Fast airborne aerosol size and chemistry measurements above Mexico City and Central Mexico during the MILAGRO campaign P. F. DeCarlo1,2,*, E. J. Dunlea1, J. R. Kimmel1, A. C. Aiken1,3, D. Sueper1, J. Crounse4, P. O. Wennberg4, L. Emmons5, Y. Shinozuka6, A. Clarke6, J. Zhou6, J. Tomlinson7, D. R. Collins7, D. Knapp5, A. J. Weinheimer5, D. D. Montzka2, T. Campos5, and J. L. Jimenez1,3 1Cooperative Institute for Research in Environmental Science (CIRES) University of Colorado, Boulder, CO, USA 2Department of Atmospheric and Oceanic Science, University of Colorado at Boulder, Boulder, CO, USA 3Department of Chemistry and Biochemistry, University of Colorado at Boulder, Boulder, CO, USA 4California Institute of Technology, Pasadena, CA, USA 5National Center for Atmospheric Research, Boulder, CO, USA 6Department of Oceanography, University of Hawaii, USA 7Department of Meteorology, Texas A&M University, College Station, TX, USA *now at: Laboratory of Atmospheric Chemistry, Paul Scherrer Institute, Switzerland
Abstract. 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.
Citation: DeCarlo, P. F., Dunlea, E. J., Kimmel, J. R., Aiken, A. C., Sueper, D., Crounse, J., Wennberg, P. O., Emmons, L., Shinozuka, Y., Clarke, A., Zhou, J., Tomlinson, J., Collins, D. R., Knapp, D., Weinheimer, A. J., Montzka, D. D., Campos, T., and Jimenez, J. L.: Fast airborne aerosol size and chemistry measurements above Mexico City and Central Mexico during the MILAGRO campaign, Atmos. Chem. Phys., 8, 4027-4048, doi:10.5194/acp-8-4027-2008, 2008.