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

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.