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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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AIR QUALITY INDEX & VISIBILITY
The Air Quality Index (AQI) is a scale to estimate
the health impact of air pollution
In Northern Thailand, the AQI is almost only determined by PM2.5
Most of the year is 'healthy' to 'moderate' and
becomes 'unhealthy' to 'hazardous' during the burning season
Air pollution decrease visibility down to a 10-20km with mild pollution and sometimes below 1km during intense haze events
Air Quality Index
The Air Quality Index (AQI) is a quick estimation tool designed by environmental authorities in different countries to assess the effects of air pollution on human health and inform the public with a simple scale (A similar scale exist for water, the WQI). The scale is not simply proportional to the concentration of pollutants as the index is assessed on the health effects that each measured pollutant has on the human body. As a result, the correlation between AQI and pollutants is not continuous and change when certain thresholds are passed. It also has no cumulative effect. Only the highest pollutant is used to calculate the AQI, regardless of the concentration of the other pollutants.
Components that enter into the AQI determination are fine particulate (PM2.5), coarse particulate (PM10), ozone (O3), carbon monoxide (CO), Nitrogen oxides (NOx), Sulfur dioxide (SO2) and in some scales (i.e. India, Australia), other pollutants such as ammonia (NH3) and lead (Pb) are also measured. The different pollutants are computed into a piecewise linear function with various breakpoint (5 to 10) providing a result ranging from clean air to very hazardous air pollution. For particulate matter, the AQI considers an average of PM-related health effects established by government agencies and reassessed every few years based on recent research and changes in air pollution in the region where the AQI is applied.
Figure 12: Equivalence between particulate matter concentration (PM2.5 and PM10) and ozone and the resulting AQI. The different break points are visible in the change of slope for different pollutants.
In Chiang Mai, the AQI is determined by particulate matter as it is by far the most dominant pollutant. Most of the time, PM2.5 concentrations dictates the AQI but in some rare cases, when nearby forest fires are present, PM10 can take over temporarily. Other pollutants are almost never a main concern and their independent AQI value is at its worst, moderate during the burning season while PM is high in the unhealthy to hazardous range, resulting in an AQI permanently solely defined by PM concentrations.
Since AQI is a non-linear index based on health factors, different scales are used with different values and breaking points. The US AQI is very similar to the Chinese AQI and scales used in Asia also have a similar range of values (0 to 500) but with slightly different definitions. Other scales used in Europe, UK, Australia, India, etc. can have different indices (0 to 10, 0 to 100) but health warnings for similar pollution levels remain essentially the same. In recent years, an homogenization of world data is available on the World Air Quality Index Project (waqi.org) where all data is recaculated on the US AQI to avoid confusion.
Some misunderstanding and inaccuracies can still occur. The close proximity between actual concentrations and AQI for particulate matter can be confusing and is often seen in the media; the mixing up of average values over different time periods is also common place in the media and even official sources of information. There is also an inherent issue with AQI that assumes all dust air pollution are the same (urban, industrial, desert dust, biomass burning) while in reality the AQI is mostly calibrated on urban-industrial pollution which is considerably different to biomass burning of Northern Thailand (as an extreme example, the AQI during the Bhopal disaster in 1984 would not have registered any high values, despite the thousands of deaths that occurred from polluted air). For more information on the local toxicity of air pollution in Northern Thailand, please refer to the tobacco equivalence page)
A Thai AQI scale exists as a local index to estimate the health risk of air pollution. It differs from the typical US AQI or Chinese AQI by having a higher tolerance for low and medium level of pollution (up to 400%) while providing higher AQI numbers in the unhealthy levels of air pollution (15 to 25%). The index is poorly defined (no official or scientific publication) and it is unclear in which condition the Thai AQI scale is used
Figure 13: Comparison between the US AQI and the Thailand AQI scale for PM2.5 and PM10. Divergences are shown relative to equivalence shown by the black dotted line.
In recent years, in absolute terms, Thailand has lowered its warning level from 50 to 37.5 μg/m3 (AQI 135 to AQI 100 on the US scale; AQI 100 to AQI 50 on Thai scale) for daily averages. These values are worth comparing to WHO guidelines for yearly average pollution which are 15 μg/m3 for PM10 and 5 μg/m3 for PM2.5 with an upper daily limit of 15 μg/m3. Both PM categories have very unrealistic goals to be reached in South-East Asia since the natural background is relatively high to 10-20 μg/m3 in the rainy season.
AQI variations
Outside the dry season, local variations can be extreme (up to 100x background) for a single pollutant. When sources are investigated and found, it is the result of a local emission polluting near a monitoring station but defective monitors have also been noticed.
Less extreme variations (<10x background) are likely explained by a local source (burning, house fire, industrial or urban activity) or a specific environmental feature (winds, topography, etc.) that can cause a short-lasting anomaly. Particulate matter size distribution and chemical composition can also change within these anomalies.
