Characterization and Source Apportionment of Organic Aerosols in Delhi, India

Delhi is one of the world’s most polluted city and experiences very high levels of particulate matter throughout the year. It significantly affects radiative forcing, increases mortality, and causes other deleterious effects on human health. Generally, there is a lack of consensus on the dominant sources of organic aerosol driving these high aerosol concentrations. In particular, there is a need to elucidate the relative importance of primary vs. secondary sources and formation mechanisms of secondary aerosols. In order to answer questions pertaining to the sources, formation mechanisms, and atmospheric transformations of organic aerosol, we deployed for the first time in Delhi, the recently developed extractive electrospray ionization long-time-of-flight mass spectrometer (EESI-TOF) in Delhi. This was deployed along with a high-resolution long-time-of-flight aerosol mass spectrometer (AMS), and a Chemical Ionisation Mass Spectrometer fitted with a Filter Inlet for Gases and AEROsols (FIGAERO-CIMS). These measurements were further supported by measurements of black carbon and gaseous species such as CO, NO_x etc.

The EESI-TOF provides in real-time information on the chemical composition at the near-molecular level without thermal desorption and fragmentation (Lopez-Hilfiker et al., 2019). It was operated in Delhi from December to February 2019. Measurements by the AMS showed persisting high levels of particulate matter. We attributed the relative contributions of different sources to total non-refractory PM mass by performing source apportionment analysis by means of positive matrix factorization (PMF). A strong day-night variability was clearly seen in all the AMS species. High levels of secondary sources during daytime e.g., low-volatility oxygenated organic aerosol (LVOOA) and an increase of factors associated with primary sources i.e., hydrocarbon-like organic aerosol (HOA) and aerosol from solid fuel combustion (SFC) during morning and evening hours was observed. A similar trend as for the primary aerosol was seen for the semi-volatile oxygenated organic aerosol (SVOOA) mainly due to increased temperature during daytime which may leads to evaporation of semi-volatile species. A unique feature observed was a significant contribution of chloride to the non-refractory (NR) PM_2.5 mass. On average, chloride contributed 18-20% to total NR-mass. Similar trends have also been seen in a recent study (Gani et al., 2018). The effect of these high chlorine concentrations during the night and early morning on atmospheric chemistry as well as on secondary organic aerosol formation will be presented.