Atmospheric Chemistry and Physics Discussions
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10.5194/acpd-9-25915-2009
http://www.atmos-chem-phys-discuss.net/9/25915/2009/
http://www.atmos-chem-phys-discuss.net/9/25915/2009/acpd-9-25915-2009.html
http://www.atmos-chem-phys-discuss.net/9/25915/2009/acpd-9-25915-2009.pdf
25915
25981
2009-12-02
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 modern carbon fraction
A. C. Aiken
B. de Foy
C. Wiedinmyer
P. F. DeCarlo
I. M. Ulbrich
M. N. Wehrli
S. Szidat
A. S. H. Prevot
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
jose.jimenez@colorado.edu
Dept. of Chemistry and Biochemistry, University of Colorado, Boulder, CO, USA
Cooperative Institute for Research in the Environmental Sciences (CIRES), University of Colorado, Boulder, CO, USA
Saint Louis University, St. Louis, MO, USA
National Center for Atmospheric Research, Boulder, CO, USA
Dept. of Atmospheric and Oceanic Sciences, University of Colorado, Boulder, CO, USA
Dept. of Chemistry and Biochemistry, University of Bern, Berne, Switzerland
Laboratory of Atmospheric Chemistry, Paul Scherrer Institut, Villigen, Switzerland
Dept. of Chemistry, Atmospheric Science, University of Gothenburg, Gothenburg, Sweden
Institute for Particle Physics, ETH Hönggerberg, Zurich, Switzerland
Texas A&M University, College Station, TX, USA
Brookhaven National Laboratory, Upton, NY, USA
Pacific Northwest National Laboratory, Richland, USA
Dept. of Physics, Univ. of Nevada and the Desert Research Institute, Reno, NV, USA
Molina Center for Energy and the Environment and Massachusetts Institute of Technology, USA
Instituto Mexicano del Petróleo, Mexico City, Mexico
IDAEA, Consejo Superior de Investigaciones CientÃficas, Barcelona, Spain
now at: ETH-Zurich, Switzerland
now at: Paul Scherrer Institut, Switzerland
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 OA (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 index correlates well with the observed BBOA, CH<sub>3</sub>CN,
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 ppt,
and PM<sub>2.5</sub> potassium having a background of
~160 ng m<sup>−3</sup> (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
predicted fire impacts. 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 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 fire impacts
or fire tracers, indicating that it is dominated by urban and/or
regional sources and not by the fires near the MCMA. A new
<sup>14</sup>C aerosol dataset is presented. Both this new and
a previously published dataset of <sup>14</sup>C analysis suggest
a similar BBOA contribution as the AMS and chemical mass balance
(CMB), resulting in 15% higher modern 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, 37% of organic carbon (OC) and 30% total
carbon (TC) are from modern sources, suggesting the importance of
urban and regional modern carbon sources other than the fires, such as
food cooking and regional biogenic SOA.
<br><br>
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. The ambient
BBOA/ΔCH<sub>3</sub>CN ratio is much higher in the afternoon
when the wildfires are most intense than during the rest of the
day, which may explain some disagreements between BB impacts from afternoon aircraft
flights and those from 24-h ground measurements. Finally, we show that there
are large differences in the
contributions of the different OA components to the surface
concentrations vs. the integrated column amounts.
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