Aerosol mass spectrometry: particle–vaporizer interactions and their consequences for the measurements

摘要

The Aerodyne aerosol mass spectrometer (AMS) is a frequently used instrumentfor on-line measurement of the ambient sub-micron aerosol composition. Withthe help of calibrations and a number of assumptions on the flashvaporization and electron impact ionization processes, this instrumentprovides robust quantitative information on various non-refractory ambientaerosol components. However, when measuring close to certain anthropogenicor marine sources of semi-refractory aerosols, several of these assumptionsmay not be met and measurement results might easily be incorrectlyinterpreted if not carefully analyzed for unique ions, isotope patterns, andpotential slow vaporization associated with semi-refractory species.Here we discuss various aspects of the interaction of aerosol particles withthe AMS tungsten vaporizer and the consequences for the measurement results:semi-refractory components – i.e., components that vaporize but do notflash-vaporize at the vaporizer and ionizer temperatures, like metal halides (e.g.,chlorides, bromides or iodides of Al, Ba, Cd, Cu, Fe, Hg, K, Na, Pb, Sr, Zn)– can be measured semi-quantitatively despite their relatively slowvaporization from the vaporizer. Even though non-refractory components (e.g.,NHNO or (NH)SO) vaporize quickly, undercertain conditions their differences in vaporization kinetics can result inundesired biases in ion collection efficiency in thresholded measurements.Chemical reactions with oxygen from the aerosol flow can have an influenceon the mass spectra for certain components (e.g., organic species). Finally,chemical reactions of the aerosol with the vaporizer surface can result inadditional signals in the mass spectra (e.g., WOCl-relatedsignals from particulate Cl) and in conditioning or contamination of thevaporizer, with potential memory effects influencing the mass spectra ofsubsequent measurements.Laboratory experiments that investigate these particle–vaporizerinteractions are presented and are discussed together with field results,showing that measurements of typical continental or urban aerosols are notsignificantly affected, while measurements of semi-refractory aerosol in thelaboratory, close to anthropogenic sources or in marine environments, can bebiased by these effects.