References

ACP

Atmospheric Chemistry and Physics

ACP

1680-7324

Copernicus GmbH

Göttingen, Germany

10.5194/acp-8-4027-2008

Fast airborne aerosol size and chemistry measurements above Mexico City and Central Mexico during the MILAGRO campaign

DeCarlo

P. F.

¹ ² ⁸ Dunlea

E. J.

¹ Kimmel

J. R.

¹ Aiken

A. C.

¹ ³ Sueper

¹ Crounse

⁴ Wennberg

P. O.

⁴ Emmons

⁵ Shinozuka

⁶ Clarke

⁶ Zhou

⁶ Tomlinson

⁷ Collins

D. R.

⁷ Knapp

⁵ Weinheimer

A. J.

⁵ Montzka

D. D.

² Campos

⁵ Jimenez

J. L.

¹ ³

Cooperative Institute for Research in Environmental Science (CIRES) University of Colorado, Boulder, CO, USA

Department of Atmospheric and Oceanic Science, University of Colorado at Boulder, Boulder, CO, USA

Department of Chemistry and Biochemistry, University of Colorado at Boulder, Boulder, CO, USA

California Institute of Technology, Pasadena, CA, USA

National Center for Atmospheric Research, Boulder, CO, USA

Department of Oceanography, University of Hawaii, USA

Department of Meteorology, Texas A&M University, College Station, TX, USA

now at: Laboratory of Atmospheric Chemistry, Paul Scherrer Institute, Switzerland

25 07 2008

8 14 4027 4048

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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The full text article is available as a PDF file from http://www.atmos-chem-phys.net/8/4027/2008/acp-8-4027-2008.pdf

The concentration, size, and composition of non-refractory submicron aerosol (NR-PM1) was measured over Mexico City and central Mexico with a High-Resolution Time-of-Flight Aerosol Mass Spectrometer (HR-ToF-AMS) onboard the NSF/NCAR C-130 aircraft as part of the MILAGRO field campaign. This was the first aircraft deployment of the HR-ToF-AMS. During the campaign the instrument performed very well, and provided 12 s data. The aerosol mass from the AMS correlates strongly with other aerosol measurements on board the aircraft. Organic aerosol (OA) species dominate the NR-PM1 mass. OA correlates strongly with CO and HCN indicating that pollution (mostly secondary OA, SOA) and biomass burning (BB) are the main OA sources. The OA to CO ratio indicates a typical value for aged air of around 80 μg m−3 (STP) ppm−1. This is within the range observed in outflow from the Northeastern US, which could be due to a compensating effect between higher BB but lower biogenic VOC emissions during this study. The O/C atomic ratio for OA is calculated from the HR mass spectra and shows a clear increase with photochemical age, as SOA forms rapidly and quickly overwhelms primary urban OA, consistent with Volkamer et al. (2006) and Kleinman et al. (2008). The stability of the OA/CO while O/C increases with photochemical age implies a net loss of carbon from the OA. BB OA is marked by signals at m/z 60 and 73, and also by a signal enhancement at large m/z indicative of larger molecules or more resistance to fragmentation. The main inorganic components show different spatial patterns and size distributions. Sulfate is regional in nature with clear volcanic and petrochemical/power plant sources, while the urban area is not a major regional source for this species. Nitrate is enhanced significantly in the urban area and immediate outflow, and is strongly correlated with CO indicating a strong urban source. The importance of nitrate decreases with distance from the city likely due to evaporation. BB does not appear to be a strong source of nitrate despite its high emissions of nitrogen oxides, presumably due to low ammonia emissions. NR-chloride often correlates with HCN indicating a fire source, although other sources likely contribute as well. This is the first aircraft study of the regional evolution of aerosol chemistry from a tropical megacity.

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