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

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.