Progress of Online Measurement for Atmospheric Organic Aerosols Based on Proton Transfer Reaction Time-of-Flight Mass Spectrometry

Organic aerosol (OA) represents a substantial fraction of aerosols in the atmosphere, accounting for up to 20%-45% of the total mass concentration. OA plays a crucial role in reducing atmospheric visibility, influencing air quality and climate, as well as posing risks to human health. Due to the diverse sources and complex chemical composition of OA, the measurement of OA composition at the molecular level is essential for understanding its sources, chemical transformation processes, and environmental, climatic, and health effects. Offline analytical techniques, such as gas chromatography-mass spectrometry (GC-MS) and high-performance liquid chromatography-mass spectrometry (HPLC-MS), have been previously used to measure the molecular composition of OA. However, offline methods are influenced by potential contamination during transport, pretreatment, and time-consuming analytical procedures, thereby limiting the ability to capture the dynamic sources and atmospheric evolution of OA at the molecular level. Proton transfer reaction time-of-flight mass spectrometry (PTR-TOF MS) has been widely utilized for real-time monitoring of gas-phase organic compounds at the molecular level in the atmosphere due to its soft ionization feature, high mass resolution (up to 10 000) and ng/m³ detection limit. However, PTR-TOF MS cannot directly measure particulate-phase organic compounds of OA at the molecular level. To overcome this limitation, coupling PTR-TOF MS with thermal desorption inlets for OA sampling enables the real-time detection and quantitative analysis of particulate-phase organic compounds. This review summarizes commonly used thermal desorption inlets including thermal desorption (TD), chemical analysis of aerosols online (CHARON), and aerosol collection module (ACM), all coupled with PTR-TOF MS, named TD-PTR-TOF MS, CHARON-PTR-TOF MS and ACM-PTR-TOF MS, respectively. Although the three thermal desorption inlets allow real-time measurement of OA, their detection limits, time resolutions, and volatility coverage are substantially different. This review focuses on the systematic description and comparison of TD-PTR-TOF MS, CHARON-PTR-TOF MS, and ACM-PTR-TOF MS in terms of their design principles, analytical performance, and applications in both laboratory and field studies. TD-PTR-TOF MS provides a low detection limit for OA ranging from 0.1 to 0.6 ng/m³, enabling sensitive detection of trace-level OA in both laboratory and field studies. CHARON-PTR-TOF MS shows high time resolution (10-60 s), allowing real-time tracking of rapid changes in secondary OA formation and evolution in both laboratory studies and field environments. ACM-PTR-TOF MS has flexible temperature-programmed desorption, enabling in-depth investigations into the relationship between OA formation, volatility, and oxidation state in laboratory chamber studies. In addition, optimizing thermal desorption inlet conditions, using alternative reagent ions (e.g., NO⁺, O₂⁺) and diversifying ionization sources for PTR-TOF MS have been proposed to improve the detection selectivity and sensitivity for low-volatility OA. Finally, the integration of machine learning and artificial intelligence approaches offers new opportunities for efficient interpretation of complex mass spectral data and facilitates the identification of unknown and emerging compounds in measurements using PTR-TOF MS coupled with thermal desorption inlets.