March when the ground temperature can be 10ºC lower than a thousand meters above. The strong contrast of density between layers of air strongly limit vertical movements and hills surrounding valleys limit any horizontal flow and any pollution is basically trapped in that space. As the ground heats up during the day, the thermal inversion eventually dissipates in the afternoon which explain why air pollution is generally a bit better at that time of the day.
Another related factor is the humidity saturation and the difference between air temperature and dew point. In the weather conditions of the dry season, with a thermal inversion, there is very little chance to reach conditions where humidity could precipitate into rainfall and capture particulate matter in the process. The most obvious effect is the common absence of clouds in the sky during the burning season.
Daily Variations
The change in atmospheric conditions throughout the day has a direct impact on air pollutants and near 10-fold daily variations (70 to 600 μg/m3), dependent on emission sources, are recorded. Particulate matter highest concentrations occurs at 9 am in January-February to shift to earlier time (6-7 am) by mid-April. Lowest concentrations are met in mid afternoon (3 pm)
As the sun rises, higher atmospheric layers are warmed up while ground in valleys is still in the shade, producing the strong thermal inversion trapping pollutants that couldn't escape during the night due to the suppression of the mixing layer. By 8 to 10 am, the atmosphere is warming up, initiating a slight decrease in pollutants until mid-afternoon. Ozone (and sulfates) shows an opposite trend, increasing after sunrise when the stable nocturnal layer break up and reaches a peak at 3 pm due to photochemical reactions. At night, ozone is decomposed or deposited and reacting with surfaces.
Figure 11: Schematic representation of the state of the atmosphere during the burning season (top) and outside (bottom). A fire producing smoke is dispersed over a considerable atmospheric column in the bottom case while burning season conditions trap this smoke in valleys due to low mixing layer and thermal inversion
SOURCES OF AIR POLLUTION
The source of air pollution in Northern Thailand
is biomass burning (forests, grassland, crops)
In Chiang Mai during the burning season, forest fires
produce ~90% of the smoke
In Chiang Mai, in December-January and May,
ricefields are the dominant source of smoke
In some areas of Northern Thailand, agricultural burning
can represent 50% of smoke production for the whole season
Transborder pollution only contributes to background levels
and not intense haze
Historical data and more recent scientific reports have identified the source of air pollution in Northern Thailand as biomass burning (forest & grassland fires, agricultural residues) and excludes any major contribution from other sources such as ground dust, traffic, industrial and agro-industrial emissions.
Based on emission of particulate matter, biomass burning produces 85 to 97% of air pollution in Northern Thailand. Detailed chemical studies and ground & satellite land surveys provide a very precise information on air pollution sources. From these data sets, identification of sources such as the type of forest that burn, the type of plant species (trees & crops), the type of fuel (leaves, stems, seeds, sheath, stubbles, etc.), the type of fire (flaming, smoldering, etc.), the contribution of each of these elements to the air pollution and to specific chemicals, variations between districts and over a few days; all of it can be reasonably determined using proper analysis that is too long and complicated to expose here. Claims that diverge from the scientific consensus (i.e. dominance of corn burning, peak pollution in Chiang Mai due to farmers, excessive emphasis on transborder sources, etc.) can easily be dismissed by a simple look at the data available in numerous studies.
Figure 17: Simplified land use map of Thailand showing the surface occupied by forest in the north and the presence of significant agricultural areas only in the eastern part of uppermost Northern Thailand.
Forest fires
In Chiang Mai province, around 80% is covered by forest with 5% of land allocated to rice paddies and another 5% to other crops. Of these 80%, 70% are considered by the Forest Fire Control Division as subject to fire, with dipterocarp & deciduous forests carrying the highest risk and also representing the largest surface (86% of forests). At the national level, only 46% of haze is linked to forest fire but the same dataset show that two-third of forest fires occur in uppermost northern Thailand and more particularly in Chiang Mai, Tak and Mae Hon Song provinces.
In Chiang Mai province, it is estimated that forest fires represent 80 to 92% of air pollution emission during the strong haze episode between mid-February to May. The correlation between forest fires and haze is clearly established for decades with zero fires from June to November, a handful of events in December and May, hundreds in January, February and April and thousands of fire events in March. In the past two decades, satellite data has significantly strengthen this observation in real time, showing an overwhelming distribution of hot spots in forested areas.
The causes of forest fires are monitored through surveys for the past two decades and are mostly unchanged. 40 to 75% of fires are started to help the collection of non-timber products (mushrooms, bamboos, herbs, honey), 10 to 25% for hunting activities, 6 to 20% for agriculture (slash & burn, land clearing, animal farming), 0.2 to 2% for illegal logging, 1 to 10% for criminal or accidental causes and a significant percentage of unclear causes. In recent years, surveys show that the collection of plant and animal products from forests represents 73 to 82% of identified causes of fires.
