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| Titel |
The importance of aerosol mixing state and size-resolved composition on CCN concentration and the variation of the importance with atmospheric aging of aerosols |
| VerfasserIn |
J. Wang, M. J. Cubison, A. C. Aiken, J. L. Jimenez, D. R. Collins |
| 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 ; 10, no. 15 ; Nr. 10, no. 15 (2010-08-06), S.7267-7283 |
| Datensatznummer |
250008685
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| Publikation (Nr.) |
 copernicus.org/acp-10-7267-2010.pdf |
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| Zusammenfassung |
| Aerosol microphysics, chemical composition, and CCN concentrations were
measured at the T0 urban supersite in Mexico City during Megacity
Initiative: Local and Global Research Observations (MILAGRO) in March 2006.
The aerosol size distribution and composition often showed strong diurnal
variation associated with traffic emissions and aging of aerosols through
coagulation and local photochemical production of secondary aerosol species.
CCN concentrations (NCCN) are derived using Köhler theory from the
measured aerosol size distribution and various simplified aerosol mixing
state and chemical composition, and are compared to concurrent measurements
at five supersaturations ranging from 0.11% to 0.35%. The influence of
assumed mixing state on calculated NCCN is examined using both aerosols
observed during MILAGRO and representative aerosol types. The results
indicate that while ambient aerosols often consist of particles with a wide
range of compositions at a given size, NCCN may be derived within
~20% assuming an internal mixture (i.e., particles at a given size are
mixtures of all participating species, and have the identical composition)
if great majority of particles has an overall κ (hygroscopicity
parameter) value greater than 0.1. For a non-hygroscopic particle with a
diameter of 100 nm, a 3 nm coating of sulfate or nitrate is sufficient to
increase its κ from 0 to 0.1. The measurements during MILAGRO
suggest that the mixing of non-hygroscopic primary organic aerosol (POA) and
black carbon (BC) particles with photochemically produced hygroscopic
species and thereby the increase of their κ to 0.1 take place in a
few hours during daytime. This rapid process suggests that during daytime, a
few tens of kilometers away for POA and BC sources, NCCN may be derived
with sufficient accuracy by assuming an internal mixture, and using bulk
chemical composition. The rapid mixing also indicates that, at least for
very active photochemical environments such as Mexico City, the timescale
during daytime for the conversion of hydrophobic POA and BC to hydrophilic
particles is substantially shorter than the 1–2 days used in some global
models. The conversion time scale is substantially longer during night. Most
POA and BC particles emitted during evening hours likely remain
non-hygroscopic until efficiently internally mixed with secondary species in
the next morning. The results also suggest that the assumed mixing state
strongly impacts calculated NCCN only when POA and BC represent a large
fraction of the total aerosol volume. One of the implications is that while
physically unrealistic, external mixtures, which are used in many global
models, may also sufficiently predict NCCN for aged aerosol, as the
contribution of non-hygroscopic POA and BC to overall aerosol volume is
often substantially reduced due to the condensation of secondary species. |
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