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| Titel |
Determination of particulate lead using aerosol mass spectrometry: MILAGRO/MCMA-2006 observations |
| VerfasserIn |
D. Salcedo, T. B. Onasch, A. C. Aiken, L. R. Williams, B. Foy, M. J. Cubison, D. R. Worsnop, L. T. Molina, J. L. Jimenez |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1680-7316
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| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Chemistry and Physics ; 10, no. 12 ; Nr. 10, no. 12 (2010-06-20), S.5371-5389 |
| Datensatznummer |
250008559
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| Publikation (Nr.) |
 copernicus.org/acp-10-5371-2010.pdf |
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| Zusammenfassung |
| We report the first measurements of particulate lead (Pb) from Aerodyne
Aerosol Mass Spectrometers, which were deployed in and around Mexico City
during the Megacity Initiative: Local and Global Research Observations
(MILAGRO)/Mexico City Metropolitan Area 2006 (MCMA-2006) field campaigns.
The high resolution mass spectrometer of one of the AMS instruments (HR-AMS)
and the measured isotopic ratios unequivocally prove the detection of Pb in
ambient particles. A substantial fraction of the lead evaporated slowly from
the vaporizer of the instruments, which is indicative of species with low
volatility at 600 °C. A model was developed in order to estimate the
ambient particulate Pb entering the AMS from the signals in the "open" and
the "closed" (or "background") mass spectrum modes of the AMS. The model
suggests the presence of at least two lead fractions with ~25% of
the Pb signal exhibiting rapid evaporation (1/e decay constant, τ<0.1 s)
and ~75% exhibiting slow evaporation (τ~2.4 min) at the T0 urban supersite and a different fraction (70% prompt
and 30% slow evaporation) at a site northwest from the metropolitan area
(PEMEX site). From laboratory experiments with pure Pb(NO3)2
particles, we estimated that the Pb ionization efficiency relative to
nitrate (RIEPb) is 0.5. Comparison of time series of AMS Pb with other
measurements carried out at the T0 supersite during MILAGRO (using Proton
Induced X-ray Emission (PIXE), Inductively-Coupled Plasma Mass Spectrometry
(ICP-MS) and single-particle counts from an Aerosol Time-of-Fight Mass
Spectrometer (ATOFMS)) shows similar levels (for PIXE and ICP-MS) and
substantial correlation. During part of the campaign, sampling at T0 was
alternated every 10 min with an Aerosol Concentrator, which enabled the
detection of signals for PbCl+ and PbS+ ions. PbS+ displays
the signature of a slowly evaporating species, while PbCl+ appears to
arise only from fast evaporation, which is likely due to the higher vapor
pressure of the compounds generating PbCl+. This is consistent with the
evaporation model results. Levels of particulate Pb measured at T0 were
similar to previous studies in Mexico City. Pb shows a diurnal cycle with a
maximum in the early morning, which is typical of primary urban pollutants.
Pb shows correlation with Zn, consistent with previous studies, while the
sources of Pb appear to be at least partially disjoint from those of
particulate chloride. Back trajectory analysis of the T0 Pb data suggests
the presence of sources inside the urban area SSW and N of T0, with
different chemical forms of Pb being associated with different source
locations. High signals due to particulate lead were also detected in the
PEMEX site; again, no correlation between Pb and chloride plumes was
observed, suggesting mostly different sources for both species. |
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