New insight into the formation and aging processes of organic aerosol from positive matrix factorization (PMF) analysis of ambient FIGAERO-CIMS thermograms

Abstract. Secondary organic aerosol (SOA) is an important component of organic aerosol (OA), yet its evolution of volatility remains unclear. We investigated SOA volatility at a downwind site of the Pearl River Delta (PRD) region in the fall of 2019, using a time-of-flight chemical ionization mass spectrometer coupled with a Filter Inlet for Gases and Aerosol (FIGAERO-CIMS). Positive matrix factorization (PMF) analysis was performed on the thermogram data of organic compounds measured by the FIGAERO-CIMS. Eight factors were resolved, including six daytime chemistry related factors, a biomass burning related factor (BB-LVOA, 10 %), and a nighttime chemistry related factor (Night-LVOA, 15 %) along with their corresponding volatility. Day-HNO_x-LVOA (12 %) and Day-LNO_x-LVOA (11 %) were mainly formed through gas-particle partitioning, with higher NO_x promoting more volatile OA. Two aged OA factors (Day-aged-LVOA, 16 %; Day-aged-ELVOA, 11 %) reflected daytime photochemical aging, while Day-urban-LVOA (16 %) and Day-urban-ELVOA (7 %) were linked to urban plumes. Results show that both gas-particle partitioning (36 %) and photochemical aging (30 %) accounted for a major fraction in the afternoon during the urban air masses period, especially for high-NO_x-like pathway (~21 %). In general, the six daytime OA factors collectively explain the majority (82 %) of daytime SOA identified by an aerosol mass spectrometer (AMS), while the highly oxygenated OA and hydrocarbon-like OA cannot be identified with FIGAERO-CIMS in this study. In summary, our results show that the volatility of OA is strongly governed by its formation pathways and subsequent atmospheric aging processes.