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 atmospheric evolution and impacts on volatility remain poorly understood. In this study, we investigated the volatility of different types of SOA 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 (referred as FIGAERO-OA) measured by the FIGAERO-CIMS. Eight factors were resolved, including six daytime chemistry related factors, a biomass burning related factor (BB-LVOA, 10 % of the FIGAERO-OA), and a nighttime chemistry related factor (Night-LVOA, 15 %) along with their corresponding volatility. Day-HNOx-LVOA (12 %) and Day-LNOx-LVOA (11 %) were mainly formed through gas-particle partitioning. Increasing NOx levels mainly affected SOA formation through gas-particle partitioning, suppressing the formation of low-volatile organic vapors, and thus promoting the formation of relatively high volatile OA with a higher N : C ratio. Two aged OA factors, Day-aged-LVOA (16 %) and Day-aged-ELVOA (11 %), were attributed to daytime photochemical aging of pre-existing OA. In addition, the daytime formation of Day-urban-LVOA (16 %) and Day-urban-ELVOA (7 %) could only observed in the urban plume. Results show that both gas-particle partitioning (36 %) and photochemical aging (30 %) accounted for a major fraction in FIGAERO-OA in the afternoon during the urban air masses period, especially for high-NOx-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 BB-LVOA and Night-LVOA accounted for 13 % of biomass burning OA and 48 % of nighttime chemistry OA observed by AMS, respectively. Our PMF analysis also demonstrated that 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.