Effect of vaporizer temperature on ambient non-refractory submicron aerosol composition and mass spectra measured by the aerosol mass spectrometer

Abstract

Aerodyne Aerosol Mass Spectrometers (AMS) are routinely operated with a constant vaporizer temperature (Tvap) of 600°C in order to facilitate quantitative detection of non-refractory submicron (NR-PM1) species. By analogy with other thermal desorption instruments, systematically varying Tvap may provide additional information regarding NR-PM1 chemical composition and relative volatility, and was explored during two ambient studies. The performance of the AMS generally and the functional integrity of the vaporizer were not negatively impacted during vaporizer temperature cycling (VTC) periods. NR-PM1 species signals change substantially as Tvap decreases with that change being consistent with previous relative volatility measurements: large decreases in lower volatility components (e.g., sulfate, organic aerosol [OA]) with little, if any, decrease in higher volatility components (e.g., nitrate, ammonium) as Tvap decreases. At Tvap < 600°C, slower evaporation was observed as a shift in particle time-of-flight distributions and an increase in "particle beam blocked" (background) concentrations. Some chemically reduced (i.e., CxHy+) OA ions at higher m/z are enhanced at lower Tvap, indicating that this method may improve the analysis of some chemically reduced OA systems. The OA spectra changes dramatically with Tvap; however, the observed trends cannot easily be interpreted to derive volatility information. Reducing Tvap increases the relative O:C and CO2 C , contrary to what is expected from measured volatility. This is interpreted as continuing decomposition of low volatility species that decreases more slowly (as Tvap decreases) than does the evaporation of reduced species. The reactive vaporizer surface and the inability to reach Tvap much below 200°C of the standard AMS limit the ability of this method to study the volatility of oxidized OA species.