Journal
ATMOSPHERIC CHEMISTRY AND PHYSICS
Volume 8, Issue 14, Pages 3761-3768Publisher
COPERNICUS GESELLSCHAFT MBH
DOI: 10.5194/acp-8-3761-2008
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Measurements of atmospheric gases and fine particle chemistry were made in the Mexico City Metropolitan Area (MCMA) at a site similar to 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 (R-2=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 (similar to 75-85%) by secondary formation, with a smaller contribution (similar to 15-25%) from the entrainment of air from the free troposphere. For NO3-, a majority (similar to 60%) of the midday concentration decrease was caused by dilution from boundary layer expansion, though a significant fraction (similar to 40%) of the NO3- loss was due to particle evaporation. The WSOC concentration decrease was due largely to dilution (similar to 75%), but volatilization did have a meaningful impact (similar to 25%) on the decrease, as well. The results provide an estimate of ambient SOA evaporation losses and suggest that a significant fraction (similar to 35%) of the fresh MCMA secondary organic aerosol (SOA) measured at the surface volatilized.
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