References

ACP

Atmospheric Chemistry and Physics

ACP

1680-7324

Copernicus GmbH

Göttingen, Germany

10.5194/acp-10-5315-2010

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

Aiken

A. C.

¹ ² de Foy

³ Wiedinmyer

⁴ DeCarlo

P. F.

² ⁵ ¹⁷ Ulbrich

I. M.

¹ ² Wehrli

M. N.

⁶ Szidat

⁶ Prevot

A. S. H.

⁷ Noda

⁸ ¹⁸ Wacker

⁹ Volkamer

¹ ² Fortner

¹⁰ Wang

¹¹ Laskin

¹² Shutthanandan

¹² Zheng

¹⁰ Zhang

¹⁰ Paredes-Miranda

¹³ Arnott

W. P.

¹³ Molina

L. T.

¹⁴ Sosa

¹⁵ Querol

¹⁶ Jimenez

J. L.

¹ ²

Department 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

Department of Atmospheric and Oceanic Sciences, University of Colorado, Boulder, CO, USA

Department of Chemistry and Biochemistry, University of Bern, Berne, Switzerland

Laboratory of Atmospheric Chemistry, Paul Scherrer Institut, Villigen, Switzerland

Department 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, University 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 Científicas, Barcelona, Spain

now at: Paul Scherrer Institut, Switzerland

now at: the Rakuno Gakuen University, Japan

16 06 2010

10 12 5315 5341

This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

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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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