Figure 18: Source contribution estimation to the air pollution in Chiang Mai province. The haze episode (February to April) is dominated by forest fires while the pre-haze period has higher contributions from agriculture burning
Agricultural fires
This emission source includes the burning of farming residues, clearing of cultivated fields and some pre-harvest crop preparation. Wild grassland is typically included in forest burning and is dominant in some provinces such as Tak. Agricultural burning is applied to rice, sugarcane, cassava, corn, soybean and potato as well as orchard clearing and is practiced at any time of the year. However, locally, such as uppermost Northern Thailand, the burning can highly seasonal. For example, 60% of rice burning occurs during December-January while only 3% happen in February to April. A second post-harvest burning in April to June represents another 30%.
Agricultural burning is a major emission source in Isaan and Central provinces and is given overwhelming media airtime due to its impact on Bangkok air quality. But it is important to note that agricultural burning is significant but generally a minor pollution source in the North. Substantial variations exist between provinces however and while heavy haze is very strongly associated with forest fires in Chiang Mai; Eastern provinces (Chiang Rai, Nan) have significant agricultural contributions that represent up to 60% of local air pollution emissions.
The reasons behind agricultural burning are mostly traditional and similar over all South-East Asia, Southern China and India. Farmers believe that it makes tilling easier, control insects, diseases and weeds, release nutrients and increase yield. Some of these benefits have been scientifically demonstrated but there is equally scientific evidence that burning lead to loss of organic soil, degradation, compaction, erosion and a lower soil fertility with a loss of nutrient and medium-term lower crop production with higher need for fertilizers. Regardless of agricultural and pedological advantages and drawbacks, the main reason for open-burning is economical or technical as it is the cheapest option to clear a field, the most efficient in regard of contract farming (with a short crop rotation) and in some mountainous areas, the only suitable option to clear land.
In the North, it is estimated that 6 to 11% of rice straw is burned but some regions have up to 50% burning. Corn burning is frequently blamed as a significant source of air pollution but scientific evidence shows that while 25 to 75% of corn residues are burned, the contribution to air pollution in Chiang Mai province is almost insignificant as corn is not a particularly abundant crop. Sugarcane pre-harvest burning is a very important cause of air pollution in Thailand but with the exception of some fields in Tak, Phrae and Uttaradit provinces, sugarcane is a rare crop in uppermost Northern Thailand.
Industry
There are no large industrial complexes in Northern Thailand. Most factories in the Chiang Mai-Lamphun area are small-scale companies with only a handful of metal-producing, chemical and non-metal (ceramic, concrete, pottery) factories that would produce very small amounts of pollutants. One local exception is the Mae Moh lignite-fired power plant in Lampang that used to be a very significant source of toxic pollution but conditions have dramatically improved in the last couple of decades. No relationship has been found between busy commercial and industrial areas in the North and the contribution is estimated to be <0.1% during the burning season.
Traffic & Urban sources
In large urban centres, traffic can have a significant influence on air pollution and its composition. Isotopic studies show that fossil fuels form up to ~40% of particulate matter during the rainy season in Chiang Mai city but it is down to a couple of percent during pollution peak in the burning season (and 0.1% in rural areas). However, for some harmful chemicals such as PAHs, traffic can represent a very significant contribution. Urban activities also contribute to some ground-like dust production (roads, concrete) and more specific and potentially toxic pollutants (commercial cooking, garbage burning)
Trans-boundary pollution
Dominant winds during the burning season brings air from India and Myanmar to Thailand at an average rate of 300±100 km/day at 1.5 km high. The emission sources are similar to Northern Thailand, mostly forest and agricultural fires and their impact over Thailand will depend on weather conditions in source areas and wind patterns.
In Chiang Mai, it is estimated that 60 to 90% of background air pollution is provided by long-distance transport from Mae Hon Song and Myanmar. However, during high haze levels, transborder sources contributions are unchanged while proximal emission sources soar due to local (within the province) fires. It makes the contribution of transborder pollution an important factor to consider but not as significant as it seems during very polluted days of March and April.
It is also worth noting that the origin of transborder sources is variable spatially and while Myanmar & Mae Hon Song are important polluters for Chiang Mai province; in Chiang Rai, a significant amount of smoke is actually produced in Northern Laos due to different wind patterns.
Figure 19: Probability map for the point of origin of pollutants in Chiang Mai after 3 days for different altitudes (10, 1000 and 1500 meters above ground) and the three months of the burning season.