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Aerosol Modeling Challenges: Gas-Particle Dynamics & SOA Formation

Explore challenges in modeling gas-particle partitioning of organic gases to aerosols and understand SOA formation processes. Delve into solutions, model frameworks, and future directions in the field.

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Aerosol Modeling Challenges: Gas-Particle Dynamics & SOA Formation

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  1. Modeling Dynamic Partitioning of Semi-volatile Organic Gases to Size-Distributed Aerosols Rahul A. Zaveri Richard C. Easter Pacific Northwest National Laboratory International Workshop on Air Quality Forecasting Research November 29, 2011 Potomac, MD, USA

  2. Outline • Motivation • SOA formation processes • SOA modeling challenges • MOSAIC aerosol modeling framework • Gas-particle partitioning of organic gases (new) • Sample results • Future Directions 2

  3. Indirect Radiative Forcing Direct Radiative Forcing Nucleation Activation chemistry H2SO4, HNO3, Organic aerosol Resuspension Oxidation Oxidation Hydrocarbons, NOx, SO2, NH3, POA H2SO4, MSA Forest Fires Terpenes Oxidation Soot Dust Sea-salt Dimethylsulfide Phytoplankton Need to efficiently and reliably model aerosol size, number, mass, composition, and their climate related properties at urban to global scales Battelle Proprietary 3

  4. SOA Formation Processes • Up to 90% of submicron aerosol mass is composed of organics (Kanakidou et al., 2005; Zhang et al., 2007) • SOA formation is quite rapid (within a few hours) during daytime (Volkamer et al., 2006; Kleinman et al., 2007; de Gouw et al., 2008) • SOA from oxidation of SVOCs from diesel exhaust may help explain some of the missing organic aerosol mass in models (Robinson et al., 2007) • Observed rapid growth of newly formed particles (via homogeneous nucleation) is thought to be by SOA condensation (Kuang et al., 2008) • Anthropogenic and biogenic SOA precursors may interact to enhance the overall SOA yield (Weber et al., 2007) • Particle-phase reactions of absorbed VOCs within inorganic particles can form SOA (Jang et al., 2003; Kroll et al., 2005; Liggio et al., 2005) • Accretion reactions, including aldol condensation, acid dehydration, and gem-diol condensation can transform VOCs into oligomeric compounds (Gao et al., 2004; Jang et al., 2003; Kalberer et al., 2004) 4

  5. SOA Modeling Challenges • Gas-particle partitioning processes are still poorly understood at a fundamental level • How do we handle the complexities in the gas-phase VOC chemistry? • Should we use Raoult’s Law or some sort of reactive uptake formulation as driving force for gas-particle mass transfer? • Should we use Henry’s Law if the organics are dissolved in the aqueous phase? • Are the organic particles liquid or solid? Virtanen et al. (2011) and Vaden et al. (2011) suggest that SOA particles are solid. • How do we treat organic-inorganic interactions and the associated phase transitions? • How do we treat particle-phase reactions? What are the time scales? • What are the anthropogenic-biogenic interactions? How do we reliably represent them in models? 5

  6. General Problem Solving Approach • Develop a comprehensive aerosol model framework that includes all the processes that we think are (or might be) important • Evaluate the roles of specific processes using appropriate laboratory and field observations • Simplify, parameterize, and optimize the process model as much as possible to increase computational efficiency and decrease memory requirements 6

  7. MOSAIC Aerosol Module • Model for Simulating Aerosol Interactions and Chemistry (Zaveri et al., 2008) • Comprehensive aerosol module for air quality and climate modeling • Flexible framework for coupling various gas and aerosol processes • Robust, accurate, and highly efficient custom numerical solvers for several processes • Suitable for 3-D regional and global models • Implementation in: • Weather Research and Forecasting Model (WRF-Chem) – done • Global model: Community Atmosphere Model (CAM5) – in progress • EPA’s CMAQ – planned

  8. Thermodynamics & Mass Transfer Treatments in MOSAIC HCl Kinetic mass transfer between the gas and size-distributed particles (1 to 10,000 nm) NH3 HNO3 H2SO4 H+ OH- NH4+ Cl- Na+ Solid Salts NO3- Thermodynamic equilibriumwithin the particle phase (depends on composition and RH) Particle Phase Ca2+ SO4- OM, BC H2O Reversible Gas-Particle Reactions Aqueous Phase • Custom numerical techniques have been developed to solve these equations efficiently and accurately

  9. Gas-Particle Partitioning of Organics • Organic-inorganic interactions within the particle to determine water uptake and phase separation(partially implemented) • Size-distributed, dynamic mass transfer between gas and particles • Raoult’s Law in the absence ofaqueous phase (implemented) • Reactive uptake that instantly converts VOC to non-volatile products (implemented) • Henry’s Law in the presence of aqueous phase (future) • Particle-phase reactions (future) Gas Phase HCl NH3 HNO3 Organics H2SO4 H+ OH- NH4+ Cl- Na+ Salts, BC, Org NO3- Ca2+ SO4- Org H2O Oligomer Particle Phase 9

  10. Raoult’s Law vs. Reactive Uptake Reactive Uptake Mass Transfer Raoult’s Law Based Mass Transfer HC + oxidant  a1 G1 + a2 G2 HC + oxidant  a1 G1 + a2 G2 A1 A2 A1 A2 xi = mole fraction dGi dt dGi dt = -ki (Gi – xiPi0) = -kiGi vapor pressure 10

  11. Sample Results • Idealized Case • Sacramento Urban Air Case

  12. Idealized Case Initial aerosol composition: mass ratio OA/(NH4)2SO4 = 1

  13. Idealized Case Initial aerosol composition: mass ratio OA/(NH4)2SO4 = 1

  14. Idealized Case Initial aerosol composition: mass ratio OA/(NH4)2SO4 = 1

  15. Sacramento June 6, 2010 Initial aerosol composition: mass ratio OA/(NH4)2SO4 = 10

  16. Sacramento June 6, 2010 Initial aerosol composition: mass ratio OA/(NH4)2SO4 = 10

  17. Sacramento June 6, 2010 Initial aerosol composition: mass ratio OA/(NH4)2SO4 = 10

  18. Sacramento June 6, 2010 Aitken mode mass ratio OA/(NH4)2SO4 = 10 Accumulation mode mass ratio OA/(NH4)2SO4 = 0.01

  19. Sacramento June 6, 2010 Aitken mode mass ratio OA/(NH4)2SO4 = 10 Accumulation mode mass ratio OA/(NH4)2SO4 = 0.01

  20. Future Directions • Perform additional constrained Lagrangian model analyses to test different SOA formation mechanisms • Use carefully designed chamber experiments to constrain and evaluate different formulations • Extend model analyses to mixtures of organic and inorganic species at different relative humidities • Implement and evaluate new SOA formulations in WRF-Chem using urban to regional field observations

  21. Funding for this work was provided by Department of Energy (DOE) Atmospheric System Research (ASR) Program Thank you for your attention

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