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

F Drewnick,S Borrmann,J.-M Diesch,P Faber

doi:10.5194/amt-8-3811-2015

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 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.

Highlights

Flash vaporization – i.e., very rapid vaporization of a sample by contact with a hot surface – in combination with mass spectrometry has been used since decades for the analysis of thermally fragile (e.g., Lincoln, 1965; Chinn and Lagow, 1984) or environmental (e.g., de Leeuw et al, 1986) samples with little or no pre-treatment needed
Within the first 10 s after blocking the aerosol beam, the signal decreases by 50 %, and after re-opening the particle beam the signal increases to 40 % of its final level within the same time interval
Various experiments to investigate the interaction of aerosol particles with the vaporizer of the Aerodyne aerosol mass spectrometer (AMS) and resulting consequences for the measured mass spectra were conducted

Summary

Introduction

Flash vaporization – i.e., very rapid vaporization (here on the order of a few tens of microseconds) of a sample by contact with a hot surface – in combination with mass spectrometry has been used since decades for the analysis of thermally fragile (e.g., Lincoln, 1965; Chinn and Lagow, 1984) or environmental (e.g., de Leeuw et al, 1986) samples with little or no pre-treatment needed. In on-line aerosol mass spectrometry flash vaporization was used from the very beginning on in combination with surface ionization (Davis, 1973; Stoffels and Lagergren, 1981) and in later experiments with electron impact ionization (Allen and Gould, 1981; Sinha et al, 1982). F. Drewnick et al.: Particle–vaporizer interactions and their consequences for the measurements pact ionization of the vapor (Jayne et al, 2000). Drewnick et al.: Particle–vaporizer interactions and their consequences for the measurements pact ionization of the vapor (Jayne et al, 2000) Common to these applications is the general assumption that flash vaporization quickly converts the solid sample material into a vapor without alteration – besides thermal decomposition – of the sample

Methods

Results

Discussion

Conclusion