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| Titel |
Size-resolved measurement of the mixing state of soot in the megacity Beijing, China: diurnal cycle, aging and parameterization |
| VerfasserIn |
Y. F. Cheng, H. Su, D. Rose, S. S. Gunthe, M. Berghof, B. Wehner, P. Achtert, A. Nowak, N. Takegawa, Y. Kondo, M. Shiraiwa, Y. G. Gong, M. Shao, M. Hu, T. Zhu, Y. H. Zhang, G. R. Carmichael, A. Wiedensohler, M. O. Andreae, U. Pöschl |
| 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. 10 ; Nr. 12, no. 10 (2012-05-22), S.4477-4491 |
| Datensatznummer |
250011162
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| Publikation (Nr.) |
 copernicus.org/acp-12-4477-2012.pdf |
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| Zusammenfassung |
Soot particles are the most efficient light
absorbing aerosol species in the atmosphere, playing an
important role as a driver of global warming. Their climate
effects strongly depend on their mixing state, which
significantly changes their light absorbing capability and
cloud condensation nuclei (CCN) activity. Therefore, knowledge
about the mixing state of soot and its aging mechanism becomes
an important topic in the atmospheric sciences.
The size-resolved (30–320 nm diameter) mixing state of soot
particles in polluted megacity air was measured at a suburban
site (Yufa) during the CAREBeijing 2006 campaign in Beijing,
using a volatility tandem differential mobility analyzer
(VTDMA). Particles in this size range with non-volatile
residuals at 300 °C were considered to be soot
particles. On average, the number fraction of internally mixed
soot in total soot particles (Fin), decreased
from 0.80 to 0.57 when initial Dp increased from
30 to 320 nm. Further analysis reveals that: (1)
Fin was well correlated with the aerosol
hygroscopic mixing state measured by a CCN counter. More
externally mixed soot particles were observed when particles
showed more heterogeneous features with regard to
hygroscopicity. (2) Fin had pronounced diurnal
cycles. For particles in the accumulation mode
(Dp at 100–320 nm), largest Fin
were observed at noon time, with "apparent" turnover
rates (kex → in) up to 7.8% h−1. (3)
Fin was subject to competing effects of both
aging and emissions. While aging increases Fin
by converting externally mixed soot particles into internally
mixed ones, emissions tend to reduce Fin by
emitting more fresh and externally mixed soot
particles. Similar competing effects were also found with air
mass age indicators. (4) Under the estimated emission
intensities, actual turnover rates of soot (kex → in)
up to 20% h−1 were derived, which
showed a pronounced diurnal cycle peaking around noon
time. This result confirms that (soot) particles are
undergoing fast aging/coating with the existing high levels of
condensable vapors in the megacity Beijing. (5) Diurnal cycles
of Fin were different between Aitken and
accumulation mode particles, which could be explained by the
faster growth of smaller Aitken mode particles into larger size bins.
To improve the Fin prediction in regional/global
models, we suggest parameterizing Fin by an air
mass aging indicator, i.e., Fin = a + bx, where
a and b are empirical coefficients determined from
observations, and x is the value of an air mass age
indicator. At the Yufa site in the North China Plain, fitted
coefficients (a, b) were determined as (0.57, 0.21),
(0.47, 0.21), and (0.52, 0.0088) for x (indicators) as
[NOz]/[NOy], [E]/[X]
([ethylbenzene]/[m,p-xylene]) and ([IM] + [OM])/[EC]
([inorganic + organic matter]/[elemental carbon]),
respectively. Such a parameterization consumes little
additional computing time, but yields a more realistic
description of Fin compared with the simple
treatment of soot mixing state in regional/global models. |
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