Wintertime chemical characteristics of aerosol and their role in light extinction during clear and polluted days in rural Indo Gangetic plain

Highlights: • Effects of clear and polluted days on the aerosol loading, chemical constituents, OM/OC, and light extinction was examined. • Fine aerosols controlled visibility during daytime while fog droplets in nighttime. • OM, (NH₄)₂SO₄, and NH₄NO₃ combined contributed 87% to the light extinction. • OM/OC ratio averaged 2.2 and 1.9 during polluted days and clear days, respectively. • Nitrate particles extinction contribution Increased from 9% on clear days to 20% on polluted days. This paper reports the chemical and light extinction characteristics of fine aerosol (PM_2.5) during the winter period (2017–18) at Lumbini, Nepal, a rural site on the Indo Gangetic Plains. A modified IMPROVE algorithm was employed to reconstruct light extinction by chemical constituents of aerosol. The fine aerosol levels impacted visibility adversely during daytime, but during nighttime visibility was controlled by fog droplets rather than by aerosols. The PM_2.5 chemical constituents showed varying characteristics during clear and polluted days. The average NO₃⁻/SO₄²⁻ concentration ratio was 0.57 during clear and 1.36 and polluted days, signifying a change in secondary inorganics and formation processes mainly due to decreasing photochemical production and due to increased partitioning of nitrate particles at a lower temperature. The increased secondary organics contribution and the higher OM/OC ratio (2.2) during polluted days showed the vital role of aqueous processing and biomass burning emissions in determining the concentration of organics. Total light extinction was 2.3 times higher on polluted days compared to clear days, while the PM_2.5 mass concentration was 1.5 times higher. This variation in mass and extinction order signifies that various chemical components in fine particles have a more considerable impact on light extinction. On clear days we found that carbonaceous particles (OM and EC) made a major contribution to light extinction. In contrast, the extinction contribution by secondary inorganic (especially NH₄NO₃) increased significantly during polluted days, with hygroscopic growth and enhanced scattering efficiency at higher RH conditions playing a major role. The comparison between clear and polluted days altogether suggests that regulating the nitrate sources can help significantly in improving the visibility levels and restrict fog haze development during wintertime in rural IGP.