Measurements of atmospheric gases and fine par- ticle chemistry were made in the Mexico City Metropolitan Area (MCMA) at a site ∼ 30 km down wind of the city cen- ter. Ammonium nitrate (NH 4 NO 3 ) dominated the inorganic aerosol fraction and showed a distinct diurnal signature char- acterized 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) concentra- tions increased and decreased in a manner consistent with that of NO − 3 , and the two were highly correlated ( R 2 =0.88) during this time. A box model was used to analyze these be- haviors and showed that, for both NO − 3 and WSOC, the con- centration increase was caused primarily ( ∼ 75–85%) by sec- ondary formation, with a smaller contribution ( ∼ 15–25%) from the entrainment of air from the free troposphere. For NO − 3 , a majority ( ∼ 60%) of the midday concentration de- crease was caused by dilution from boundary layer expan- sion, though a significant fraction ( ∼ 40%) of the NO − 3 loss was due to particle evaporation. The WSOC concen- tration 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 frac- tion ( ∼ 35%) of the fresh MCMA secondary organic aerosol (SOA) measured at the surface volatilized.