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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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DESCRIPTION OF AIR POLLUTION (PHYSICAL)
Air pollution in Northern Thailand is haze (not a smog) made of
micro-ashes from biomass combustion
The pollution is made of micro-particulates ranging from 0.02mm to molecular size but particularly abundant between 0.00002 and 0.001 mm
The amount of micro-particulates increases by ten times between
the rainy season and burning season
The worst month is March and air pollution is at its highest in the early monring
Microscopic properties of air pollution
The air pollution in Northern Thailand is a thick haze. It is a dry air pollution made of small particulates with relatively insignificant amount of gases. Micro-dust or smoke are terms that are acceptable (due to the nature and source of the pollution), PM2.5 is a lot more specific but not incorrect; but it is not a smog, despite often label as such. A smog is a sort of fog that results from the precipitation of humidity and toxic gases (as droplets, like clouds) producing an aggresive wet air pollution (as droplets, like clouds). Significant smog is very rare in Thailand and non-existent in the North (and to hammer the point, the PM2.5 of any smog has a value of zero). In December , it is also quite common to confuse haze with fog or mist (a decrease of visibility due to water droplets) as humidity can be very high in mornings (see the section on visibility to see this effect).
The particulate matter that forms most of the air pollution is generally described as 'PM' followed by a number indicating the maximum particulate size. PM2.5 has been more or less high-jacked by the media has synonym of air pollution, but it doesn't have to be and some high PM2.5 values can be completely harmless (see toxicity). In Chiang Mai, for the general public, the air pollution is simplified in 3 categories:
PM10: Coarse particulates with a diameter of 10 microns or less (<0.01 mm, so <1/10th of the thickness of a hair). It is historically the first fraction to be monitored since detection is easier.
PM2.5: Fine particulates and the most commonly reported fraction in public announcement and monitoring devices as it represents a significant part of air pollution with health effects. These particulates are smaller than 2.5 microns (<0.0025 mm, or <1/40th of the thickness of a hair).
PM0.1: Nanoparticulates and the smallest non-gaseous fraction of air pollution. These particulates are smaller than 0.0001 mm and are difficult to collect and analyze. Despite being a potentially significant health concern, this fraction is rarely reported due to detection difficulties.
Gases: It includes ozone (O3), sulfur dioxide (SO2), nitrogen oxides (NOx), CO2 and carbon monoxide (CO). These are present in minor quantities compared to particulate matter.
The type of particulate matter produced in a wildfire depends on several parameters such as the type of fire, the intensity of the different burning stages (ignition, flaming, smoldering), the fuel condition (live, dead, wet), its distribution (dense/light, flat/sloped) and type (grasses, leaves, branches), humidity, wind, available oxygen, displacement after emission, etc. As a result, the exact particulate size distribution (and chemical composition) changes over time and distance regarding to the emission source.
Figure 2: Distribution of particulate matter according to size. The dotted green curve is the mass distribution and dominated by coarse particulates (a single 10 μm dust grain has the same mass than 1000 dust grain of 1 μm) . The blue curve represents the number of particulates and the smaller they are, the more abundant. The red curve is the surface area distribution and is particularly important for health studies as most bioavailable toxic chemicals are on the outside of dust particulates. Units are relative to the total concentration.
Most changes affecting physical properties of smoke are solely of academic interest and minor compared with other types of air pollution (ex: fossil fuel combustion, traffic, industrial, intense firestorms, etc.). On average, around 20% (in mass) of particulate matter is PM10, 42% is PM2.5 and 23% is PM0.1 with the remainder above 10 microns and mostly found in direct proximity of fires as they eventually settle down. The PM2.5/PM10 ratio often used to characterise air pollution is quite constant in Northern Thailand as dust gets homogenised. The only exception are fires occurring within a few kilometers of measurements (ex: Samoeng, 2019; Doi Suthep-Pui 2020) leading to coarser smoke and higher concentration of PM10 relative to PM2.5. It has an effect on the resulting AQI that can be determined on PM10 instead of PM2.5 (see Air Quality Index).
