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Single-Particle Detection Efficienciesof Aerosol Time-of-Flight MassSpectrometry during the NorthAtlantic Marine Boundary LayerExperiment

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During the North Atlantic marine boundary layerexperiment (NAMBLEX) sampling campaign at MaceHead, Ireland, both continental and maritime air masseswere sampled. Aerosol was characterized both with aTSI 3800 time-of-flight mass spectrometer (ATOFMS) anda MOUDI microorifice impactor, and particle numbercounts were measured independently with an aerodynamicparticle sizer. The data have been analyzed in order toelucidate factors determining the particle detectionefficiencies of the ATOFMS. These are broken downaccording to the efficiency of the inlet system, the hitefficiency on particles which enter the sensing zone ofthe instrument and the sensitivity of the measured ionsignal to the chemical species. A substantial matrixeffect depending on the chemical composition of theaerosol sampled at the time was found, which isreflected in variations in the hit efficiency of particlesentering the sensing zone of the instrument with the maindesorption−ionization laser. This is in addition to thestrong inverse power-law dependence of inlet transmissionefficiency on particle diameter. The variation in hitefficiency with particle type is likely attributable todifferences in the energetics of laser energy absorption,ablation, and ion formation. However, once variations in bothinlet transmission and hit efficiencies are taken intoaccount, no additional matrix dependence of ATOFMSresponse is required to obtain a linear relationship betweenthe ion signal and the concentration of a particularchemical species. The observations show that a constantmass of material is ionized from each particle, irrespectiveof size. Consequently the integrated ion signal for a givenchemical component and particle size class needs to beincreased by a factor related to the cube of particle diameterin order to correlate with the airborne mass of thatcomponent.

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