On the Purity of Laboratory-Generated Sulfuric Acid Droplets and Ambient Particles Studied by Laser Mass Spectrometry

Particle analysis by laser mass spectrometry (PALMS) was used to examine sulfuric acid particles representative of stratospheric sulfate aerosols (SSAs) and ambient tropospheric aerosols. Sulfuric acid particles were generated in the laboratory by condensing sulfuric acid vapors in a flow stream of particle-free dry air or nitrogen. The purest particles were produced using filtered, high-purity nitrogen in a clean glass and stainless steel system. In contrast, generation techniques using filtered compressed air, Tygon tubing, or Viton o-rings resulted in detectable organic impurities in the positive ion spectra of sulfuric acid droplets. With the PALMS instrument, the lower limit of our detection of organics is at most 0.02 wt% a-tocopherol in sulfuric acid, which in terms of surface coverage corresponds to less than one monolayer on a 0.2-lJ-m-diameter particle. When we eventually deploy the PALMS instrument in the upper troposphere and lower stratosphere, we should be able to detect the presence of very small amounts of organic compounds in SSAs. In the atmosphere, residence times are long and tropospheric concentrations of reactive gas-phase organic compounds are high, implying that ambient sulfuric acid particles are unlikely to be pure. Other experiments using internally mixed tridecane/sulfuric acid particles confirm that ambient particles from Boulder, CO and Idaho Hill, CO contain internal mixtures of oxidized organic compounds and sulfate.