Atmospheric Measurement Techniques Chlorine activation by N2O5: simultaneous, in situ detection of

Abstract. We report a new method for the simultaneous in situ detection of nitryl chloride (ClNO2) and dinitrogen pen-toxide (N2O5) using chemical ionization mass spectrometry (CIMS). The technique relies on the formation and detection of iodide ion-molecule clusters, I(ClNO2) − and I(N2O5)−. The novel N2O5 detection scheme is direct. It does not suf-fer from high and variable chemical interferences, which are associated with the typical method of nitrate anion detec-tion. We address the role of water vapor, CDC electric field strength, and instrument zero determinations, which influ-ence the overall sensitivity and detection limit of this method. For both species, the method demonstrates high sensitivity (>1 Hz/pptv), precision (∼10 % for 100 pptv in 1 s), and ac-curacy (∼20%), the latter ultimately determined by the nitro-gen dioxide (NO2) cylinder calibration standard and charac-terization of inlet effects. For the typically low background signals (<10 Hz) and high selectivity, we estimate signal-to-noise (S/N) ratios of 2 for 1 pptv in 60 s averages, but uncer-tainty associated with the instrumental zero currently leads to an ultimate detection limit of ∼5 pptv for both species. We validate our approach for the simultaneous in situ measure-ment of ClNO2 and N2O5 while on board the R/V Knorr as part of the ICEALOT 2008 Field Campaign.