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| Titel |
Characterizing black carbon and brown carbon aerosols by their optical and microphysical properties in an urban Mediterranean area. |
| VerfasserIn |
Francesca Costabile, Francesca Barnaba, Federico Angelini, Gian Paolo Gobbi |
| Konferenz |
EGU General Assembly 2013
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 15 (2013) |
| Datensatznummer |
250077970
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| Zusammenfassung |
| Sub-micrometer carbonaceous aerosols in the atmosphere are of serious concern for air
pollution and climate change. They consist of particles with diameters from a few to several
hundred nanometers made of organic carbon (OC), black carbon (BC), plus additional trace
elements. Combustion sources contribute to primary carbonaceous aerosols (BC-
and OC-rich) in a overwhelming way. Combustion also contributes to secondary
particles, i.e. particles deriving from chemical transformation of primary particles,
or from gas-to-particle transformation of gaseous emissions (mainly OC-rich).
Combustion carbonaceous aerosols show huge number concentrations in the ultrafine
particle range (UFPs, with diameters < 100 nm); they also show relatively large light
absorption, whose spectral behaviour depends on the relative BC-to-OC abundance. The
combination of these parameters is therefore important to identify and characterize such
particles.
In this work, we discuss an original approach to characterize BC and OC aerosols based
on their spectral optical and microphysical properties. Total absorption (Ïăa) is decomposed in
the sum of three terms: an absorption due to dust (ÏăaD), a spectrally constant absorption
(ÏăaBC) due to BC, and a spectrally variable absorption (ÏăaOC) due to “brown carbon”. ÏăaD
is evaluated through a proper combination of scattering angstrom exponent, single scattering
albedo, and particle number concentration. ÏăaOC is obtained as the fraction of the
dust-free absorption (Ïăa - ÏăaD) increasing with increasing Absorption Angstrom
exponent. ÏăaBC is finally calculated as Ïăa - ÏăaD - ÏăaOC, and used to obtain an
equivalent BC mass concentration (BCeq). Through Mie theory simulations, particle
diameters of aerosol populations with large absorbing OC equivalent contents (OCeq)
or BCeq contents are calculated. This approach is applied to a dataset of spectral
light absorption (3 wavelength PSAP) and scattering (3 wavelength nephelometer)
measurements in the visible region, and (butanol-based CPC) measurements of total
particle number concentrations (N). Measurements were carried out in 2010-2011
in the Central Mediterranean area of Rome (Italy) at three sites representative of
conditions ranging from rural background to urban pollution to aircraft/road traffic
emissions.
Findings show ÏăaOC to be surprisingly similar to ÏăaBC regardless of measurement site
and season. For both ÏăaBC and ÏăaOC, accumulation mode particles with 100-300 nm
diameters show significant contributions, in particular at the urban background site. The role
of UFPs is as well not negligible, in particular at the aircraft traffic site, where N is of the
order of 106cm-3 : here, ÏăaBC apportioned to soot mode particles, and ÏăaOC
apportioned to Aitken mode particles reach, respectively, up to 20 and 10 Mm-1. Future
evaluations of the OCeq and BCeq mass absorption efficiency varying with varying
particle size down to UFPs become necessary to unravel the true role of brown carbon
aerosols, this role being here indicated as potentially comparable to the one of BCeq. |
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