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| Titel |
Aerosol mixing state, hygroscopic growth and cloud activation efficiency during MIRAGE 2006 |
| VerfasserIn |
S. Lance, T. Raatikainen, T. B. Onasch, D. R. Worsnop, X.-Y. Yu, M. L. Alexander, M. R. Stolzenburg, P. H. McMurry, J. N. Smith, A. Nenes |
| 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 ; 13, no. 9 ; Nr. 13, no. 9 (2013-05-15), S.5049-5062 |
| Datensatznummer |
250018656
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| Publikation (Nr.) |
 copernicus.org/acp-13-5049-2013.pdf |
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| Zusammenfassung |
Observations of aerosol hygroscopic growth and CCN activation spectra for
submicron particles are reported for the T1 ground site outside of Mexico
City during the MIRAGE 2006 campaign. κ-Köhler theory is used to
evaluate the characteristic hygroscopicity parameter, κ*, for the
CCN active aerosol population using both size-resolved HTMDA and
size-resolved CCNc measurements. Organic mass fractions
(forg) are evaluated from size-resolved aerosol mass
spectrometer (AMS) measurements, from which predictions of the hygroscopicity
parameter are compared against κ*.
Strong diurnal changes in aerosol water uptake parameters and aerosol
composition are observed. We find that new particle formation (NPF) events
are correlated with an increased κ* and CCN-active fraction during
the daytime, with greater impact on smaller particles. During NPF events, the
number concentration of 40 nm particles acting as CCN at 0.51% ± 0.06%
supersaturation can surpass by more than a factor of two the corresponding concentrations
of 100 nm particles. We also find that at
06:00–08:00 LT throughout the campaign, fresh traffic emissions
result in substantial changes to the chemical distribution of the aerosol,
with on average 65% externally mixed fraction for 40 nm particles and
30% externally mixed fraction for 100 nm particles, whereas at midday
nearly all particles of both sizes can be described as "internally mixed".
Average activation spectra and growth factor distributions are analyzed for
different time periods characterizing the daytime (with and without NPF
events), the early morning "rush hour" and the entire campaign. We show
that κ* derived from CCNc measurements decreases as a function of
size during all time periods, while the CCN-active fraction increases as a
function of size. Size-resolved AMS measurements do not predict the observed
trend for κ* versus particle size, which can be attributed to
unresolved mixing state and the presence of refractory material not measured
by the AMS. Measured κ* typically ranges from 0.2 to 0.35, and
organics typically make up 60–85 % of the aerosol mass in the size range
studied. We show that κAMS is able to
describe CCN concentrations reasonably well, provided mixing-state
information is available, especially at the highest CCN concentrations. This
is consistent with other CCN studies carried out in urban environments, and
is partly due to the fact that the highest CCN concentrations occur during
the daytime when the aerosol is internally mixed. During the early
morning rush hour, however, failing to account for the aerosol mixing state results
in systematic overestimation of CCN concentrations by as much as 50–100%
on average. |
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