ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-9-2141-2009 Thermodynamic characterization of Mexico City aerosol during MILAGRO 2006 Fountoukis C. 1 7 Nenes A. 1 2 Sullivan A. 2 8 Weber R. 2 Van Reken T. 3 9 Fischer M. 4 Matías E. 5 Moya M. 5 Farmer D. 6 Cohen R. C. 6 School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, Atlanta, GA, USA School of Earth & Atmospheric Sciences, Georgia Institute of Technology, Atlanta, GA, USA National Center for Atmospheric Research, Boulder, CO, USA Environmental Energy Technologies Division, Lawrence Berkeley National Laboratory, Berkeley, CA, USA Centro de Ciencias de la Atmosfera, Universidad Nacional Autonoma de Mexico, Mexico City, Mexico Department 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 24 03 2009 9 6 2141 2156 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys.net/9/2141/2009/acp-9-2141-2009.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/9/2141/2009/acp-9-2141-2009.pdf

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. PM<sub>2.5</sub> 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.

 References Amundson, N. R., Caboussat, A., He, J. W., Martynenko, A. V., Savarin, V. B., Seinfeld, J. H., and Yoo, K. Y.: A new inorganic atmospheric aerosol phase equilibrium model (UHAERO), Atmos. Chem. Phys., 6, 975–992, 2006. Ansari, A. S. and Pandis, S. N.: The effect of metastable equilibrium states on the partitioning of nitrate between the gas and aerosol phases, Atmos. Environ., 34, 157–168, 2000. Ansari, A. S. and Pandis, S. N.: Prediction of multicomponent inorganic atmospheric aerosol behavior, Atmos. Environ., 33, 745–757, 1999a. Ansari, A. S. and Pandis, S. N.: An analysis of four models predicting the partitioning of semivolatile inorganic aerosol components, Aerosol Sci. Technol., 31, 129–153, 1999b. Capaldo, K. P., Pilinis, C., and Pandis, S. N.: A computationally efficient hybrid approach for dynamic gas/aerosol transfer in air quality models, Atmos. Environ., 34, 3617–3627, 2000. Cruz, C. N., Dassios, K. G., and Pandis, S. N.: The effect of dioctyl phthalate films on the ammonium nitrate aerosol evaporation rate, Atmos. Environ., 34, 3897–3905, 2000. Dassios, K.G., and Pandis, S.N.: The mass accommodation coefficient of ammonium nitrate aerosol, Atmos. Environ., 33, 2993-3003, 1999. Day, D. A., Wooldridge, P. J., Dillon, M., Thornton, J. A., and Cohen, R. C.: A Thermal dissociation-laser induced fluorescence instrument for in situ detection of NO<sub>2</sub>, peroxy(acyl)nitrates, alkyl nitrates, and HNO<sub>3</sub>, J. Geophys. Res., 107(D5–6), 4046, doi:10.1029/2001JD000779, 2002. Doran, J. C., Arnott, W. P., Barnard, J. C., Cary, R., Coulter, R., Fast, J. D., Kassianov, E. I., Kleinman, L., Laulainen, N. S., Martin, T., Paredes-Miranda, G., Pekour, M. S., Shaw, W. J., Smith, D. F., Springston, S. R., and Yu, X.-Y.: The T1-T2 study: evolution of aerosol properties downwind of Mexico City, Atmos. Chem. Phys., 7, 1585–1598, 2007. Farmer, D. K., Wooldridge, P. J., and Cohen, R. C.: Thermal-dissociation laser induced fluorescence (TD-LIF) as a new technique for measurement of HNO<sub>3</sub>, $§igma$ Alkyl nitrates, $§igma$ peroxy nitrates, and NO<sub>2</sub> eddy covariance fluxes, Atmos. Chem. Phys., 6, 3471–3486, 2006. Fast, J. D., de Foy, B., Acevedo