Understanding the sources and formation of organic aerosol in Delhi using non-target analysis

Exposure to PM_2.5 is the leading environmental risk to healthin India, where the National Capital Territory of  Delhi experiences annual mean concentrations of ~110 μg m^-3. During the post-monsoon season, severe air pollution events are frequent, with extreme levels exceeding 1000 µg  m⁻³. A large fraction of PM_2.5 in Delhi is organic aerosol (OA) derived from a wide range of primary and secondary sources. Recent studies investigating the composition and sources of OA in Delhi using online aerosol mass spectrometry (AMS), followed by positive matrix factorisation have highlighted the dominance of primary sources over secondary production during the very polluted post monsoon period. These studies suggest the resolved traffic and burning-related sources were the largest contributors, however, significant oxidised organic aerosol is present across most of the year, and the dominant sources of this material cannot be resolved using this approach.

 

High-resolution mass spectrometry (HRMS) allows detailed investigation of the molecular complexity of OA composition. Previous studies have focussed on key tracers of specific OA sources such as biomass burning or biogenic volatile organic compound oxidation. However, targeted analysis reveals a biased and incomplete picture of the chemical composition which limits our ability to detect emerging pollutants. Here we harness recent advances in the analysis of complex environmental samples via non-target analysis (NTA), coupled with advanced suspect screening and a novel semi-quantification method to investigate the complex composition of OA within Old Delhi during the post-monsoon period of 2018. Using high time resolution filter sampling and an automated analysis workflow, the temporal evolution of the OA  could be studied. Hierarchical cluster analysis of this high resolution data identified six separate OA factors. Two factors peaked at night and were dominated by primary oxidised traffic and wood combustion emissions. The other four factors peaked during the day and could be linked to different types of secondary organic aerosol that peak under different oxidative and meteorological conditions. These species showed different temporal profiles to the oxidised factors, MO-OOA and LO-OOA, measured using AMS, providing novel insights into the sources and factors that control local SOA production in Delhi. This offline NTA approach provides complementary information to the online AMS observations, while only requiring the deployment of a filter sampler to the observation location.