Sulfur trioxide (SO3) is a crucial compound for atmospheric sulfuric acid (H2SO4) formation, acid rain formation, and other atmospheric physicochemical processes. During the daytime, SO3is mainly produced from the photo-oxidation of SO2by OH radicals. However, the sources of SO3during 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 SO3and H2SO4. Our results show the level of SO3was higher during the winter than during the summer, with high SO3levels 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 SO2, NOx, black carbon, traffic flow, and atmospheric ions, we suggest SO3could be formed from the catalytic oxidation of SO2on the surface of traffic-related black carbon. This previously unidentified SO3source results in significant H2SO4formation in the early morning and thus promotes sub-2.5 nm particle formation. These findings will help in understanding urban SO3and formulating policies to mitigate secondary particle formation in Chinese megacities.