Transboundary sources dominated PM_2.5 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 (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.