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January 14, 2010 | Many of the smallest particles in the atmosphere emerge through a different process than previously thought, according to analysis from a team led by NCAR's Jim Smith. The team has found that aminium salts make up as much as half of the mass of newly formed particles in places as diverse as Atlanta, Mexico City, northeast Colorado, and Finland. The work appeared online on 7 January in the Proceedings of the National Academy of Sciences.
While many particles enter the air as dust, salt, or soot, others take shape as extremely small particles borne in the atmosphere from reactions among gases. These nanoparticles (classified as those less than 50 nanometers in diameter) can eventually grow large enough to serve as cloud nuclei. Until recently, only sulfuric acid had been identified as a source of nanoparticle growth, but field observations hinted that other chemical species must be involved.
Studying nanoparticles in the field is difficult due to their tiny size and rapid formation. Smith and colleagues employed a unique instrument, the Thermal Desorption Chemical Ionization Mass Spectrometer (TDCIMS), developed by NCAR over the last decade through a collaboration with the University of Minnesota. According to Smith, the TDCIMS is the world's only instrument that can directly measure the molecular composition of particles as small as 10 nm.
In the urban areas studied, the team found that aminium salts, which are mixtures of amines with organic acids, comprised almost half of the identified ions within nanoparticles. Sulfates were far less prevalent. This leaves a large part of the nanoparticle growth still unexplained, Smith says. He and colleagues hope to better analyze the sources of amines. Large livestock operations are one important source, he says, but more observations are needed.
"Even a small amount of amines in the air can have a big effect on nanoparticle formation," Smith says. "These newly discovered aminium compounds give us hope that we can eventually understand and predict the impacts of new particle formation on health and climate."