During a haze episode (the scientific term for burning season), the concentration of PM2.5 and PM10 increases by 10 times from 20-30 μg/m3 monthly average in the rainy season to background levels as high as 200 μg/m3 with heavy haze events peaking at up to 600 μg/m3 hourly averages and obviously even higher values in direct proximity of fires. PM0.1 behaves slightly differently and the increase is around three-fold between rainy and dry seasons. The high background of PM0.1 indicates that in Chiang Mai, around 1/3 of PM0.1 has a source associated with permanent urban activities and unrelated to the burning season.
Figure 3: Air pollution situation in 2019 (end of March) when large wildfires occurred in Samoeng valley. The proximity of fires pushed PM10 to be the dominant pollutant while PM2.5 is generally the factor defining air quality. Most AQI scale are limited to 500 and the value 989 (PM10) and 796 (PM2.5) are unnapproved extrapolation of AQI usage (see Air Quality Index).
Variations through time
The hourly concentration of air pollution is strongly controlled by atmospheric characteristics but the actual maxima and minima can depend on the location. In Chiang Mai, particulate matter concentration increases slowly overnight to reach a maximum in the early morning between 6 and 10 am. Minimum concentrations are reached at 3 pm and coincide with the hottest part of the day. It is not unusual for PM concentration to double or triple throughout the day.
Figure 4: Average variations of PM2.5 (red) over 24 hours show stable levels at night, a strong increase in the early morning and the lowest values in the second half of the afternoon. Ozone (green), a minor pollutant, shows a high doubling of concentration at sunrise (vertical line) followed by a slow increase until 3 pm. These curves.are weather controlled as both PM2.5 and O3 show a relationship with sunrise and temperature (yellow).
Weekly changes can have 3-4 fold variations and it is not uncommon to go from a 100 μg/m3 background to reach 300 μg/m3 daily average a few days later. These variations are mostly related to emission sources and some weather patterns.
Monthly variations are related to human activities and weather patterns. The concentration in particulate matter starts to increase dramatically over the whole region in the second half of January ±2 weeks and is often considered as the beginning of the burning season. Some authors make it start earlier in December but there is a general agreement that intense haze, when visibility is constantly reduced and health symptoms are significant, covers the entirety of February, March and April with only a few rare (if any) days with acceptable levels of pollution.
The yearly average of PM2.5 and PM10 is 30 to 60 μg/m3 and these values are heavily controlled by haze episodes, which despite representing less than 15% of the year above 50 μg/m3, can have concentrations in hundreds of μg/m3. The very low rainy season background contrasting with pollution peaks in the burning season is partly responsible for the different psychological response of northern Thailand residents compared to cities with similar or worse annual pollution (ex: Dubai, Athens, Milano) (see Public Opinion).
The wide inter-annual variations, essentially dictated by the intensity of biomass burning, are dependent on the global 'climate' and particularly the state of the El Niño - Southern Oscillation (ENSO). El Niño and Neutral ENSO brings relatively high level of air pollution due to optimal meteorological conditions for burning and accumulation of aerosols while La Niña is characterized by more humidity and wind patterns limiting burning and aerosol confinement in valleys (See Atmospheric factors).
Long term trends is a minor scientific topic of discussion, yet, it is omnipresent in the media and always described as a worsening situation. The truth is that it is unclear and if anything, insignificant. Serious research on this subject shows trends from a slight increase of 0.3 μg/m3 per year to a slight decrease of 0.8 μg/m3 per year. Considering the variability of monthly averages during the haze season from less than 100 μg/m3 (2011) to >350 μg/m3 (2007), a 10% increase or decrease would take decades to be statistically visible in monthly or annual averages and it would take even longer to be felt as a difference in daily averages. The only conservative conclusion is that the haze season varies too much from year to year to identify a significant trend that could be experienced by Northern Thailand residents.
