Journal
PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA
Volume 109, Issue 43, Pages 17354-17359Publisher
NATL ACAD SCIENCES
DOI: 10.1073/pnas.1209071109
Keywords
atmospheric chemistry; chemical aging; organic aerosol; collection efficiency
Categories
Funding
- Office of Science (BES), US Department of Energy [DE-FG02-08ER64529]
- Japan Society for the Promotion of Science (JSPS)
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The interconversion of atmospheric organic particles among solid, semisolid, and liquid phases is of keen current scientific interest, especially for particles of secondary organic material (SOM). Herein, the influence of phase on ammonia uptake and subsequent particle-phase reactions was investigated for aerosol particles of adipic acid and a-pinene ozonolysis SOM. The nitrogen content of the particles was monitored by online mass spectrometry for increasing ammonia exposure. Solid and semisolid adipic acid particles were inert to the ammonia uptake for low RH (<5%). For the solid particles, ammonia exposure at high relative humidity (RH; >94%) induced a first-order deliquescence phase transition into aqueous particles. Solid particles exposed to supersaturated (RH >100%) conditions and cycled back to high RH (>94%), thereby becoming acidic metastable particles, underwent a gradual second-order transition upon ammonia exposure to form aqueous, partially neutralized particles. For a-pinene SOM, ammonia exposure at low RH increased the particle-phase ammonium content by a small amount. Mass spectrometric observations suggest a mechanism of neutralization and co-condensation of acidic gas-phase species, consistent with a highly viscous semisolid upon which adsorption occurs. At high RH the ammonium content increased greatly, indicative of rapid diffusion and absorption in a liquid environment. The mass spectra indicated the production of organonitrogen compounds, possibly by particle-phase reactive chemistry. The present results demonstrate that phase can be a key regulator of the reactivity of atmospheric SOM particles.
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