Artículos de revistas
Measured and modelled cloud condensation nuclei (CCN) concentration in São Paulo, Brazil: the importance of aerosol size-resolved chemical composition on CCN concentration prediction
Fecha
2014Registro en:
Atmospheric Chemistry and Physics, Mainz, v. 14, p. 7559-7572, 2014
1680-7367
10.5194/acp-14-7559-2014
Autor
Almeida, G. P.
Ferreira, Jose Roberto Brito
Rodriguez, Carlos Augusto Morales
Andrade, Maria de Fatima
Netto, Paulo Eduardo Artaxo
Institución
Resumen
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 μgm−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