Figure 5: Example of weekly variations of PM10 between 2010 and 2017 showing the extreme variability that exist between good years *ex: 2011) and bad years (ex: 2010)
DESCRIPTION OF AIR POLLUTION (CHEMICAL)
Air pollution is made of biomass burning smoke and is mostly (~60%) carbon
Air pollution contains a large number of mostly non-toxic chemical compounds
but useful to identify the sources of emissions
Harmful chemicals are present but concentrations are not very high
compared to urban pollution
Carbonaceous components
Carbon is the main component of particulate matter. It forms 50-60% of it and is divided into two main groups: elemental and organic carbon.
Elemental carbon (EC) is a form of graphite-like black carbon that is very abundant in polluted cities but not as much in Northern Thailand haze. It can either be a residue of combustion (char-EC) or direct precipitation from combustion gases (soot-EC). These two components have slightly different compositions and absorb/scatter sunlight differently and therefore have different role in global warming.
Organic carbon (OC) includes complex molecules combined with nitrogen, oxygen and hydrogen. It is the result of incomplete combustion of biomass burning that would normally be almost totally decomposed in CO2 and H2O. Aside from the chemical classification of OC components, an optical classification of OC also exists to explain sunlight absorbing properties of haze.
The OC/EC ratio is indicative of emission sources. While fossil fuel burning has a very high EC content (OC/EC ~1), wood burning has OC/EC between 0.18 and 0.33 and undefined biomass burning between 0.1 and 0.2. The OC/EC range in Chiang Mai air pollution is 0.12 to 0.29 with small variations due to burning conditions and sources throughout the burning season.
Figure 6: Summary of chemical composition of particulate matter. The central pie graph shows the main component (carbonaceous, metals and anionic groups). Side pie charts are specific chemicals (inorganic compounds (blue & green in the central pie), carboxylic acids, PAH, dioxins, sugar alcohols (part of the yellow in the central pie)) found in particulate matter with their respective concentrations (in %). Fractions show in orange to red are carcinogenic compounds
Polycyclic Aromatic Hydrocarbons
Among organic carbon are the PAHs, some benzene-like compounds, which are of particular concern due to their carcinogenic properties. Concentrations in Chiang Mai are ~10±3 ng/m3 *<1 to 26). It's a very small value but to put these concentrations in perspective, Bangkok PAHs vary between 36 and 55 ng/m3 and Beijing 30 to 279 ng/m3 which shows that their presence in Chiang Mai pollution is not a major public health concern but is nevertheless constantly monitored. The variety of PAHs gives some information on sources and show that during the burning season, 55% are produced by traffic & fossil fuels, 15% are produced by agriculture and 30% by forest burning. This type of information is interesting as it show that even if biomass burning was absent, ideally, reducing PM2.5 by more than 95%, the remaining 5% would still keep 50% of its carcinogenic properties (see sources).
Anhydrosugars
An interesting group of organic molecules that is not a health concern. They form around 1% of particulate matter and are a bit the equivalent of caramel (dehydrated common sugar) for plants as a result of incomplete combustion of cellulose, starch, etc. The proportions between different species of anhydrosugars give some information on biomass emission sources and and can show the contribution of different forests, rice, grass, sugarcane, etc. and fire characteristics to the production of smoke. Some ratio such as high potassium/levoglucosan are indicative of a significant contribution of agriculture since potassium is associated with fertilizers.
Figure 7: Example of how detailed chemistry can help understanding the sources of haze in Chiang Mai and elsewhere. Levoglucosan, mannosan and galactosan are three common anhydrosugars. The burning of different plant species will produce different ratio that can directly be compared with the composition of haze in Northern Thailand.
