Unprecedented Ambient Sulfur Trioxide (SO(3)) Detection: Possible Formation Mechanism and Atmospheric Implications

[Image: see text] Sulfur trioxide (SO(3)) is a crucial compound for atmospheric sulfuric acid (H(2)SO(4)) formation, acid rain formation, and other atmospheric physicochemical processes. During the daytime, SO(3) is mainly produced from the photo-oxidation of SO(2) by OH radicals. However, the sources of SO(3) during the early morning and night, when OH radicals are scarce, are not fully understood. We report results from two field measurements in urban Beijing during winter and summer 2019, using a nitrate-CI-APi-LTOF (chemical ionization-atmospheric pressure interface-long-time-of-flight) mass spectrometer to detect atmospheric SO(3) and H(2)SO(4). Our results show the level of SO(3) was higher during the winter than during the summer, with high SO(3) levels observed especially during the early morning (∼05:00 to ∼08:30) and night (∼18:00 to ∼05:00 the next day). On the basis of analysis of SO(2), NO(x), black carbon, traffic flow, and atmospheric ions, we suggest SO(3) could be formed from the catalytic oxidation of SO(2) on the surface of traffic-related black carbon. This previously unidentified SO(3) source results in significant H(2)SO(4) formation in the early morning and thus promotes sub-2.5 nm particle formation. These findings will help in understanding urban SO(3) and formulating policies to mitigate secondary particle formation in Chinese megacities.