High concentrations of N2O5 and NO3 observed in daytime with a TD-CIMS: chemical interference or a real atmospheric phenomenon?

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. To our surprise, very high concentrations of N₂O₅ +NO₃ were frequently observed in daytime, with 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 LT) N₂O₅ +NO₃ signal at our site. However, evidence exists for the presence of elevated daytime N₂O₅, in addition to the daytime signal at 62amu. This includes: (1) daytime N₂O₅ measured via the I(N₂O₅)⁻ cluster ion with an unheated inlet, which subjects 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 un-tested potential interfering chemicals in the real atmosphere, we caution the use of 62amu 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. [ABSTRACT FROM AUTHOR]