The Fate of Synthetic Nanoparticles in the Atmosphere.

Any particle emitted into the atmosphere is immediately subject to two strong processes: condensation of low vapor-pressure compounds formed in the atmosphere, and attack by oxidants including hydroxyl radical (OH) and ozone.  Collectively, these aging processes will transform the properties of the particles and also control their residence time in the atmosphere.  The major loss process for fine particles (smaller than about 2.5 micrometers) in the atmosphere is precipitation (wet deposition), which occurs mostly when the particles grow to a sufficient size and contain enough solute to serve as cloud droplet condensation nuclei (CCN).  All particles in the atmosphere grow rapidly by condensation of inorganics (sulfate and nitrate, neutralized by ammonia to form ammonium sulfate and ammonium nitrate if there is excess ammonia after sulfate neutralization) and a rich mixture of organics.  These coatings may also chemically modify the original particles.  We have investigated the aging chemistry of several model systems in a smog chamber, including polystyrene spheres and silver nanoparticles, to better understand how these processes will occur in the atmosphere.