Articles | Volume 8, issue 14
Atmos. Chem. Phys., 8, 3761–3768, 2008

Special issue: MILAGRO/INTEX-B 2006

Atmos. Chem. Phys., 8, 3761–3768, 2008

  16 Jul 2008

16 Jul 2008

On the volatility and production mechanisms of newly formed nitrate and water soluble organic aerosol in Mexico City

C. J. Hennigan1, A. P. Sullivan2,*, C. I. Fountoukis3, A. Nenes2,3, A. Hecobian2, O. Vargas2, R. E. Peltier2,**, A. T. Case Hanks2, L. G. Huey2, B. L. Lefer4, A. G. Russell1, and R. J. Weber2 C. J. Hennigan et al.
  • 1School of Civil and Environmental Engineering, Georgia Institute of Technology, Atlanta, GA, 30332-0340, USA
  • 2School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, GA, 30332-0340, USA
  • 3School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, Atlanta, GA, 30332-0340, USA
  • 4Geosciences Department, University of Houston, Houston, TX, 77204-5007, USA
  • *now at: Colorado State University, Ft. Collins, Colorado, USA
  • **now at: New York University, School of Medicine, USA

Abstract. Measurements of atmospheric gases and fine particle chemistry were made in the Mexico City Metropolitan Area (MCMA) at a site ~30 km down wind of the city center. Ammonium nitrate (NH4NO3) dominated the inorganic aerosol fraction and showed a distinct diurnal signature characterized by rapid morning production and a rapid mid-day concentration decrease. Between the hours of 08:00–12:45, particulate water-soluble organic carbon (WSOC) concentrations increased and decreased in a manner consistent with that of NO3, and the two were highly correlated (R2=0.88) during this time. A box model was used to analyze these behaviors and showed that, for both NO3 and WSOC, the concentration increase was caused primarily (~75–85%) by secondary formation, with a smaller contribution (~15–25%) from the entrainment of air from the free troposphere. For NO3, a majority (~60%) of the midday concentration decrease was caused by dilution from boundary layer expansion, though a significant fraction (~40%) of the NO3 loss was due to particle evaporation. The WSOC concentration decrease was due largely to dilution (~75%), but volatilization did have a meaningful impact (~25%) on the decrease, as well. The results provide an estimate of ambient SOA evaporation losses and suggest that a significant fraction (~35%) of the fresh MCMA secondary organic aerosol (SOA) measured at the surface volatilized.

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