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Introduction to CAMx

Introduction to CAMx. Presented by Chris Emery ENVIRON International Corporation June 2003. Topics. Conceptual Overview Ozone Chemistry Modeling Atmospheric Dispersion Description of CAMx Model Design Comparison to Other Models Application of CAMx Model Setup

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Introduction to CAMx

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  1. Introduction to CAMx Presented by Chris Emery ENVIRON International Corporation June 2003

  2. Topics • Conceptual Overview • Ozone Chemistry • Modeling Atmospheric Dispersion • Description of CAMx • Model Design • Comparison to Other Models • Application of CAMx • Model Setup • Replication of Historical Case/Performance Evaluation • Evaluation of Future Case/Control Strategies

  3. Conceptual OverviewIntroduction to Ozone Chemistry • Sources and sinks for tropospheric ozone • Ozone formation from VOC and NOx • Control strategy implications • Sensitivity to VOC and/or NOx • VOC reactivity • NOx suppression • Condensed chemical mechanisms

  4. Conceptual OverviewIntroduction to Ozone Chemistry • Sources • Smog chemistry involving VOCs and NOx • Global methane and CO oxidation • Stratosphere • Sinks • Chemical reactions (e.g., NO, alkenes) • Deposition (dry and wet)

  5. Conceptual OverviewOzone Formation from VOC and NOx no sunlight  no ozone productionno NOx no ozone productionno VOC  no ozone production

  6. Conceptual OverviewCentral Role of Hydroxyl Radical (•OH)

  7. Conceptual OverviewOzone and Precursor Relationships: EKMA Diagram

  8. Conceptual OverviewOzone and Precursor Relationships: Ozone vs. NOz • NOz = NOy - NOx • Observed ozone/NOz relationship for a rural location in eastern U.S. • Interpret slope as production efficiency for ozone from NOx NOz ppb

  9. Conceptual OverviewControl Strategy Implications • Sensitivity to emission reductions • VOC sensitive, NOx sensitive, or both • VOC reactivity • Depends upon chemical nature: reaction rate, radical yields, products • Approximated by reactivity scales, e.g., Carter MIRS

  10. Conceptual OverviewControl Strategy Implications • NOx Suppression • If Ozone is highly VOC sensitive, NOx reduction may incur disbenefits by two effects: (1) direct titration (or scavenging) of ozone by fresh NO emissions (2) scavenging of radicals by NO2 (inhibits ozone production)

  11. Conceptual OverviewCondensed Chemical Mechanisms • Hundreds of VOCs and thousands of reactions in the atmosphere • Need to condense this chemistry for modeling • Condensation strategies: • Focus on the most important reaction pathways • Lump together VOCs with similar reactions

  12. Conceptual OverviewVOC Lumping Strategies: Lumped Structure (CB4) CH3 - CH2 - CH3 CH3-CH = CH2 CH3 - CH2 -CH2 - CH3 CH3 - CH2- CH = CH2 PAR OLE

  13. Conceptual OverviewVOC Lumping Strategies: Lumped Molecule (SAPRC) CH3 - CH2 - CH3 CH3 -CH = CH2 CH3 - CH2 -CH2 - CH3 CH3 - CH2 - CH = CH2 ALK1 OLE1

  14. Conceptual OverviewLumped Mechanisms for Modeling

  15. Conceptual OverviewIntroduction to Atmospheric Dispersion • All models solve some form of the Continuity Equation • Relates changes in pollutant concentration to: • “Dispersion” • Advection (transport by mean/resolved wind) • Turbulent diffusion (transport by unresolved motion) • Chemical reactions • Deposition • Emissions

  16. Conceptual OverviewGeneral Form of the Continuity Equation(Eulerian Form) Mathematical solution (integration) of the general form is difficult • Simplifying assumptions are required

  17. Conceptual OverviewThree General Types of Air Quality Models • Lagrangian types -- Coordinate system follows air parcels • Eulerian types – Coordinate system is fixed in space • Hybrid types – Incorporate features of Lagrangian types into an Eulerian framework

  18. Conceptual OverviewLagrangian Models • Many simplifying assumptions • Air parcel coherency • Break down quickly, especially in complex wind flows • Cost-effective solution at relatively close range for a relatively small number of sources • Readily produce source-receptor relationships • Severe technical limitations for: • Large numbers of sources • Regional-scale transport applications • Non-linearly reactive pollutants

  19. Conceptual OverviewLagrangian Models • Gaussian Plume Models • The earliest air models • Many simplifying assumptions provide closed-form analytical solutions • Steady-state (i.e., time invariant) • Spatially uniform (homogeneous) dispersion • Inert or first-order decay • Examples include: • ISC • COMPLEX • RTDM • AERMOD

