Biomass burning sources control ambient particulate matter but traffic and industrial sources control VOCs and secondary pollutant formation during extreme pollution events in Delhi

Volatile organic compounds (VOCs) and particulate matter (PM) are major constituents of smog. Delhi experiences severe smog during post-monsoon season, but a quantitative understanding of VOCs and PM sources is still lacking. Here, we source-apportioned VOCs and PM, using a high-quality recent (2022) dataset of 111 VOCs, PM2.5, and PM10 using positive matrix factorization. Contrasts between clean-monsoon and polluted-post-monsoon air, VOC source fingerprints, molecular-tracers, enabled differentiating paddy-residue burning from other biomass-burning sources, which has hitherto been impossible. Fresh paddy-residue burning and residential heating & waste-burning contributed the highest to observed PM10 (25 % & 23 %), PM2.5 (23 % & 24 %), followed by heavy-duty CNG-vehicles 15 % PM10 and 11 % PM2.5. For ambient VOCs, ozone, and SOA formation potentials, top sources were petrol-4-wheelers (20 %, 25 %, 30 %), petrol-2-wheelers (14 %, 12 %, 20 %), mixed-industrial emissions (12 %, 14 %, 15 %), solid fuel-based cooking (10 %, 10 %, 8 %) and road construction (8 %, 6 %, 9 %). Emission inventories tended to overestimate residential-biofuel emission (>2) relative to the PMF output. The major source of PM pollution was regional biomass burning, whereas traffic and industries governed VOC and secondary pollutant formation. Our novel source-apportionment method quantitatively resolved even similar biomass and fossil-fuel sources, offering insights into both VOC and PM sources affecting extreme-pollution events. It represents a notable advancement over current source apportionment approaches, and would be of great relevance for future studies in other polluted cities/regions of the world with complex source mixtures.