References

ACPD

Atmospheric Chemistry and Physics Discussions

ACPD

1680-7375

Copernicus GmbH

Göttingen, Germany

10.5194/acpd-11-30419-2011

The variability of urban aerosol size distributions and optical properties in São Paulo – Brazil: new particle formation events occur at the site

Backman

¹ Rizzo

L. V.

² Hakala

¹ Nieminen

¹ Manninen

H. E.

¹ Morais

³ Aalto

P. P.

¹ Siivola

¹ Carbone

⁴ Hillamo

⁴ Artaxo

³ Petäjä

¹ Kulmala

Division of Atmospheric Sciences, Department of Physics, University of Helsinki, Finland

Department of Earth and Exact Sciences, Federal University of São Paulo, Brazil

Institute of Physics, University of São Paulo, Brazil

Finnish Meteorological Institute, Helsinki, Finland

11 11 2011

11 11 30419 30455

This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

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The quest to reduce the dependence on fossil fuel has increased the use of bio-ethanol as an additive to gasoline. The metropolitan area of São Paulo (population 20 million) is a unique laboratory to study the ambient aerosol population caused by the use of bio-fuels because 55% of the fuel used is ethanol. The use of ethanol as an additive to fossil fuel is known to increase aldehyde emissions and when photo chemically oxidized, result in smog. In order to characterize this smog problem total particle number concentration, particle number size distribution, light scattering and light absorption measurement equipment were deployed at the University of São Paulo campus area. Here we present the results from three months of measurements from 10 October 2010 to 10 January 2011. The median total particle number concentration for the sub-micron aerosol typically varies between 1×104–3×104 cm−3 frequently exceeding 5×104 cm−3 during the day. Median diurnal values for light absorption and light scattering vary between 12–33 Mm−1 and 21–64 Mm−1, respectively. The hourly median single-scattering albedo varied between 0.63 and 0.85 indicating a net warming effect on a regional scale. A total of ten new particle formation (NPF) events were observed. During these events, growth rates ranged between 9–25 nm h−1. On average, a calculated sulphuric acid vapour abundance of 2.6× 108 cm−3 would have explained the growth with a vapour production rate of 2.8×106 cm−3 s−1 to sustain it. The estimated sulphuric acid concentration, calculated from global irradiance and sulphur dioxide measurements, accounted for only a fraction of the vapour concentration needed to explain the observed growth rates. This indicates that also other condensable vapours participate in the growth process. During the events, the condensation sink was calculated to be 12× 10−3 s−1 on average.

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