Identifying the Sources of Winter Air Pollution in Bangkok Part I
By: Worasom Kundhikanjana
Tags: Data Visualization, Thailand, Data Science, Air Pollution, Environment

Air Pollution Map near Bangkok in January 2019

Air pollution is a serious environmental threat in many Asian countries. In Thailand, this issue has recently gained prominence due to the high levels of air pollution in Bangkok during the winter of 2019. Air pollution is reported through the air quality index (AQI), with higher values indicating worse air pollution. Being informed about the current outdoor air quality can help you to protect yourself.

Tourists and locals probably have questions like: Is air pollution getting worse? Is there a time of day or day of week dependence? What about provinces outside Bangkok? In Part I of this blog post, I will answer these questions through visualizing the publicly available air pollution data. From the data, we can form some hypotheses about the sources of the air pollution. In Part II of this blog post, I will model the air pollution data.

The code used to generate these plots can be found in my notebook on GitHub. Inspiration for this work came from a Facebook post by Data-addict. Check out my earlier blog post detailing how to scrap air pollution data from the government website.

How is AQI calculated?

Environmental agencies report air pollution measurements using the air quality index (AQI). Details on how to convert these measurements to numbers depend on the local laws. In general, the environmental agencies need to monitor four pollutant sources: ground level ozone(O₃), particle pollution (PM2.5 and PM10), and toxic gases (NO₂, CO, SO₂). The snapshot below shows reports of CO, NO₂, O₃, SO₂, PM10 and PM2.5 levels from various monitoring stations in Thailand on February 2, 2019, as provided by the Thailand Environmental Agency website. Air pollution indices are normal for most locations except for high levels of PM2.5 in the northern provinces, shown in red.

An AQI value is calculated for each of the major types of pollutant (each with their own formula), and the highest AQI value averaged over 24 hours is reported¹. Since the data above indicates that PM2.5 pollutants are high, we will focus our analysis on the PM2.5 data.

PM2.5 is a measurement of particles in the atmosphere smaller than 2.5 micron. In the US, a PM2.5 concentration higher than 35.4 µg/m³ corresponds to an AQI of 100, and is considered moderate for the general population and unhealthy for sensitive individuals (children, pregnant women, and elderly). PM2.5 above 55.4 µg/m³ corresponds to an AQI of 150, and is considered unhealthy.

What caused the high levels of air pollution in Bangkok last winter? The major sources of PM2.5 particles are exhaust from car engines, power plants, and agricultural waste burning, which lead to a number of theories - stagnant air flow, old diesel cars, coal power plants, and agricultural burning from neighboring provinces. As the public attention on this issue increased, people also started wondering whether the air pollution was getting worse. The government proposed some short term measures to address the issue. But for the most effective mitigations, we will need to understand the sources of the pollution.

Winter haze in Bangkok

PM 2.5 pollution in Bangkok over time( the gray vertical bands are guide to the eyes), and the corresponding google trend

Let us start by looking for general trends. This data is from the Berkeley Earth database. The air pollution in Bangkok seems to be seasonal, with surges in the winter. It appears that this is not a new problem, but public awareness has only increased recently. The air quality in Bangkok is actually still better than many Asian countries.

Is air pollution getting worse? I took the average of each year end (September — May) and counted the number of day with high PM 2.5 AQI:

Yearly average PM 2.5 AQI and the number of bad air days

The yearly average for PM2.5 AQI seems to slightly increase in winter 2018–2019, while pollution in the previous years are comparable. This could be due to the smaller number of measurements, since the data for the rest of winter 2018-2019 is yet to be measured. For the numbers of days plot, I used the number of days at the same time in the previous years to estimate the missing data. In terms of the number of bad air days, winter 2017-2018 had 70 days with very high AQI, 10 days more than the previous year. Without more data, it’s hard to judge from this graph if the situation is getting worse.

If you’re concerned about air quality, as a tourist visiting Bangkok, which months should you avoid? As a resident, what is the best time to go for a jog? Is the air pollution better during the holiday when everyone is away? To answer these questions, I plotted the daily average for different months using data from 2017 and 2018.

Daily average of PM2.5 pollution in 2017 and 2018

We again see the seasonal trend of the PM2.5 pollution. The pollution starts around late October and lasts until early April for both 2017 and 2018. The air quality seems to be better during the New Year break in 2017, but data is missing in 2018, thus inconclusive. On average tourists should avoid Bangkok between late October and early April. You may be thinking, “But that’s when I have my holiday! Where else I can go?” May I suggest Phuket or Samui Island? The south of Thailand has low AQI all year long, as you can see in the map below.

What about local residents? Is the air quality better in the morning or during the weekend? Does the pollution spike during rush hour? To answer these questions, let’s look at the hourly average during the winter. Note that I plotted normalized data for better comparison.

Normalized average PM 2.5 for different days

The air pollution is not better during the weekend. This may be because the traffic is equally bad (which is true, the traffic is VERY bad on Saturdays). Or perhaps the high AQI is due to other factors. First, let’s look closely at how air pollution is related to traffic.

Hourly average of PM2.5 pollution

There are some spikes in the morning rush hour (6– 10 am), but not in the evening rush hour. From this, we can hypothesis that other factors such as temperature and wind play more important roles in determining the air pollution level.

People suffer more in the north

With the start of the monsoon season, the weather in Bangkok will get better, and public attention on this issue will likely fade. However, without immediate policy measures, the problem will come back again next winter. In fact, the Bangkok region is not the only place that suffers from particle air pollution. Below, I show a map of particle pollution in February 2019.

particle pollution map in February 2019

The seasonal pollution is not limited to the central region. While the air pollution in Bangkok peaked in January, the pollution moved to the northern provinces. In fact, their situation is much worse as judged by the maximum pollution level across the country shown below. Note that this is a plot of the maximum from monthly averages, to get rid of outliers.

maximum particle pollution in each province

The northern provinces, such as Lampang, Chiang Mai, and Chiang Rai, suffer from worse air pollution than Bangkok, but receive much less media attention. The southern provinces, such as Phuket, Songkhla, and Trang, enjoy good air quality all year round.

In summary, PM 2.5 pollution has a seasonal trend in Bangkok and the northern provinces, but have only recently received public attention. During this winter haze, the harmful particle pollution level is high throughout the day, including on weekends. In Part II of this blog post, I will try to use machine learning to identify the source of the air pollution in Bangkok.

Thank you for reading, and stay tuned for Part II! If you have any suggestions, please leave your comment below. I am going to leave you with a gif of the monthly average particle pollution across Thailand.