Size-segregated aerosols over a high altitude Himalayan and a tropical urban metropolis in Eastern India: Chemical characterization, light absorption, role of meteorology and long range transport

Abstract

A study has been conducted focusing on the chemical and optical characterization of water soluble inorganic and organic components of aerosols at different sizes, over a high altitude Himalayan station, Darjeeling (27.1° N and 88.15° E, 2200 m amsl) and a tropical urban metropolis, Kolkata (22.5° N, 88.3° E, ~6 m amsl) for two-year long period (March 2016–February 2018). It was observed that local meteorology and long range transport of pollution plumes have played the pivotal role in governing the temporal variation of the mass distribution and concentration of aerosols and its various components over both the sites. The aerosol mass-size distributions were found to be bimodal in nature with the relative dominance of accumulation mode (0.1–1.0 μm) over coarse mode (1.8–10 μm) over both the stations, indicating dominance of the anthropogenic emissions. Among the size classes, accumulation mode aerosols alone contributed 55–75% over Darjeeling and 40–60% over Kolkata to PM10. The coarse mode aerosols were mainly consisted of primary inorganic species over Kolkata and water soluble organic carbons (WSOC) over Darjeeling whereas the fine mode aerosols (accumulation: 0.1–1.0 μm) were primarily composed of the secondary inorganic aerosols for both the stations. Irrespective of the seasons and stations, SO42− and NH4+ exhibited peaks in the mass-size distribution at 1.0–0.1 μm whereas that of NO3− varied with the seasons. The photochemical oxidation and the aqueous phase oxidation of SO2 (g) were the pathways for SO42− formation over Darjeeling whereas the later dominated over Kolkata. Concentration weighted trajectory (CWT) model has revealed that the secondary inorganic aerosols and WSOC were local/regional in origin over Kolkata megacity whereas Darjeeling was influenced by mixed sources. The accumulation mode aerosols were found to be the highest light absorbing over both the stations irrespective of the seasons. Over Darjeeling, the absorption coefficient (babs_365) and the mass absorption efficiency (MAE) of WSOC were maximum for local biomass burning aerosols than the transported plumes whereas those over Kolkata were maximum for transported biomass burning plumes from Eastern Ghats.