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

Abstract

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 ambient aerosol components. However, when measuring close to certain anthropogenic sources or in marine environments, several of these assumptions may not be met and measurement results might easily be misinterpreted. 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, their differences in vaporization kinetics can result in undesired biases in ion collection efficiency in the measurements. Chemical reactions with water vapor and oxygen from the aerosol flow can have an influence on the mass spectra for certain components (e.g. NH4NO3, (NH4)2SO4, organic species). Finally, chemical reactions of the aerosol with the vaporizer surface can result in additional signals in the mass spectra (e.g. WO2C2-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 laboratory experiments, measurements close to anthropogenic sources or in marine environments can be biased by these effects.

The effects of molecular weight and thermal decomposition on the sensitivity of a thermal desorption aerosol mass spectrometer

“…This is not consistent with electron impact ionization of gas phase ammonia. The vaporization pulse lengths of the ammonium fragments are also very similar to that of the NO 2 + ion (Drewnick et al 2015), supporting the concept that they come from the same neutral molecule. Whether or not that is intact ammonium nitrate or another vaporization product, these calculations show that the sensitivity to ammonium can change by more than a factor of 4 depending on how ammonium nitrate thermally decomposes on the vaporizer.…”

“…For example, from Figure 2 one can infer that that the molecule responsible for the NO 2 C peak has a molecular weight considerably less than 200. Studies along the lines of Drewnick et al (2015) for many more peaks and aerosol types may put limits on thermal decomposition. It should be possible to put bounds on the molecular weight of the neutral molecules in the ion source with the ion fragments providing a lower limit and the vaporization length providing an upper limit.…”

“…The evaporation time for ammonium nitrate particles must be much faster than the observed~25 ms pulses of ions from single particles (Jayne et al 2000;Drewnick et al 2015). It seems unlikely that particles will evaporate faster than the time it takes for heat transfer from the side of the particle to the center.…”

“…The match in shape and approximate length supports the use of a free molecular flow model. The exact length and especially the tail of the evaporation pulse will be due to the exact dimensions of the ion source as well as molecules getting trapped in pores or temporarily adsorbing elsewhere on either the vaporizer or surfaces in the ion source (Drewnick et al 2015). Complete dimensions of the AMS ion source are not published but the electron beam is as close as feasible to a 3.8-mm diameter vaporizer (Drewnick et al 2005).…”

Simultaneous Aerosol Mass Spectrometry and Chemical Ionisation Mass Spectrometry measurements during a biomass burning event in the UK: Insights into nitrate chemistry

“…Figure 3 shows the absorption coefficients for wood burning babs_470wb (blue) and traffic babs_950tr (red), both increasing around 17:00-18:00 hrs to values lower than 100 However, mineral nitrate salts tend to be large particles (Allan et al, 2006;Chakraborty et al, 2016) and also have low vaporisation efficiency (Drewnick et al, 2015), which makes it unlikely to be measured by the AMS in large quantities. The …”