Sources of organic aerosol in polluted urban environments and the heatwave impacts

Secondary formation of OA in polluted urban environments involves diverse precursors and complex pathways. When the severity and frequency of heatwaves are expected to rise, little is about the heatwave impacts on the OA formation in such polluted environments. Here we deployed a long time-of-flight aerosol mass spectrometer (LTOF-AMS) to measure the real-time chemical composition of submicron aerosols along with advanced time-of-flight chemical ionization mass spectrometers (TOF-CIMS) to detect gaseous and particulate oxidation products in Beijing in recent years. Six process-level secondary OA (SOA) factors are resolved and unique molecular tracers for each of the six processes are identified. The six SOA factors can be explained by intensified photochemical and heterogeneous reactions with higher volatile organic compounds emissions and oxidant level, increased aerosol surface, stronger aerosol acidity, and higher ammonia concentration etc. The six-factor SOA seperation provides a machanistic understanding of the net enhancement of SOA during heatwave events, which have been observed in many places worldwide in summer. We further applied machine learning methods to identify the key drivers of the SOA enhancement and used the simulated key parameters from the GEOS-Chem model to perturb the SOA formation during the heatwave episode under clean air actions and climate change scenarios. Our results suggest that the SOA enhancement due to heatwave will be increasingly important in the future. This study underscores the urgency of validating temperature responses of organic aerosol in chemical transport models to facilitate air quality management in a warming world.