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

[摘要] This paper shows that most of the so far identified constituents of thetropospheric organic particulate matter belong to a semivolatile fraction forwhich gas phase diffusion in the lower troposphere is sufficiently fast toestablish thermodynamic equilibrium between aerosol particles. For the firsttime analytical expressions for this process are derived. Inspection of vaporpressure data of a series of organic substances allows a rough estimate forwhich substances this mixing process must be considered. As general benchmarkswe conclude that for typical aerosol radii between 0.1 and 1 µm thismixing process is efficient at 25°C for polar species with molecularweights 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.The extent of mixing of this semivolatile fraction is governed by equilibriumthermodynamics, leading to a selectively, though not completely, internallymixed aerosol. The internal mixing leads to a systematic depression of meltingand deliquescence points of organic and mixed organic/inorganic aerosols, thusleading to an aerosol population in the lower troposphere which ispredominantly liquid.

[学科分类] 大气科学