On the volatility and production mechanisms of newly formed nitrate and water soluble organic aerosol in Mexico City
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, CO, 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. The concentration increase was due to both secondary formation and entrainment from the free troposphere. A majority (approximately two-thirds) of the midday concentration decrease was caused by dilution from boundary layer expansion, however a significant fraction (approximately one-third) of the nitrate loss was due to particle evaporation. The water-soluble organic carbon fraction of fine particles (WSOC) and nitrate were highly correlated (R2=0.80) for the entire three-day analysis period, however the WSOC-nitrate correlation was highest (R2=0.88) between the hours of 08:00–12:45, indicating similar sources and processing during this period. The results show that WSOC also experienced evaporation losses and that a significant fraction of the MCMA secondary organic aerosol (SOA) measured at the surface was semi-volatile.