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| Titel |
Deliquescence, efflorescence, and phase miscibility of mixed particles of ammonium sulfate and isoprene-derived secondary organic material |
| VerfasserIn |
M. L. Smith, A. K. Bertram, S. T. Martin |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1680-7316
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| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Chemistry and Physics ; 12, no. 20 ; Nr. 12, no. 20 (2012-10-23), S.9613-9628 |
| Datensatznummer |
250011525
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| Publikation (Nr.) |
 copernicus.org/acp-12-9613-2012.pdf |
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| Zusammenfassung |
| The hygroscopic phase transitions of ammonium sulfate mixed with
isoprene-derived secondary organic material were investigated in aerosol
experiments. The organic material was produced by isoprene photo-oxidation at
40% and 60% relative humidity. The low volatility fraction of the
photo-oxidation products condensed onto ammonium sulfate particles. The
particle-phase organic material had oxygen-to-carbon ratios of 0.67 to 0.74
(±0.2) for mass concentrations of 20 to 30 μg m−3. The
deliquescence, efflorescence, and phase miscibility of the mixed particles
were investigated using a dual arm tandem differential mobility analyzer. The
isoprene photo-oxidation products induced deviations in behavior relative to
pure ammonium sulfate. Compared to an efflorescence relative humidity (ERH)
of 30 to 35% for pure ammonium sulfate, efflorescence was eliminated for
aqueous particles having organic volume fractions ϵ of 0.6 and
greater. Compared to a deliquescence relative humidity (DRH) of 80% for
pure ammonium sulfate, the DRH steadily decreased with increasing
ϵ, approaching a DRH of 40% for ϵ of 0.9.
Parameterizations of the DRH(ϵ) and ERH(ϵ) curves
were as follows: DRH(ϵ)= ∑i ci,d ϵi
valid for 0 ≤ ϵ ≤0.86 and ERH(ϵ)= ∑ i
ci,e ϵi valid for 0 ≤ ϵ ≤ 0.55 for the
coefficients c0,d= 80.67, c0,e = 28.35, c1,d = −11.45,
c1,e = −13.66, c2,d = 0, c2,e = 0, c3,d = 57.99, c3,e
= -83.80, c4,d = −106.80, and c4,e = 0. The molecular description
that is thermodynamically implied by these strongly sloped DRH(ϵ) and ERH(ϵ) curves is that the organic isoprene
photo-oxidation products, the inorganic ammonium sulfate, and water form a
miscible liquid phase even at low relative humidity. This phase miscibility
is in contrast to the liquid-liquid separation that occurs for some other
types of secondary organic material. These differences in liquid-liquid
separation are consistent with a prediction recently presented in the
literature that the bifurcation between liquid-liquid phase separation versus
mixing depends on the oxygen-to-carbon ratio of the organic material. The
conclusions are that the influence of secondary organic material on the
hygroscopic properties of ammonium sulfate varies with organic composition
and that the degree of oxygenation of the organic material, which is a
measurable characteristic of complex organic materials, is an important
variable influencing the hygroscopic properties of mixed organic-inorganic
particles. |
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