Assessing the high-resolution PM_2.5 measurements over a Central Himalayan site: impact of mountain meteorology and episodic events

The pristine Himalayan region is often influenced by continental and transboundary intrusions of pollutants over the year. Here, for the first time, systematic high-resolution measurements of PM2.5 are made during March 2018-February 2020 at a high-altitude station Manora Peak, Nainital (29.36 N, 79.46 E, 1950 m amsl) in the Central Himalayan region. The annual mean values of PM2.5 are observed to be 18 +/- 17 mu g m(-3) and 14 +/- 11 mu g m(-3) for 2018 and 2019. Seasonal variability consists of peak levels during summer (26 +/- 39, 17 +/- 25 mu g m(-3)) and the lowest levels during the monsoon (11 +/- 12, 10 +/- 10 mu g m(-3)). Convergent cross-mapping analysis between meteorological parameters and PM2.5 identified the wind speed (skill factor ".": 0.47), temperature (0.44), and boundary layer height (0.39) as major factors for the frequent poor air quality incidences (88/606 days), exceeding daily limit of 25 mu g m(-3). Such episodic deterioration of air quality is mainly caused by dust, biomass burning, and daytime enhancements in pollutants during winter. The day-night variation of PM2.5, in conjunction with boundary layer height and zonal/meridional wind cross-section, revealed that distinct transport mechanisms are responsible for pollutants reaching higher altitudes in summer and winter. To decipher the processes, two case studies, each of dust storms ( PM2.5: 340 mu g m(-3)) and biomass burning ( PM2.5: 110 mu g m(-3)), are presented that highlight the necessity of continuous PM2.5 measurements to be potentially used for regional climate models and pollution mitigation strategies.