Thermodynamic characterization of Mexico City aerosol during MILAGRO 2006 C. Fountoukis1,*, A. Nenes1,2, A. Sullivan2,**, R. Weber2, T. Van Reken3,***, M. Fischer4, E. Matías5, M. Moya5, D. Farmer6, and R. C. Cohen6 1School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, Atlanta, GA, USA 2School of Earth & Atmospheric Sciences, Georgia Institute of Technology, Atlanta, GA, USA 3National Center for Atmospheric Research, Boulder, CO, USA 4Environmental Energy Technologies Division, Lawrence Berkeley National Laboratory, Berkeley, CA, USA 5Centro de Ciencias de la Atmosfera, Universidad Nacional Autonoma de Mexico, Mexico City, Mexico 6Department of Chemistry, University of California Berkeley, Berkeley, CA, USA *now at: Institute of Chemical Engineering and High Temperature Chemical Processes (ICEHT), Foundation for Research and Technology Hellas (FORTH), Patras, Greece **now at: Department of Atmospheric Science, Colorado State University, Fort Collins, CO, USA ***now at: Laboratory for Atmospheric Research, Department of Civil & Environmental Engineering, Washington State University, Pullman, Washington, USA
Abstract. Fast measurements of aerosol and gas-phase constituents coupled with the
ISORROPIA-II thermodynamic equilibrium model are used to study the
partitioning of semivolatile inorganic species and phase state of Mexico
City aerosol sampled at the T1 site during the MILAGRO 2006 campaign.
Overall, predicted semivolatile partitioning agrees well with measurements.
PM2.5 is insensitive to changes in ammonia but is to acidic
semivolatile species. For particle sizes up to 1μm diameter,
semi-volatile partitioning requires 15–30 min to equilibrate; longer time is
typically required during the night and early morning hours. Aerosol and
gas-phase speciation always exhibits substantial temporal variability, so
that aerosol composition measurements (bulk or size-resolved) obtained over
large integration periods are not reflective of its true state. When the
aerosol sulfate-to-nitrate molar ratio is less than unity, predictions
improve substantially if the aerosol is assumed to follow the deliquescent
phase diagram. Treating crustal species as "equivalent sodium" (rather
than explicitly) in the thermodynamic equilibrium calculations introduces
important biases in predicted aerosol water uptake, nitrate and ammonium;
neglecting crustals further increases errors dramatically. This suggests
that explicitly considering crustals in the thermodynamic calculations is
required to accurately predict the partitioning and phase state of aerosols.
Citation: Fountoukis, C., Nenes, A., Sullivan, A., Weber, R., Van Reken, T., Fischer, M., Matías, E., Moya, M., Farmer, D., and Cohen, R. C.: Thermodynamic characterization of Mexico City aerosol during MILAGRO 2006, Atmos. Chem. Phys., 9, 2141-2156, doi:10.5194/acp-9-2141-2009, 2009.