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Abstract. Here we describe the design and performance of a newwater cluster chemical ionization–atmospheric pressure interface time-of-flight mass spectrometer (CI-APi-TOF). The instrument selectively measurestrace gases with high proton affinity such as ammonia and dimethylamine,which are important for atmospheric new particle formation and growth.Following the instrument description and characterization, we demonstratesuccessful measurements at the CERN CLOUD (Cosmics Leaving OUtdoor Droplets)chamber where very low ammonia background levels of ∼4  pptvwere achieved (at 278 K and 80 % RH). The limit of detection of the watercluster CI-APi-TOF is estimated to be ∼0.5  pptv for ammonia.Although no direct calibration was performed for dimethylamine (DMA), weestimate its detection limit is at least 3 times lower. Due to the shortion–molecule reaction time and high reagent ion concentrations, ammoniamixing ratios up to at least 10 ppbv can be measured with the instrumentwithout significant reagent ion depletion. Besides the possibility tomeasure compounds like ammonia and amines (dimethylamine), we demonstratethat the ionization scheme is also suitable for the measurement of tracegases containing iodine. During CLOUD experiments to investigate theformation of new particles from I2 , many different iodine-containingspecies were identified with the water cluster CI-APi-TOF. The compoundsincluded iodic acid and neutral molecular clusters containing up tofour iodine atoms. However, the molecular structures of theiodine-containing clusters are ambiguous due to the presence of an unknownnumber of water molecules. The quantification of iodic acid ( HIO3 )mixing ratios is performed from an intercomparison with a nitrateCI-APi-TOF. Using this method the detection limit for HIO3 can beestimated as 0.007 pptv. In addition to presenting our measurements obtainedat the CLOUD chamber, we discuss the applicability of the water clusterCi-APi-TOF for atmospheric measurements.

Measurement of ammonia, amines and iodine compounds using protonated water cluster chemical ionization mass spectrometry