Rosas, F., Caetano, E., Carmichael, G., Emmons, L., McKenna, D., Mena, M., Skamarock, W., Tie, X., Coulter, R. L., Barnard, J. C., Wiedinmyer, C., and Madronich, S.: A meteorological overview of the MILAGRO field campaigns, Atmos. Chem. Phys., 7, 2233–2257, 2007. Fountoukis, C., and Nenes, A.: ISORROPIA II: A computationally efficient thermodynamic equilibrium model for K$^+$-Ca$^2+$-Mg$^2+$-NH$_4^+$-Na$^+$-SO$_4^2-$-NO$_3^-$-Cl$^-$-H<sub>2</sub>O aerosols, Atmos. Chem. Phys., 7, 4639–4659, 2007. Jacobson, M. Z.: Studying the effect of calcium and magnesium on size-distributed nitrate and ammonium with EQUISOLV II, Atmos. Environ., 33, 3635–3649, 1999. Heitzenberg, J.: Fine particles in the global troposphere: a review, Tellus 41B, 149–160, 1989. Hennigan, C. J., Sullivan, A. P., Fountoukis, C. I., Nenes, A., Hecobian, A., Vargas, O., Case, A. T., Hanks, L., Huey, G., Lefer, B. L., and Weber, R. J.: On the Volatility and Production Mechanisms of Newly Formed Nitrate and Water Soluble Organic Aerosol in Mexico City, Atmos. Chem. Phys., 8, 3761–3768, 2008. Malm, W. C., Sisler, J. F., Huffman, D., Eldred, R. A., and Cahill, T. A.: Spatial and seasonal trends in particle concentration and optical extinction in the United States, J. Geophys. Res., 99, 1347–1370, 1994. Marcolli, C., Luo, B., and Peter, T.: Mixing of the Organic Aerosol Fractions: Liquids as the Thermodynamically Stable Phases, J. Phys. Chem. A, 108, 2216–2224, 2004. Martin S. T., Han, J. H., and Hung H. M.: The size effect of hematite and corundum inclusions on the efflorescence relative humidities of aqueous ammonium sulfate particles, Geophys. Res. Lett., 28(13), 2601–2604, 2001. Meng, Z. Y., Seinfeld, J. H., Saxena, P., and Kim, Y. P.: Atmospheric gas – aerosol equilibrium IV. Thermodynamics of carbonates, Aerosol Sci. Technol., 23, 131–154, 1995. Metzger, S., Mihalopoulos, N., and Lelieveld, J.: Importance of mineral cations and organics in gas-aerosol partitioning of reactive nitrogen compounds: case study based on MINOS results, Atmos. Chem. Phys., 6, 2549–2567, 2006. Meng, Z. and Seinfeld, J. H.: Time scales to achieve atmospheric gas aerosol equilibrium for volatile species, Atmos. Environ., 30, 2889–2900, 1996. Moya, M., Fountoukis, C., Nenes, A., Matías, E., and Grutter, M.: Predicting diurnal variability of fine inorganic aerosols and their gas-phase precursors near downtown Mexico City, Atmos. Chem. Phys. Discuss., 7, 11257–11294, 2007. Moya, M., Grutter, M., and Baez, A.: Diurnal variability of size-differentiated inorganic aerosols and their gas-phase precursors during January and February of 2003 near downtown Mexico City, Atmos. Environ., 38, 5651–5661, 2004. Moya, M., Pandis, S. N., and Jacobson, M. Z.: Is the size distribution of urban aerosols determined by thermodynamic equilibrium? An application to Southern California, Atmos. Environ., 36, 2349–2365, 2002. Moya, M., Ansari, A. S., and Pandis, S. N.: Partitioning of nitrate and ammonium between the gas and particulate phases during the 1997 IMADA-AVER study in Mexico City, Atmos. Environ., 35, 1791–1804, 2001. Nowak, J. B., Huey, L. G., Russell, A. G., Tian, D., Neuman, J. A., Orsini, D., Sjostedt, S. J., Sullivan, A. P., Tanner, D. J., Weber, R. J., Nenes, A., Edgerton, E., and Fehsenfeld, F. C.: Analysis of urban gas phase ammonia measurements from the 2002 Atlanta Aerosol Nucleation and Real-Time Characterization Experiment (ANARChE), J. Geophys. Res., 