Chemical Composition of Atmospheric Brown Carbon From the Indo-Gangetic Plain: Evidence of Near-Infrared Absorption

Abstract

Brown Carbon (BrC), the light-absorbing fraction of organic aerosols, plays a critical yet underappreciated role in atmospheric radiative forcing. Over the Indo-Gangetic Plain (IGP), BrC is predominantly emitted from biomass burning associated with widespread biofuel use for heating and cooking, agricultural residue burning, and unregulated domestic waste burning. Despite its ubiquity, the molecular composition and optical behavior of BrC in this region is poorly understood. In this study, we present new molecular-level evidence revealing substantial Near-Infrared (NIR) light absorption by organic acids in the IGP. We investigated the chemical composition of nighttime BrC collected at a suburban site in the northwestern IGP by comparing two different seasons—post-monsoon and winter, and two wind-sectors—urban and rural. Aerosol samples were collected at the IISER Mohali Atmospheric Chemistry Facility, India. Compositional information was obtained by employing Direct-Analysis in Real-Time High-Resolution-Mass-Spectrometry to analyze samples of organic aerosols collected on the filter spots of a 7-wavelength aethalometer. Pronounced compositional differences appeared between urban-influenced and rural-influenced samples, although no significant seasonal variations were observed. Overall, organic aerosols in the region are dominated by nitrogen-containing species. Urban-influenced air-masses are strongly affected by open burning of domestic waste, as reflected by the abundance of reduced-nitrogen species. Notably, the presence of long-chain organic acids, observed particularly in urban-influenced samples, was found to enhance light absorption at longer wavelengths, extending into NIR, a spectral region traditionally attributed to black carbon (BC). This has critical implications for aerosol optical measurements, suggesting potential underestimation of BrC and overestimation of BC contributions.