81% of Bhutan’s air pollution comes from our neighbors

While our air is better than our neighbors it is not as clean as we think it is

Except for the last few days the air quality in the last few weeks have been noticeably bad with a bluish grey haze covering the skies of Bhutan, including in Thimphu.

However, only around 19 percent of this is of Bhutan’s making with the rest coming from Bhutan’s various neighbors.

The World Bank in its recent report on Bhutan titled ‘Bhutan Country Environment Analysis: Taking the Green Growth Agenda Forward,’ that looks at green growth opportunities in its air pollution segment stated that nonlocal emissions dominate the PM 2.5 concentrations in Thimphu.

The trajectories of originating sources were allocated to south or southwest, with India contributing around 44%, followed by Bangladesh at around 19%, Bhutan itself at around 19 %, and China (Tibet) contributing around 16%. The same study indicated significant contributions from transboundary sources to PM 2.5 concentrations in Thimphu, Bhutan.

The World Bank report in turn is referring to a study titled ‘Transboundary sources dominated PM2.5 in Thimphu, Bhutan,’ published in the International Journal of Environmental Science and Technology by S. Sharma, R. Sharma, S.K Sahu and S.H Kota.

This paper says during the COVID-19 lockdown imposed from August 11 to September 21 in Bhutan in 2020 only a marginal reduction of 4% in the PM 2.5 concentrations was observed, indicating that nonlocal emissions dominate the PM 2.5 concentrations in Thimphu, Bhutan. 

This study estimates the potential source regions contributing to PM 2.5 in the capital city of Thimphu, Bhutan, during the years 2018–2020 using the ground-based data, followed by the HYSPLIT back trajectory analysis.

The average PM 2.5 concentration in the entire study period was 32.47 µg/m3 which is three times of the World Health Organization recommended limit of 10 µg/m3. Less than half of the days in pre-monsoon at 43.47% and post-monsoon at 46.41%, and no days in winter were within the 24-h average WHO guideline of 25 μg/m3.

The paper said very few studies, if any, were carried out on the air quality of Bhutan with a focus to understand if the pollutants are of local or non-local origin. Also, contribution of air pollution to mortality and morbidity in Bhutan is not well established. Thus, the main aim of this study is to understand the status of PM 2.5 pollution and possible source origins in the capital city of Bhutan.

Thimphu, the capital and the largest city of Bhutan, was selected for this study as it is the only city in Bhutan for which historical ground-based PM 2.5 data were available

The study period of April 2018 to June 2020 was divided into four seasons, viz. pre-monsoon (March, April, and May), monsoon (June, July, and August), post-monsoon (September, October and November), and winter (December, January and February).

For this study, archived global reanalysis data from National Oceanic and Atmospheric Administration (NOAA) were used as the meteorological input data for back trajectory analysis.

The PM 2.5 data based on regionally interpolated real-time monitoring station data were downloaded from Berkeley Earth.

Air pollution severe in Thimphu

The study found that air pollution is severe in Thimphu.

The average concentration in the study period was 32.5 µg/m3, which is three times of the WHO recommended limit of 10 µg/m3. There was a clear increase in concentrations in the months of September to March, with peaks in January 2020 (182.1 µg/m3), December 2019 (170.4 µg/m3), November 2018 (168.4 µg/m3), and February 2020 (100.6 µg/m3), which was observed.

The averaged concentrations in pre-monsoon, monsoon, post-monsoon, and winter during the entire study period were 30.4, 15.8, 29.7, and 55.6 µg/m3, respectively.

43.5%, 98.2%, and 46.4% days in pre-monsoon, monsoon, and post-monsoon were within the 24-h average WHO limit of 25 μg/m3.

However, in winters, all days exceeded the 24-h average WHO recommended PM 2.5 concentration limit of 25 μg/m3.

Daily averaged PM 2.5 concentrations exceeded the WHO guideline of 25 μg/m3 on 38.3%, 63.4%, 39.4% days in 2018, 2019, and 2020, respectively. Among all the months, the 24-hr averaged PM 2.5 concentration in the August of 2018 and 2019 did not exceed the guideline on any day, unlike January of 2019 and 2020, where the concentrations exceeded the limit on all days.

December had the maximum PM 2.5 concentrations in 2018 throughout the day with the highest concentrations observed at 4 am (65.2 µg/m3) and 5 pm (72.4 µg/m3).

In 2019, the month of January had the maximum PM 2.5 concentration during early mornings at 4 am (76.0 µg/m3) and December had the maximum PM 2.5 concentration with the peak during the evening at 5 p.m. (90.7 µg/m3).

In 2020, the highest concentrations were observed for the month of January with the peaks observed at 5 a.m. (63.7 µg/m3) in the morning and 5 p.m. (92.2 µg/m3) in the evening. Therefore, it can be concluded that in winter season, particularly December and January months, concentrations peaked during mornings and evenings.

Local emissions role limited

The study found that local emission changes in Bhutan have minor effects.

Bhutan confirmed its first COVID-19 case on 6 March 2020. The first nationwide lockdown in Bhutan was enforced on 11 August 2020 which ended on 21 September 2020. During the lockdown, travel and goods transport between districts were banned. All schools, nonessential government facilities, and commercial establishments were closed.

