 |
| Titel |
Measured and modelled cloud condensation nuclei (CCN) concentration in São Paulo, Brazil: the importance of aerosol size-resolved chemical composition on CCN concentration prediction |
| VerfasserIn |
G. P. Almeida, J. Brito, C. A. Morales, M. F. Andrade, P. Artaxo |
| Medientyp |
Artikel
|
| Sprache |
Englisch
|
| ISSN |
1680-7316
|
| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Chemistry and Physics ; 14, no. 14 ; Nr. 14, no. 14 (2014-07-29), S.7559-7572 |
| Datensatznummer |
250118907
|
| Publikation (Nr.) |
 copernicus.org/acp-14-7559-2014.pdf |
|
|
|
|
|
| Zusammenfassung |
Measurements of cloud condensation nuclei (CCN), aerosol size distribution
and non-refractory chemical composition were performed from 16 to 31 October
2012 in the São Paulo Metropolitan Area (SPMA), Brazil. CCN measurements
were performed at 0.23, 0.45, 0.68, 0.90 and 1.13% water
supersaturation and were subsequently compared with the Köhler theory,
considering the chemical composition. Real-time chemical composition has
been obtained by deploying, for the first time in the SPMA, an aerosol chemical
ionization monitor (ACSM). CCN closure analyses were performed considering
internal mixtures.
Average aerosol composition during the studied period yielded (arithmetic
mean~± standard deviation) 4.81 ± 3.05, 3.26 ± 2.10,
0.30 ± 0.27, 0.52 ± 0.32, 0.37 ± 0.21 and 0.04 ± 0.04 μg m−3
for organics, BC, NH4, SO4, NO3 and Cl,
respectively. Particle number concentration was 12 813 ± 5350 cm−3,
with a dominant nucleation mode. CCN concentrations were on
average 1090 ± 328 and 3570 ± 1695 cm−3 at
SS = 0.23% and SS = 1.13%, respectively.
Results show an increase in aerosol hygroscopicity in the afternoon as a
result of aerosol photochemical processing, leading to an enhancement of
both organic and inorganic secondary aerosols in the atmosphere, as well as
an increase in aerosol average diameter.
Considering the bulk composition alone, observed CCN concentrations were
substantially overpredicted when compared with the Köhler theory (44.1 ± 47.9%
at 0.23% supersaturation and 91.4 ± 40.3% at
1.13% supersaturation). Overall, the impact of composition on the
calculated CCN concentration (NCCN) decreases with decreasing
supersaturation, partially because using bulk composition introduces less
bias for large diameters and lower critical supersaturations, defined as the
supersaturation at which the cloud droplet activation will take place.
Results suggest that the consideration of only inorganic fraction improves
the calculated NCCN.
Introducing a size-dependent chemical composition based on filter
measurements from previous campaigns has considerably improved simulated
values for NCCN (average overprediction error 14.8 ± 38.6% at
0.23% supersaturation and 3.6 ± 21.6% at 1.13%
supersaturation). This study provides the first insight on aerosol real-time
composition and hygroscopicity at a site strongly impacted by emissions of a
unique vehicular fleet due to the extensive biofuel usage. |
| |
|
| Teil von |
|
|
|
|
|
|
|
|