Minor variations, (often mentioned by social media commenters, sometimes within a pinch of conspiracy), are the result of natural variability in air pollution, not so different from cloud distribution in the sky, but at ground level, affecting the distribution of haze. The position of detectors in the urban environment also creates variability when these are placed near traffic, buildings, different elevation, semi-closed spaces, etc.). Detectors also have inherent concept issues that create inconsistency in available data. All commercially available AQI-meter are poorly calibrated instruments with low-accuracy and low robustness (meaning large systematic errors) and are also affected by time-drift (an new AQI-meter behaves differently than an old one, an AQI-meter experiencing very high pollution has some memory of it) . The precision of these instruments is around 10-20%. It creates significant scattering in the data recorded but has no health impact on the everyday interpretation of results and decisions that would follow (i.e. when an AQI is stated as 'hazardous' (>300), it does not really matter if your device is measuring an AQI of 380 or 420). Public sources providing AQI maps of a region where no AQI-meter is present are based on satellite measurement of atmospheric transparency. The techniques has its own advantages and disadvantages. It is not uncommon for a private AQI-meter to measure values significantly different from satellite measurements.
Another source of variation and misunderstanding lies in the use of different scales and measurement characteristics. The scientific approach exclusively express air pollution in mass concentration per volume (ex: microgrrams per cubic meter), avoiding the AQI that is non-linear, based on health factors that can be modified and not consistent between indices. However, for AQI and concentration measurements, the time scale factor also has to be considered. All data (instantaneous, hourly, daily, monthly, yearly) cannot be compared for different time period due to the associated variability. It is not uncommon to see some media and public sources displaying instantaneous or hourly averages as daily averages, which can present the air pollution as a lot worse than it actually is.
Visibility
Along with health effects, low atmospheric visibility is the other main consequence of air pollution experienced by all residents of Northern Thailand. Since Doi Suthep is visible from most parts of Chiang Mai metropolitan area when the weather allows it, it is often used as the first indication of the presence of air pollution. As particulate matter absorb and scatter sunlight, there is a strong correlation between pollution and how far you can see
Figure 14: View of Doi Suthep from Dr Artima Medical Clinic in Mae Hia, (7.5 km from the ridge line). a. In good atmospheric conditions (Low AQI, medium humidity). b. In mildly polluted conditions (AQI ~150) and medium humidity. c. In mildly polluted conditions (AQI ~150) but back lit by sunlight. d. In mildly polluted conditions (AQI ~150) but high humidity (~95%).
Atmospheric transparency is dependent on a few parameters but the most important ones are particulate matter content and humidity. Carbonaceous matter emitted by biomass fires has strong absorption and scattering properties in the visible spectrum. From the ground, the sun appears redder than normal when light is passing through smoke plumes due to differences between blue and red wavelengths. It also results in thick haze having an orange tinge by through transmission of light but bluish in reflection.
The reduction of visibility is directly related to these optical properties and has been described in Chiang Mai for decades. When the AQI<50, the visibility is mostly unaffected except for long distances. Between AQI75 and 200, the visibility decreases from 30 to 10 km, progressively hiding topographical features on the horizon. for AQI above 200, the visibility steadily decreases and is eventually observable within the city where light poles, bridges, buildings start to disappear in the distance.
Figure 15: Approximate visibility threshold for the ridge line of Doi Suthep (grey) as a function of AQI PM2.5. This map is only relevant between 10 am and 3 pm for relative humidity below 60%.
Although this correlation seems easy to apply, there is a strong tendency to overestimate air pollution in the pre-haze season (December-January) based on visibility. This is due to relative humidity that start to have very strong effect on atmospheric transparency for values above 80% and could then be called fog (or mist) rather than haze. The most distinctive observation between fog and haze in Northern Thailand is the colour; while haze tends towards orange-yellow colours (sunlight through or above) or bluish (sunlight behind the observer), fog is consistently grey but a mixture of the two is possible. Fog can also dissolve quite suddenly and pseudo-pollution that was there an hour ago is not completly gone.
In the early stages of the burning season (December-January), it is not uncommon to have a mild early morning fog which makes visibility not compatible with measured levels of air pollution. A 95% relative humidity in clean air over Doi Suthep gives an atmospheric transparency equivalent to an AQI of 300. Although purely observational, the misinterpretation of fog for haze is enough to cause psychosomatic symptoms for some individuals. For most of the burning season (February-April) , the humidity lies between 40 and 60% and the effect on visibility is minor.
Figure 16: Correlation between visibility (in km) and air pollution given in PM2.5 AQI. Different curves are provided for different relative humidity levels.
Contrast is also very important. Doi Suthep is always visible at dusk, even on the most polluted days. Air pollution tends to be lower at the end of the day, but more importantly, the sun is right behind the mountain seen from the city centre, creating a strong contrast. In the morning, looking at Doi Suthep from the city, the sunlight is scattering and reflecting significantly on air pollution, preventing any contrast between the ridge line and the sky. The effect is a bit similar to car headlights in a heavy fog. Hard to see an object in front of you with your own headlights but if a car comes towards you with headlights, it is more visible.
Altitude of observation has a minor effect and only becomes significant if elevation is more than 1500 m at which point air pollution start to decrease. Other pollutants (gases) concentrations are low enough to have no significant effect on visibility.
Another effect related to atmospheric transparency is the decrease in radiative flux at ground level by 100 W/m2 or even higher. As a result, solar panels efficiency in conditions with an AQI of 150 have an output reduced by 6 to 11% depending on the type of photovoltaic device used.