Transboundary sources dominated PM2.5 in Thimphu, Bhutan

Abstract

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 (43.47%) and post-monsoon (46.41%), and no days in winter were within the 24-h average WHO guideline of 25 μg/m³. During the COVID-19 lockdown imposed from August 11 to September 21 in Bhutan, 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. Most back trajectories in the analysis period were allocated to south or south-west sector. India was the major contributor (~ 44%), followed by Bangladesh (~ 19%), Bhutan itself (~ 19%) and China (~ 16%). This study confirms that there are significant contributions from transboundary sources to PM_2.5 concentrations in Thimphu, Bhutan, and the elevated PM_2.5 concentrations need to be tackled with appropriate action plans and interventions.

Keywords: Bhutan; HYSPLIT; PM2.5; Source regions; Transboundary sources.

Fig. 1

Monthly variation of average PM_2.5 (μg/m³) concentration. The red dotted line represents the WHO annual average criteria for PM_2.5. The central line in the box represents the interquartile range (IQR) (first (25 percentile, Q1) to third quartile (75 percentile (Q3)), the dark horizontal line in the box shows the median concentrations, the extended horizontal lines at the end of dashed vertical lines represent the maximum and minimum values, and the circles represent the outliers (criteria: minimum = (Q1 − 1.5 × IQR) and maximum = (Q3 + 1.5 × IQR))

Fig. 2

Percentage reduction in the PM_2.5 concentrations during the lockdown period in 2020 when compared to the average concentrations observed in 2018 and 2019. The dashed red line shows the average percentage reduction (3.71%) observed during the lockdown period. The positive values show reduction in the PM_2.5 concentrations in the year 2020, and the negative values indicate an increase in the PM_2.5 concentrations

Fig. 3

Figure showing the proportion of time in a year, a back trajectory resides in a specific sector and the mean concentration associated with the back trajectories associated with the specific sector

Fig. 4

Gridded back trajectory frequencies with hexagonal binning

Fig. 5

Gridded and smoothed back trajectory concentrations showing mean PM_2.5 concentrations using concentration weighted trajectory approach

Fig. 6

PM_2.5 contributions from different countries to total PM_2.5 in Thimphu for different seasons

Fig. 7

PM_2.5 contributions from Indian states to total PM_2.5 in Thimphu for different seasons