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| Titel |
Cloud condensation nuclei from fresh and aged air pollution in the megacity region of Beijing: size-resolved measurements and parameterization of aerosol chemical composition, hygroscopicity and CCN activity |
| VerfasserIn |
Sachin S. Gunthe |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250052394
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| Zusammenfassung |
| Atmospheric aerosol particles serving as cloud condensation nuclei (CCN) are key elements
of the hydrological cycle and climate. CCN properties were measured and characterized
during the CAREBeijing-2006 campaign at a suburban site south of the megacity
Beijing, China. Size-resolved CCN efficiency spectra recorded for a supersaturation
range of S = 0.07% to 0.86% yielded average activation diameters in the range
of 190 nm to 45 nm. The corresponding effective hygroscopicity parameters (κ)
exhibited a strong size dependence ranging from ~0.25 in the Aitken size range to
~0.45 in the accumulation size range. The campaign average value (κ = 0.3±0.1)
was similar to the values observed and modeled for other populated continental
regions.
The hygroscopicity parameters derived from the CCN measurements were consistent with
chemical composition data recorded by an aerosol mass spectrometer (AMS) and
thermo-optical measurements of elemental and organic carbon (EC/OC). The CCN
hygroscopicity and its size dependence could be parameterized as a function of AMS-based
organic and inorganic mass fractions using the simple mixing rule κp - 0.1*forg +
0.7*finorg.
When the measured air masses originated in the north and passed rapidly over the
center of Beijing (fresh city pollution), the average particle hygroscopicity was
reduced (κ = 0.2±0.1), which is consistent with enhanced mass fractions of organic
compounds (~50%) and EC (~30%) in fine particulate matter (PM1). Moreover, large
fractions of CCN-inactive particles with much lower hygroscopicity were observed
at low supersaturation (S = 0.07%), which can be explained by freshly emitted,
externally mixed soot particles. Particles in stagnant air from the industrialized
megacity region south of Beijing (aged regional pollution) were on average larger and
more hygroscopic, which is consistent with enhanced mass fractions (~60%) of
soluble inorganic ions (mostly sulfate, ammonium, and nitrate). Accordingly, the
number concentration of CCN in aged air from the megacity region was higher
((2.5-9.9)Ã103 cm-3 vs. (0.4-8.3)Ã103 cm-3) although the total aerosol particle number
concentration was lower than in fresh city outflow (1.2Ã104 cm-3 vs. 2.3Ã104 cm-3).
A comparison with related studies suggests that the fresh outflow from Chinese
megacity centers may generally contain more but smaller and less hygroscopic
aerosol particles and thus fewer CCN than the aged outflow from megacity regions. |
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