To assess the effect of these restricted emissions, PM 2.5 concentrations during the lockdown phase of August 11 to September 21, for the years 2018, 2019, and 2020, were analysed. It was observed that the average concentration for the lockdown phase for 2018 and 2019 combined was 15.1 µg/m3, and in 2020 was 13.6 µg/m3, indicating a marginal 4% reduction in the PM 2.5 concentrations due to lockdown in 2020.

However, from August 11 to 30, a significant reduction of 18.7% in the PM 2.5 concentrations was observed. This analysis shows that local emission sources do not affect the total PM 2.5 concentrations much. This suggests that regional transport of PM 2.5 is mainly responsible for the high PM 2.5  concentrations in Thimphu, Bhutan.

The role of wind

From the wind trajectory analysis using HYSPILT for 2018, 2019 and 2020 it was evident that majority of the wind trajectories reaching Thimphu, Bhutan, originate from the southern neighbouring regions of Bhutan.

Overall, from the back-trajectory analysis, it can be concluded that nearly 94% of the wind trajectories originate from Indian states of Sikkim, Assam, Meghalaya, northern West Bengal, Northern parts of Indo-Gangetic Plain; northern Bangladesh and Bhutan itself.

From the Concentration Weight Trajectory (CWT) analysis, the paper says it can be concluded that the regions outside Bhutan with north-east India, Indo-Gangetic Plain, Nepal and China (Tibet) are responsible for significant contributions to the PM 2.5 concentrations in Thimphu and can be considered as the major source regions.

For example, CWT analysis for 2019 showed that grids lying in Nepal, southern parts of Tibet, and northern region of Indian state West Bengal as well as western Bhutan were associated with concentration of PM 2.5 as high as 70 μg/m3 in winters.

PM 2.5 contributions from neighbours

In terms of contribution to the PM 2.5 to Bhutan by neighbouring states in pre-monsoon, the seasonal average of PM 2.5 observed in Thimphu was 29.11 μg/m3. India was the highest contributor to the PM 2.5 with 45% with Indian states of Bihar at 12%, and Uttar Pradesh at 10%, being the major contributors. The next major contributors were Nepal at 17%, Bangladesh at 13%, and China (Tibet) at a 12%.

During monsoon, the average PM 2.5 concentration observed during the study period was 15.9 μg/m3. India contributed 43% of the PM 2.5 concentrations with the states Assam (12%,), Meghalaya (14%), and West Bengal (10%) being the major contributors. Bangladesh contributed 31.04%, and regions of Bhutan around Thimphu contributed 21% of the PM 2.5 concentrations. This also shows that there are significant contributions of the regional transport to the total PM 2.5 concentrations.

During the post-monsoon, the average PM 2.5 concentration observed during the study period was 30.4 μg/m3. India contributed 42% to the total PM 2.5 during this season. In India, the states of West Bengal (13%), Bihar (10%), and Assam (8%,) were the major contributors. Apart from India, contribution of Bangladesh is 17%, China is 6%, regions of Bhutan also contributed 19% of the PM 2.5 concentrations in Thimphu.

The highest seasonal PM 2.5 concentration average of 50 μg/m3 was observed in the winter season. During winter, India contributed 45%. West Bengal and Bihar contributed to 10% and were the major contributing states of India. China contributed 20%, Nepal 13% and regions of Bhutan contributed 23%.

From the analysis, it can be concluded that Thimphu experiences the highest PM 2.5 concentrations during winter. Regions of Bhutan also contribute significantly to the total PM 2.5 concentrations observed in Thimphu.

As hourly ground-based pollutant concentration data are still not available for Bhutan, more ground-based continuous monitoring stations are required for monitoring different pollutant concentrations.

Also, daily hospital admission data logging is required to enable estimation of risks associated with PM 2.5 in Bhutan. Also, the study’s scope was limited due to the data availability; however, more detailed modelling studies using chemical transport models or dispersion models are required to explore the nature of PM 2.5 in Thimphu, Bhutan.

World’s worst polluted cities in South Asia

The above should not come as a surprise as a report of IQAir which tracks air quality across the world found that of the 100 cities with the world’s worst air pollution 99 are in Asia and most of them are in South Asia with India having the most of them at 83 cities.

Bangladesh, Pakistan and India are at the top of the rankings where the average PM 2.5 concentration exceeded WHO guidelines in 2023.

Central and South Asia were the worst performing regions globally, home to all four of the most polluted countries last year: Bangladesh, Pakistan, India and Tajikistan.

South Asia is of particular concern, with 29 of the 30 most polluted cities in India, Pakistan or Bangladesh. The report ranked the major population centers of Lahore in 5th, New Delhi in 6th and Dhaka in 24th place.

The only 10 countries and territories with “healthy” air quality are Finland, Estonia, Puerto Rico, Australia, New Zealand, Bermuda, Grenada, Iceland, Mauritius and French Polynesia.

Of particular problem in South Asia is the South Asian brown cloud or haze that builds up due to the combination of air pollution, weather and geography in the region and flows across borders.

While Bhutan has better air than its neighbours it is also impacted by this brown haze, especially in winter.

The Bhutanese requested air quality data from the Department of Environment and Climate Change and was asked to put in an official written request which it did. The data was not given to the paper. While weather reports are publicly available there is no similar system for air quality data.

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