Investigation of Volatility Method for Measuring Aqueous Sulfuric Acid on Mixed Aerosols
The thermal discrimination or volatility technique is a widely used method exploiting differences in aerosol volatility to discriminate between particles of different chemical composition. In recent years numerous investigators applied this technique to determine the existence and the amount of sulfuric acid in the aerosol phase of aircraft contrails forming in the upper troposphere and lower stratosphere (UT/LS). Although the potential for systematic errors due to incomplete evaporation and recondensation of volatile material as well as internal wall losses was recognized by other investigators, we are not aware of any study on polydisperse aerosol (broad size distribution) incorporating these effects into the volatility technique. Here, a tandem differential mobility analyzer (TDMA) is employed to investigate the performance of a thermal discriminator designed at the University of Missouri-Rolla (UMR). Since sulfuric acid is of particular interest for atmospheric aerosol, this study focused on aqueous sulfuric acid (H2SO4/H2O) aerosol. For an operating temperature of 300°C and an aerosol residence time of more than 0.25 s, we found that complete evaporation of H2SO4/H2O aerosol occurred up to diameters of at least 1.9 m, which is consistent with theoretical calculations. No evidence for recondensation was found for H2SO4/H2O particle surface area and mass concentrations typical for UT/LS background and aircraft plume conditions. Wall losses were measured and incorporated into a size-resolved version of the volatility method, allowing more accurate measurements of the volatile (H2SO4/H2O) volume fraction of polydisperse aerosol. The increased accuracy was demonstrated using well-characterized, mixed (partially volatile) H2SO4/H2O/NaCl aerosol.