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Dinitrogen pentoxide (N2O5) and the nitrate radical (NO3) playimportant roles in atmospheric chemistry, yet accurate measurements of theirconcentrations remain challenging. A thermal dissociation chemical ionizationmass spectrometer (TD-CIMS) was deployed to an urban site in Hong Kong tomeasure the sum of N2O5 and NO3 in autumn 2010 based on thesignals of NO3− at 62 amu which has also been adopted in previous studiesreported in literature. To our surprise, very large signals ofN2O5 + NO3 were frequently observed at 62 amu in the daytime,with equivalent N2O5 + NO3 mixing ratios in the range of200–1000 pptv. To investigate this unusual phenomenon, various interferencetests and measurements with different instrument configuration wereconducted. It was found that peroxy acetyl nitrate (PAN) contributed tomeasurable signals at 62 amu, and more importantly, this interferenceincreased significantly with co-existence of NO2. Nitric acid(HNO3), on the other hand, had little interference to the detection ofN2O5/NO3 via the NO3− ion in our TD-CIMS. According tothe test results, the interference from PAN and NO2 could havecontributed to 30–50% of the average daytime (12:00–16:00, local time)N2O5 + NO3 signal at our site. On the other hand, evidenceexists for the presence of elevated daytime N2O5, in addition tothe daytime signal at 62 amu. This includes (1) daytime N2O5measured via the I(N2O5)− cluster ion with an unheated inlet,which was subjected to minimum interferences, and (2) observation of elevateddaytime ClNO2 (a product of N2O5 hydrolysis) during afollow-up study. In view of the difficulty in accurately quantifying thecontribution from the interferences of PAN and NO2 and untestedpotential interfering chemicals in the real atmosphere, we caution the use of62 amu in the TD-CIMS for measuring ambient N2O5 in a highNOx environment like Hong Kong. Additional studies are needed tore-examine the daytime issue using other measurement techniques

atmospheric measurement techniques

Large daytime signals of N&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;O&amp;lt;sub&amp;gt;5&amp;lt;/sub&amp;gt; and NO&amp;lt;sub&amp;gt;3&amp;lt;/sub&amp;gt; inferred at 62 amu in a TD-CIMS: chemical interference or a real atmospheric phenomenon?