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Journal article

On the diurnal cycle of urban aerosols, black carbon and the occurrence of new particle formation events in springtime São Paulo, Brazil

Backman J, Rizzo L, Hakala J, Nieminen T, Manninen H, Morais F, Aalto P, Siivola E, Carbone S, Hillamo R, Artaxo P, Virkkula A, Petäjä T, Kulmala M ...see all

Atmospheric Chemistry and Physics, vol. 12, issue 23 (2012) pp. 11733-11751

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Abstract

Large conurbations are a significant source of the anthropogenic
pollution and demographic differences between cities that result in a
different pollution burden. The metropolitan area of Sao Paulo (MASP,
population 20 million) accounts for one fifth of the Brazilian vehicular
fleet. A feature of MASP is the amount of ethanol used by the vehicular
fleet, known to exacerbate air quality. The study describes the diurnal
behaviour of the submicron aerosol and relies on total particle number
concentration, particle number size distribution, light scattering and
light absorption measurements. Modelled planetary boundary layer (PBL)
depth and air mass movement data were used to aid the interpretation.
During morning rush-hour, stagnant air and a shallow PBL height favour
the accumulation of aerosol pollution. During clear-sky conditions,
there was a wind shift towards the edge of the city indicating a heat
island effect with implications on particulate pollution levels at the
site. The median total particle number concentration for the submicron
aerosol typically varied in the range 1.6 x 10(4)-3.2 x 10(4) cm(-3)
frequently exceeding 4 x 10(4) cm-3 during the day. During weekdays,
nucleation-mode particles are responsible for most of the particles by
numbers. The highest concentrations of total particle number
concentrations and black carbon (BC) were observed on Fridays. Median
diurnal values for light absorption and light scattering (at 637 nm
wavelength) varied in the range 12-33 Mm(-1) and 21-64 Mm(-1),
respectively. The former one is equal to 1.8-5.0 mu g m(-3) of BC. The
growth of the PBL, from the morning rush-hour until noon, is consistent
with the diurnal cycle of BC mass concentrations. Weekday hourly median
single-scattering albedo (omega(0)) varied in the range 0.59-0.76.
Overall, this suggests a top of atmosphere (TOA) warming effect.
However, considering the low surface reflectance of urban areas, for the
given range of omega(0), the TOA radiative forcing can be either
positive or negative for the sources within the MASP. On the average,
weekend omega(0) values were 0.074 higher than during weekdays. During
11% of the days, new particle formation (NPF) events occurred. The
analysed events growth rates ranged between 9 and 25 nm h(-1). Sulphuric
acid proxy concentrations calculated for the site were less than 5% of
the concentration needed to explain the observed growth. Thus, other
vapours are likely contributors to the observed growth.

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Authors

  • Paulo ArtaxoUniversidade de Sao Paulo Instituto de Fisica

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  • J. Backman

  • L. V. Rizzo

  • J. Hakala

  • T. Nieminen

  • H. E. Manninen

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