111, D17308, doi:10.1029/2006JD007113, 2006. Orsini, D. A., Ma, Y., Sullivan, A., Sierau, B., Baumann, K., and Weber, R. J.: Refinements to the particle-into-liquid sampler (PILS) for ground and airborne measurements of water soluble aerosol composition, Atmos. Environ., 37, 1243–1259, 2003. Pilinis, C., Capaldo, K. P, Nenes, A., and Pandis, S. N.: MADM – A New Multicomponent Aerosol Dynamics Model, Aerosol Sci. Technol., 32(5), 482–502, 2000. Salcedo, D., Onasch, T. B., Dzepina, K., Canagaratna, M. R., Zhang, Q., Huffman, J. A., DeCarlo, P. F., Jayne, J. T., Mortimer, P., Worsnop, D. R., Kolb, C. E., Johnson, K.S., Zuberi, B., Marr, L.C., Volkamer, R., Molina, L. T., Molina, M. J., Cardenas, B., Bernabe, R. M., Marquez, C., Gaffney, J. S., Marley, N. A., Laskin, A., Shutthanandan, V., Xie, Y., Brune, W., Lesher, R., Shirley, T., and Jimenez, J. L., Characterization of ambient aerosols in Mexico City during the MCMA-2003 campaign with Aerosol Mass Spectrometry: results from the CENICA Supersite, Atmos. Chem. Phys., 6, 925–946, 2006. San~Martini, F. M.,~ Dunlea, E. J., Volkamer, R., Onasch, T. B., Jayne, J. T., Canagaratna, M. R., Worsnop, D. R., Kolb, C. E., Shorter, J. H., Herndon, S. C., Zahniser, M. S., Salcedo, D., Dzepina, K., Jimenez, J. L., Ortega, J. M., Johnson, K. S., McRae, G. J., Molina, L. T., and Molina M. J.: Implementation of a Markov Chain Monte Carlo method to inorganic aerosol modeling of observations from the MCMA-2003 campaign – Part II: Model application to the CENICA, Pedregal and Santa Ana sites, Atmos. Chem. Phys., 6, 4889–4904, 2006. Seinfeld, J. H. and Pandis, S. N.: Atmospheric Chemistry and Physics: From Air Pollution to Climate Change, John Wiley & Sons, Inc., 558–574, 2006. Takahama, S., Wittig, A. E., Vayenas, D. V., Davidson, C. I., and Pandis, S. N.: Modeling the diurnal variation of nitrate during the Pittsburgh Air Quality Study, J. Geophys. Res., 109, D16S06, doi:10.1029/2003JD004149, 2004. Wexler, A. S. and Clegg, S. L.: Atmospheric aerosol models for systems including the ions H$^+$, NH$_4^+$, Na$^+$, SO$_4^2-$, NO$_3^-$, Cl$^-$, Br$^-$, and H<sub>2</sub>O, J. Geophys. Res., 107, 4207, doi:10.1029/2001JD000451, 2002. Wexler, A. S. and Seinfeld, J. H.: Second – generation inorganic aerosol model, Atmos. Environ., 25A, 2731–2748, 1991. Wexler, A. S. and Seinfeld, J. H.: Analysis of aerosol ammonium nitrate: departures from equilibrium during SCAQS, Atmos. Environ., 26A, 579–591, 1992. Yu, S., Dennis, R., Roselle, S., Nenes, A., Walker, J., Eder, B., Schere, K., Swall, J., and Robarge, W.: An assessment of the ability of three-dimensional air quality models with current thermodynamic equilibrium models to predict aerosol NO$_3^-$, J. Geophys. Res., 110, D07S13, doi:10.1029/2004JD004718, 2005. Zhang, Y., Seigneur, C., Seinfeld, J. H., Jacobson, M., Clegg, S. L., and Binkowski, F. S.: A comparative review of inorganic aerosol thermodynamic equilibrium models: similarities, differences, and their likely causes, Atmos. Environ., 34, 117–137, 2000. Zhang, J., Chameides, W. L., Weber, R., Cass, G., Orsini, D., Edgerton, E. S., Jongejan, P., and Slanina, J.: An evaluation of the thermodynamic equilibrium assumption for fine particulate composition: Nitrate and ammonium during the 1999 Atlanta Supersite Experiment, J. Geophys. Res., 107, 8414, doi:10.1029/2001JD001592, 2003.