Micromorphology of size-segregated aerosols and their airway deposition in public transport commuters

Commuters’ exposure to size-segregated fine particulates in four public transport microenvironments was assessed in the Kolkata megacity of India. Personal exposure to PM 2.5 and PM 1 varied from 130.8 & 112.1 μg m −3 in air-conditioned (AC) buses, followed by 158.5 μg m −3 & 134.3 μg m −3 in non-AC buses, 187.1 μg m −3 & 150.8 μg m −3 in non-AC cars, to 242.2 μg m −3 & 199.6 μg m −3 in 3-wheeler auto rickshaws, respectively. The exposure ratio for PM 1 /PM 2.5 was comparable in all transport modes (0.64 to 0.94, 0.83 ± 0.07). The micromorphology of fine particulates, studied by scanning electron microscopy-energy-dispersive X-ray spectrometry, revealed several morphological features in both inorganic and carbonaceous particulates with Al, Si, Ca, K, Fe, and S impregnations. Soot particles were predominantly present in PM < 0.25 , and its semi-aggregated net-like structure trapped fine and ultrafine particles. The possible formation of carbonaceous aerosols from inorganic seeds via the nucleation pathway was also captured. The estimated deposition rate in the human respiratory system translated into a total PM 2.5 respiratory deposition rate (RDR) of 25.5 ± 8.9 μg h −1 in the respiratory tract was about 26% of the entire inhalation exposure to PM 2.5 . The average RDR of PM 2.5–1.0 and PM 1.0–0.5 was 11.7 ± 5.9 μg h −1 and 4.5 ± 2.3 μg h −1 that may get preferentially deposited in the head airways of the human respiratory system (75% and 60%, respectively). While the finest particles mainly get deposited in the deepest alveolar region of the human respiratory system (the RDR for PM 0.5–0.25 and PM < 0.25 was 3.5 ± 1.5 μg h −1 (49%) and 5.8 ± 2.5 μg h −1 (79%), respectively). The highest airway deposition of PM 2.5 in auto rickshaw commuters indicates that this transport mode could be the most harmful to commuters exposing them to tailpipe emissions from on-road vehicles and resuspended road dust due to low floor height and the open nature of the vehicle. Auto rickshaw commuters should practice using PM 2.5 -restricting face masks to reduce exposure to fine particulates while commuting when this mode of commute cannot be avoided.