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Dinitrogen pentoxide (N₂O₅) and the nitrate radical (NO₃) play important roles in atmospheric chemistry, yet accurate measurements of their concentrations remain challenging. A thermal dissociation chemical ionization mass spectrometer (TD-CIMS) was deployed to an urban site in Hong Kong to measure the sum of N₂O₅ and NO₃ in autumn 2010 based on the signals of NO₃^− at 62 amu which has also been adopted in previous studies reported in literature. To our surprise, very large signals of N₂O₅ + NO₃ were frequently observed at 62 amu in the daytime, with equivalent N₂O₅ + NO₃ mixing ratios in the range of 200–1000 pptv. To investigate this unusual phenomenon, various interference tests and measurements with different instrument configuration were conducted. It was found that peroxy acetyl nitrate (PAN) contributed to measurable signals at 62 amu, and more importantly, this interference increased significantly with co-existence of NO₂. Nitric acid (HNO₃), on the other hand, had little interference to the detection of N₂O₅/NO₃ via the NO₃^− ion in our TD-CIMS. According to the test results, the interference from PAN and NO₂ could have contributed to 30–50% of the average daytime (12:00–16:00, local time) N₂O₅ + NO₃ signal at our site. On the other hand, evidence exists for the presence of elevated daytime N₂O₅, in addition to the daytime signal at 62 amu. This includes (1) daytime N₂O₅ measured via the I(N₂O₅)^− cluster ion with an unheated inlet, which was subjected to minimum interferences, and (2) observation of elevated daytime ClNO₂ (a product of N₂O₅ hydrolysis) during a follow-up study. In view of the difficulty in accurately quantifying the contribution from the interferences of PAN and NO₂ and untested potential interfering chemicals in the real atmosphere, we caution the use of 62 amu in the TD-CIMS for measuring ambient N₂O₅ in a high NO_x environment like Hong Kong. Additional studies are needed to re-examine the daytime issue using other measurement techniques.