Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 4 5 2004 10.5194/acpd-4-5789-2004 http://www.atmos-chem-phys-discuss.net/4/5789/2004/ http://www.atmos-chem-phys-discuss.net/4/5789/2004/acpd-4-5789-2004.html http://www.atmos-chem-phys-discuss.net/4/5789/2004/acpd-4-5789-2004.pdf 5789 5806 2004-09-27 Internal mixing of the organic aerosol by gas phase diffusion of semivolatile organic compounds C. Marcolli claudia.marcolli@env.ethz.ch B. Luo T. Peter F. G. Wienhold Institute for Atmospheric and Climate Science, ETH Zürich, Zürich, Switzerland This paper shows that most of the so far identified constituents of the tropospheric organic particulate matter belong to a semivolatile fraction for which gas phase diffusion in the lower troposphere is sufficiently fast to establish thermodynamic equilibrium between aerosol particles. For the first time analytical expressions for this process are derived. Inspection of vapor pressure data of a series of organic substances reveals that for typical aerosol radii between 0.1 and 1 µm this mixing process is efficient at 25°C for polar species with molecular weights up to 200 and for non-polar species up to 320. At −10°C, these values are shifted to 150 for polar and to 270 for non-polar substances. Furthermore, this semivolatile fraction is selectively, though not completely, internally mixed with other aerosol constituents, with the extent of mixing governed by equilibrium thermodynamics. The internal mixing leads to a systematic 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.