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Internal mixing of the organic aerosol by gas phase diffusion of semivolatile organic compounds

by C. Marcolli, B. P. Luo, Th. Peter, F. G. Wienhold
Atmospheric Chemistry and Physics ()
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Abstract

This paper shows that most of the so far identi- fied constituents of the tropospheric organic particulate mat- ter belong to a semivolatile fraction for which gas phase dif- fusion in the lower troposphere is sufficiently fast to establish thermodynamic equilibrium between aerosol particles. For the first time analytical expressions for this process are de- rived. Inspection of vapor pressure data of a series of organic substances allows a rough estimate for which substances this mixing process must be considered. As general benchmarks we conclude that for typical aerosol radii between 0.1 and 1┬Ámthis mixing process is efficient at 25Cfor polar species with molecular weights up to 200 and for non-polar species up to 320. At 10C, these values are shifted to 150 for po- lar and to 270 for non-polar substances. The extent ofmixing of this semivolatile fraction is governed by equilibrium ther- modynamics, leading to a selectively, though not completely, internally mixed aerosol. The internal mixing leads to a sys- tematic depression of melting and deliquescence points of organic and mixed organic/inorganic aerosols, thus leading to an aerosol population in the lower troposphere which is predominantly liquid.

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