Large daytime signals of N2O5 and NO3 inferred at 62 amu in a TD-CIMS : chemical interference or a real atmospheric phenomenon

Abstract

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 NOx environment like Hong Kong. Additional studies are needed to re-examine the daytime issue using other measurement techniques.