Chemical characterisation and source apportionment of PM2.5 in the cosmopolitan city of Bengaluru, India

Over the past two decades, Bengaluru, a major city in South India, has undergone rapid urbanization and population growth, with significant increase in vehicular traffic, construction, and industrial activities. These changes can enhance local contributions from anthropogenic biomass burning and vehicular emissions, in addition to long-range transported pollutants, adversely impacting air quality, and reducing visibility. We investigated the chemical composition and sources of particulate matter (PM_2.5) in Bengaluru using an Aerodyne Time-of-Flight Aerosol Chemical Speciation Monitor (ToF-ACSM) and two Aethalometers (AethLabs microAeth MA300 and Magee Scientific AE33). This study marks the first in-situ and high time resolution source apportionment of non-refractory PM_2.5 (NR- PM_2.5) in the city using the ToF-ACSM, commencing from post-monsoon sampling period (Aug 2024). Results thus far indicate that organic aerosols (OA) are the dominant (63%) NR-PM_2.5 species, followed by sulphate (~20%) and ammonium (~9%). Positive Matrix Factorization (PMF) analysis identified two primary organic aerosol types namely hydrocarbon-like organic aerosols (HOA) and biomass-burning organic aerosols (BBOA), and two secondary organic aerosols namely less-oxidised oxygenated organic aerosols (LO-OOA) and more-oxidised oxygenated organic aerosols (MO-OOA). The air masses from the northeast (0-90°) direction were found to be associated with elevated levels of MO-OOA, which also correlated well with increased sulphate fraction. These findings highlight the role of local sources like vehicular emissions and waste burning, as well as regional sources including thermal power plant emissions, and oxidised and aged OA. Furthermore, the study includes Diwali and Kannada Rajyotsava celebrations, periods with extensive firework activity. Despite restrictions on conventional fireworks and recommended usage of green crackers, the city witnessed significant particle pollution, with hourly PM_2.5 concentrations exceeding 100 µg/m³ and spikes up to 800 µg/m³. While NR-PM_2.5 and black carbon (BC) increased during the firework periods, the rise in PM_2.5 mass loading was much greater, resulting in incomplete mass closure (only ~16% from NR-PM_2.5 + BC) in contrast to normal periods (~80% from NR-PM_2.5 + BC). Further results and detailed analyses will be presented, including seasonal changes in PM_2.5 composition and sources as we transition from the post-monsoon to the more polluted winter season.