Pollution investigation of wintertime VOCs in the coastal atmosphere of the Yellow Sea: Insights from on-line photoionization-induced CI-TOFMS

Volatile Organic Compounds (VOCs) are pivotal in tropospheric atmospheric chemistry, significantly impacting the formation of photochemical smog, fine particle pollution, and the atmospheric oxidizing potential, all of which affect air quality. Chemical ionization time-of-flight mass spectrometry, renowned for its exceptional sensitivity, precision, and swift response time, has proven to be exceptionally effective for real-time VOCs monitoring. In this study, a custom-built photoionization-induced CI-TOFMS was deployed to individually detect 116 VOCs standard gases specified by the United States Environmental Protection Agency, quantifying their responses and establishing a distinctive mass spectrogram database. A 12-day comparative analysis with online gas chromatography coupled with a flame ionization detector and mass spectrometer demonstrated the high reliability of the PICI-TOFMS, confirming its effectiveness for field VOCs monitoring. During two months of stationary monitoring, the PICI-TOFMS could further refine the timing of pollution events with high temporal resolution and identified four distinct air pollution episodes, including local emissions, coal burning, sand and dust transport and festival-related pollution. Employing a Hysplit model, the Concentration Weighted Trajectory model, and a source apportionment method based on tracer gases, the air trajectories were calculated and contributions from different potential source areas were quantified. Our findings reveal that transport was the main contributor to pollution in December 2022, while burning was the primary source in January 2023. This study refines our understanding of the dynamics of air pollution, providing valuable insights into the temporal and spatial distribution of VOCs and their sources.