Internal mixing of the organic aerosol by gas phase diffusion of semivolatile organic compounds
- ISSN: 1680-7324
This paper shows that most of the so far identified constituents of\nthe tropospheric organic particulate matter belong to a semivolatile\nfraction for which gas phase diffusion in the lower troposphere is\nsufficiently fast to establish thermodynamic equilibrium between\naerosol particles. For the first time analytical expressions for\nthis process are derived. Inspection of vapor pressure data of a\nseries of organic substances allows a rough estimate for which substances\nthis mixing process must be considered. As general benchmarks we\nconclude that for typical aerosol radii between 0.1 and 1 mum this\nmixing process is efficient at 25 degreesC for polar species with\nmolecular weights up to 200 and for non-polar species up to 320.\nAt - 10 degreesC, these values are shifted to 150 for polar and to\n270 for non-polar substances. The extent of mixing of this semivolatile\nfraction is governed by equilibrium thermodynamics, leading to a\nselectively, though not completely, internally mixed aerosol. The\ninternal mixing leads to a systematic depression of melting and deliquescence\npoints of organic and mixed organic/inorganic aerosols, thus leading\nto an aerosol population in the lower troposphere which is predominantly\nliquid.