Semi-volatile oxygenated organics and ammonium chloride increasing sub-micron aerosol hygroscopicity, cloud condensation nuclei and PM₁ mass in the Delhi region

Abstract

Delhi-National Capital Region (Delhi-NCR) suffers from adverse air quality particularly during the winter season, thereby affecting climate, health, and economic activities, warranting the need for information on key species, sources and atmospheric pathways causing intense particulate pollution. Using over one month (February–March) of sub-micron particle (PM<inf>1</inf>) chemical composition data and κ-Köhler theory at a Delhi background site, we estimate that the water uptake ability of both PM<inf>1</inf> (kappa, κ = 0.52 ± 0.10) and its organic component (κ<inf>OA</inf> = 0.22 ± 0.04) during the late winter season are almost twice as that of global average for continental aerosols. Our results indicate that apart from previously identified ammonium chloride (NH<inf>4</inf>Cl), the semi-volatile oxygenated organic aerosols (SVOOA) directly increase the PM<inf>1</inf> water uptake ability, and undergo co-condensation with water vapor under high RH and low temperature conditions during early morning hours, thereby increasing the cloud condensation nuclei counts (CCN vs SVOOA, linear correlation R = 0.81) and total PM<inf>1</inf> mass (CCN vs PM<inf>1</inf>, R = 0.88) in the Delhi region. Positive Matrix Factorization (PMF) results of both gas and particle phase organics suggest that semi-volatile oxygenated organic compounds were mainly associated with solid-fuel and traffic-related combustion emissions, whereas correlation with source-specific tracers suggest non-combustion emissions for NH<inf>4</inf>Cl. Results suggest that semi-volatile oxygenated organic compounds produced from the photochemical oxidation of organics emitted from combustion activities likely undergo gas-to-particle partitioning in the evening and aqueous phase processing at night, leading to enhanced SOA formation, CCN and PM<inf>1</inf> mass.