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 exami ne sulfuric acid particles representative of stratospheric sulfate aer osols (SSAs) and ambient tropospheric aerosols. Sulfuric acid particle s were generated in the laboratory by condensing sulfuric acid vapors in a flow stream of particle-free dry air or nitrogen. The purest part icles were produced using filtered, high-purity nitrogen in a clean gl ass and stainless steel system. In contrast, generation techniques usi ng 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 dete ction of organics is at most 0.02 wt% alpha-tocopherol in sulfuric aci d, which in terms of surface coverage corresponds to less than one mon olayer on a 0.2-mu m-diameter particle. When we eventually deploy the PALMS instrument in the upper troposphere and lower stratosphere, we s hould be able to detect the presence of very small amounts of organic compounds in SSAs. In the atmosphere, residence times are long and tro pospheric concentrations of reactive gas-phase organic compounds are h igh, 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 H ill, CO contain internal mixtures of oxidized organic compounds and su lfate. (C) 1997 American Association for Aerosol Research.