Carboxylic acids, sugar alcohols and others
Other common organic chemicals in particulate matter are carboxylic acids (oxalates and acetates represent 2/3 of them) that form 1% of particulate matter. Like anhydrosugar, their relative proportion helps understanding sources and behaviour of haze. Carboxylic acids are not a health concern, nor a sugar alcohols that form a bit less than 1% of pollution. Other concerning organic molecules such as benzene, toluene, acetaldehyde, xylenes, acrylonitrile, etc., common in urban pollution, are not present in significant or detectable amounts during Northern Thailand haze episodes. The same applies to dioxins, a strongly carcinogenic substance, which has very low concentrations due to the absence of large industrial chlorinated combustion process in Chiang Mai.
Isotopic composition
Finally, among all organic components, isotopic studies can also shed some light on pollution sources. Carbon-14 is a type of carbon that can only be found in non-fossil organic matter. That includes all living organisms and recently dead (dead leaves, dry grasses & wood, etc.), agro-industrial products and biofuels. It is completely absent from all fossil fuels, most plastics, etc. Abundance of carbon-14 in haze show that 3 to 8% of total emitted carbon is from fossil fuels in urban areas and down to 0.1% in rural areas. Other isotopes of carbon and nitrogen show that C3-plants (trees, most grasses, most crops) are the main contributors to haze while C4-plants (ex: corn, sugarcane) are at best, a minor factor in pollution emission, contrary to what is claimed by some media and eco-activists who are misled on the source of the pollution.
Figure 8: Discriminative diagram based on carbon and nitrogen stable isotopes. Air pollution composition is shown as red dots and clearly sit in the C3-plant group. At best, traffic and C4-plants (corn, sugarcane) can only be minor contributors in the air pollution of Chiang Mai province. The blue arrow show a trend between January and May showing increasing contribution of forest burning in the total air pollution
Other components
Other major components in particulate matter are 13-15% of sulfates, 4% of nitrates and 1% of chlorides combined, among other things with ammonium, potassium, calcium, sodium, magnesium, etc.
Sulfates are mostly precipitates from gaseous components (SO2) produced by fossil fuel combustion and urban sources. Photochemical reactions combines sulfates and ammonium and prevent the formation of sulfuric acid (which would happen in a smog), but still causes the first rains following the burning season to have a pH soetimes down to 4. Ammonium itself comes from urban sources and biomass burning as it is used as a fertilizer in crops. Nitrates also have two main sources as traffic and biomass burning where it is strongly correlated with potassium due to its use as a fertilizer. The origin of chlorine is less defined and could be long distance maritime sources, solid waste burning, fertilizers, herbicides.
Cations include potassium that strongly correlates with particulate matter and form up to 2% of air pollution. The correlation with levoglucosan and nitrates points towards the use of fertilizers on crops while forests and grassland fires release potassium in lesser amount. Sodium forms 0.5% of air pollution and has an unclear source that could be marine, biomass burning, solid waste incineration or a crustal component. Calcium forms 1% and is associated mostly with soil as well as urban dust (concrete, roads). Aluminosilicates, iron, manganese and magnesium are soil or sub-soil related and best explained by wind-borne dust that can be mobilised by dry weather conditions and fires.
Trace elements include some more crustal elements such as titanium but also transition metals like chromium, vanadium, zinc, copper, cobalt, nickel and heavy metals (lead, cadmium, tin, mercury). Without exception, all these elements are below WHO threshold for aerial toxicity in Northern Thailand. All concentrations for these metals are well below typical urban and industrial emissions that would be found in large cities and not a health concern. Various explanations have been given to explain their sources such as tire wear, brake pads, roads & road painting, traffic exhaust, sugarcane pulp, garbage burning, etc.
Gases
Gaseous components include sulfur dioxide, nitrogen oxides, ammonia, carbon dioxide & carbon monoxide and ozone. With the exception of short-lived localised emissions occurring at random times during and outside the burning season, these pollutants are always present at very low levels. Despite the increase during the burning season, gas pollutants do not pose a significant health issue. Ozone concentrations have a systematic daily variation that mirror the behaviour of particulate matter as it reaches a high peak in the mid-afternoon (when PM is at its lowest) due to photochemical reactions (see figure 4).