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| Titel |
Mexico city aerosol analysis during MILAGRO using high resolution aerosol mass spectrometry at the urban supersite (T0) – Part 2: Analysis of the biomass burning contribution and the non-fossil carbon fraction |
| VerfasserIn |
A. C. Aiken, B. Foy, C. Wiedinmyer, P. F. DeCarlo, I. M. Ulbrich, M. N. Wehrli, S. Szidat, A. S. H. Prévôt, J. Noda, L. Wacker, R. Volkamer, E. Fortner, J. Wang, A. Laskin, V. Shutthanandan, J. Zheng, R. Zhang, G. Paredes-Miranda, W. P. Arnott, L. T. Molina, G. Sosa, X. Querol, 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 (2010-06-16), S.5315-5341 |
| Datensatznummer |
250008555
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| Publikation (Nr.) |
 copernicus.org/acp-10-5315-2010.pdf |
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| Zusammenfassung |
Submicron aerosol was analyzed during the MILAGRO field campaign in March 2006 at the T0 urban supersite in Mexico City with a High-Resolution Aerosol
Mass Spectrometer (AMS) and complementary instrumentation. Positive Matrix
Factorization (PMF) of high resolution AMS spectra identified a biomass
burning organic aerosol (BBOA) component, which includes several large
plumes that appear to be from forest fires within the region. Here, we show
that the AMS BBOA concentration at T0 correlates with fire counts in the
vicinity of Mexico City and that most of the BBOA variability is captured
when the FLEXPART model is used for the dispersion of fire emissions as
estimated from satellite fire counts. The resulting FLEXPART fire impact
factor (FIF) correlates well with the observed BBOA, acetonitrile
(CH3CN), levoglucosan, and potassium, indicating that wildfires in the
region surrounding Mexico City are the dominant source of BBOA at T0 during
MILAGRO. The impact of distant BB sources such as the Yucatan is small
during this period. All fire tracers are correlated, with BBOA and
levoglucosan showing little background, acetonitrile having a well-known
tropospheric background of ~100–150 pptv, and PM2.5 potassium
having a background of ~160 ng m−3 (two-thirds of its average
concentration), which does not appear to be related to BB sources.
We define two high fire periods based on satellite fire counts and
FLEXPART-predicted FIFs. We then compare these periods with a low fire
period when the impact of regional fires is about a factor of 5 smaller.
Fire tracers are very elevated in the high fire periods whereas tracers of
urban pollution do not change between these periods. Dust is also elevated
during the high BB period but this appears to be coincidental due to the
drier conditions and not driven by direct dust emission from the fires. The
AMS oxygenated organic aerosol (OA) factor (OOA, mostly secondary OA or SOA)
does not show an increase during the fire periods or a correlation with fire
counts, FLEXPART-predicted FIFs or fire tracers, indicating that it is
dominated by urban and/or regional sources and not by the fires near the
MCMA.
A new 14C aerosol dataset is presented. Both this new and a previously
published dataset of 14C analysis suggest a similar BBOA contribution
as the AMS and chemical mass balance (CMB), resulting in 13% higher
non-fossil carbon during the high vs. low regional fire periods. The new
dataset has ~15% more fossil carbon on average than the previously
published one, and possible reasons for this discrepancy are discussed.
During the low regional fire period, 38% of organic carbon (OC) and
28% total carbon (TC) are from non-fossil sources, suggesting the
importance of urban and regional non-fossil carbon sources other than the
fires, such as food cooking and regional biogenic SOA.
The ambient BBOA/ΔCH3CN ratio is much higher in the afternoon
when the wildfires are most intense than during the rest of the day. Also,
there are large differences in the contributions of the different OA
components to the surface concentrations vs. the integrated column amounts.
Both facts may explain some apparent disagreements between BB impacts
estimated from afternoon aircraft flights vs. those from 24-h ground
measurements.
We show that by properly accounting for the non-BB sources of K, all of the
BB PM estimates from MILAGRO can be reconciled. Overall, the fires from the
region near the MCMA are estimated to contribute 15–23% of the OA and
7–9% of the fine PM at T0 during MILAGRO, and 2–3% of the fine PM as
an annual average. The 2006 MCMA emissions inventory contains a
substantially lower impact of the forest fire emissions, although a fraction
of these emissions occur just outside of the MCMA inventory area. |
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