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Abstract:
The Aerodyne aerosol mass spectrometer (AMS) is a frequently used
instrument for on-line measurement of the ambient sub-micron aerosol
composition. With the help of calibrations and a number of assumptions
on the flash vaporization and electron impact ionization processes, this
instrument provides robust quantitative information on various
non-refractory ambient aerosol components. However, when measuring close
to certain anthropogenic or marine sources of semi-refractory aerosols,
several of these assumptions may not be met and measurement results
might easily be incorrectly interpreted if not carefully analyzed for
unique ions, isotope patterns, and potential slow vaporization
associated with semi-refractory species.
Here we discuss various aspects of the interaction of aerosol particles
with the AMS tungsten vaporizer and the consequences for the measurement
results: semi-refractory components -i.e., components that vaporize but
do not flash-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 slow vaporization from the vaporizer. Even though
non-refractory components (e.g., NH4NO3 or (NH4/2SO4) vaporize quickly,
under certain conditions their differences in vaporization kinetics can
result in undesired biases in ion collection efficiency in thresholded
measurements. Chemical reactions with oxygen from the aerosol flow can
have an influence on the mass spectra for certain components (e.g.,
organic species). Finally, chemical reactions of the aerosol with the
vaporizer surface can result in additional signals in the mass spectra
(e.g., WO2Cl2-related signals from particulate Cl) and in conditioning
or contamination of the vaporizer, with potential memory effects
influencing the mass spectra of subsequent measurements.
Laboratory experiments that investigate these particle-vaporizer
interactions are presented and are discussed together with field
results, showing that measurements of typical continental or urban
aerosols are not significantly affected, while measurements of
semi-refractory aerosol in the laboratory, close to anthropogenic
sources or in marine environments, can be biased by these effects.
