A study on atmospheric fine (less than 2.0 µm) particulate matter was performed for the first time over the eastern Himalayas in India in order to investigate the formation and sources in two different seasons, pre-monsoon (Mar–May) and winter (Dec–Feb), for three consecutive years, 2008, 2009 and 2010. Fine mode aerosols were further segregated into three modes: ultrafine (less than 0.32 µm), superfine (0.32–1.0 µm) and fine (1.0–2.0 µm). The study was carried out at a high altitude hill station Darjeeling (2200 m asl) in the eastern Himalayas, India, using an aerosol spectrometer (GRIMM) to measure aerosol number concentrations. It was found that the ultrafine and superfine aerosol (less than 1.0 µm) concentrations were higher in the pre-monsoon period compared to winter, whereas the concentrations of fine mode aerosols were comparable during these two seasons. Meteorological conditions like sunny/non-cloudy days with higher radiative fluxes and lower humidity favored the formation of ultrafine aerosols from their precursor gases, and the initiation of this formation were observed during 0900–1000 hours LT. Superfine aerosols were mainly found to be aerosols transported from long distances during the pre-monsoon period, and showed peak levels during late morning till late afternoon. In contrast, higher concentrations were seen at night in winter. The fine mode aerosols were mainly locally generated anthropogenic aerosols, mostly emitted from vehicular sources during the pre-monsoon period, and from biomass burning during winter, with sharp diurnal peaks during morning and evening. The long-term study on size segregated aerosol concentrations carried out in this work is of considerable importance, and can be helpful for the validation of several regional and global aerosol models, and for other climate related studies that focus on the Himalayan region.