Probing Atmospheric Aerosols by Multimodal Mass Spectrometry Techniques: Revealing Aging Characteristics of Its Individual Molecular Components

Detailed chemical characterization of biomass burning organic aerosol (OA) was performed using a synergistic combination of multimodal mass spectrometry techniques. OA was analyzed in situ using a high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) and an extractive electrospray ionization time-of-flight mass spectrometer (EESI-MS) deployed onboard the NASA DC-8 research aircraft. Additionally, complementary filter samples of OA were collected for offline laboratory analysis using high-performance liquid chromatography interfaced with a photodiode array and an electrospray ionization high-resolution mass spectrometer (HPLC-PDA-HRMS). During a research flight on August 3, 2019, which was focused on the Williams Flats Fire, WA, the onboard HR-ToF-AMS data revealed the abundant presence of organic sulfur (OS) species as prominent components of the OA. These OS species were identified based on their unique fragmentation. Further investigation using HPLC-PDA-HRMS and MSn fragmentation allowed us to identify the molecular characteristics of these unusual OS species. The dominant OS compounds detected during the research flight were found to be alkylbenzene sulfonates. Organosulfate, nitroaromatic, and oxygenated aromatic components of OA were also identified. Guided by the HRMS results, time-resolved aging profiles of selected individual OA species were retrieved from the real-time EESI-MS data sets to evaluate their aging evolution in the emission plume. Notably, the alkylbenzene sulfonate species showed remarkable stability over 8 h of atmospheric transport. In contrast, common organosulfates displayed short apparent half-life times that were as low as 1.2 h, indicating their susceptibility to aging. The nitroaromatic and oxygenated aromatic species exhibited relatively slower aging, with average apparent half-life times of 1.8 and 2.2 h, respectively.