Carbonaceous aerosol in the Brahmaputra plains: Sources, and influence from the hotspot Indo-Gangetic plains, India

Abstract

Organic carbon (OC) and elemental carbon (EC) play a significant role in aerosol mass and atmospheric processes. This study is focused on the eastern part of the Great Northern Plains of India, namely the Brahmaputra Plains, to understand the influence of regional and local contribution on the carbonaceous fraction of PM_2.5. Mean annual PM_2.5 concentrations exceeded the National Ambient Air Quality Standards (NAAQS), with values of 46.6 ± 30.0 μg/m³ in the rural area and 50.4 ± 34.4 μg/m³ in the semi-urban area. The range in monsoon-winter was found to be 22.7–71.9 μg/m³. OC and EC contribute 44–50% of the PM_2.5 mass concentration. The OC/EC ratios ranged from 3.3 to 9.3 in the rural area and from 4.3 to 6.9 in the semi-urban area, indicating significant secondary organic aerosol (SOA) formation, especially during the high photochemical period of the pre-monsoon season. Lower δ13C values were observed during winter (-27.5‰ rural, -27.3‰ semi-urban), pre-monsoon (-28.1‰ rural, -27.6‰ semi-urban), and post-monsoon (-28.2‰ rural, -28.1‰ semi-urban) periods, suggesting influences from biomass burning, fossil fuel combustion, and aged aerosols. The study employs cluster analysis of air mass trajectory, and Moderate Resolution Imaging Spectroradiometer (MODIS) fire data to determine the influence of the hotspot Indo-Gangetic Plain (IGP) and long-range transport on aerosol carbonaceous content during most seasons except the monsoon period June–September in the Brahmaputra Plains.