  20. Conceptual OverviewGaussian Plume Model

  21. Conceptual OverviewLagrangian Models • Gaussian Puff Models • Fewer simplifying assumptions • Retain plume coherency assumption • Employ analytical solutions for each puff, but: • Must track large numbers of puffs • Examples include: • CALPUFF • SCIPUFF • Some models developed for individual reactive plumes; e.g., RPM • Numerical solution methods needed for chemistry • Chemical interactions between puffs, segments, or particles cannot be fully treated

  22. Conceptual OverviewEulerian Models • Generally considered to be technically superior • Allow more comprehensive, explicit treatment of • Physical processes • Chemical processes • Interactions of numerous sources • Require sophisticated solution methods • Employ discrete time steps and operator splitting • Computational grid (“grid models”) • Relatively expensive to apply for long periods

  23. Conceptual OverviewEulerian (Grid) Model Concept

  24. Conceptual OverviewGrid Cell Processes

  25. Conceptual OverviewCoupling Between Grid Cells

  26. Conceptual OverviewEulerian (Grid) Model Results

  27. Conceptual OverviewEulerian Models • Subgrid resolution can be a limitation • As grid size and time step are reduced • Accuracy increases, but • Computational time increases • Advanced grid models can • Employ improved accuracy in critical locations • Allow cost effective application on urban to regional scales

  28. Conceptual OverviewEulerian Models • Before source apportionment techniques, many runs were required to: • Establish source-receptor relationships • Evaluate effective control strategies • Examples of early photochemical grid models include: • UAM-IV • RADM • CALGRID

  29. Conceptual OverviewHybrid Models • Incorporate features of Lagrangian models into grid model framework • Overcome many of the limitations of sub-grid processes • Provides practical advantages of Lagrangian models, by development of: • Plume-in-Grid (PiG) sub-models • Variable (nested) grid resolution • Source apportionment techniques • Capitalize on availability of low-cost high-speed computers

  30. Conceptual OverviewHybrid Models • Examples of hybrid photochemical grid models include: • CAMx • MODELS3/CMAQ • UAM-V

  31. Conceptual OverviewSummary • Photochemical hybrid models • Preferred means for addressing complex and nonlinear processes affecting reactive tropospheric air pollutants • Variable grid spacing (nested grids) • Lagrangian PiG sub-models to treat subgrid plume dispersion and chemistry • Source apportionment and process analysis tools • These models invoke fewer assumptions, but require: • More computer resources • Sophisticated numerical integration methods

  32. Description of CAMxComprehensive Air quality Model with extensions (CAMx), Version 4.00 • 3-D Eulerian/hybrid tropospheric photochemical transport model • Treats emissions, chemistry, dispersion, removal • Chemical species • Photochemical gasses (NOx, VOC, CO, O3) • Aerosols (sulfate, nitrate, organics, inert) • Applicable scales • From individual point sources (< 1 km) • To regional (>1000 km)

  33. Description of CAMx CAMx v4.00 • Unifies features required of “state-of-the-science” models • New coding of several industry-accepted algorithms • Computational and memory efficient • Easy to use • Modular framework permits easy substitution of revised and/or alternate algorithms • Publicly available (www.camx.com)

  34. Description of CAMx CAMx v4.00 • Technical features: • Grid nesting • Horizontal and vertical nesting • Supports multiple levels • Variable meshing factors • Plume-in-Grid (PiG) sub-model • Multiple, fast and accurate chemical mechanisms • Mass conservative and mass consistent transport scheme

  35. Description of CAMx CAMx v4.00 • Multiple map projections • Geodetic (latitude/longitude) • Universal Transverse Mercator (UTM) • Lambert Conformal Projection (LCP) • Rotated Polar Stereographic Projection (PSP) • Multiple probing tools • Ozone Source Apportionment Technology (OSAT) • Decoupled Direct Method (DDM) of sensitivity analysis • Process Analysis tools (IPR, IRR, CPA)

  36. Description of CAMxTechnical Approach • Overview • Solves continuity equation for each species • Time splitting operation • Each process solved individually each time step, each grid • Master time step: • Maintains stable solution of transport on master grid • Multiple nested grid steps per master step • Multiple chemistry steps per master step

  37. Description of CAMxTechnical Approach • Transport • Advection and diffusion solvers are mass conservative • 3-D advection is mass consistent • Linked via divergent atmospheric incompressible continuity equation • Order of east-west and north-south advection alternates each master step • Two options for horizontal advection solver: • Bott (1989): area-preserving flux-form solver • Colella and Woodward (1984): piecewise-parabolic method

  38. Description of CAMxTechnical Approach • Vertical transport solved with implicit scheme • Resolved vertical velocity • Mass exchange across variable vertical layer structure • Vertical diffusion solved with implicit scheme • Dry deposition rates used as surface boundary condition • Horizontal diffusion solved with explicit scheme • 2-D simultaneous

  39. Description of CAMxTechnical Approach • Pollutant removal • Dry deposition • First order removal rate based on deposition velocity (Weseley, 1989) • Dependent upon: season, land cover, solar flux, surface stability, surface wetness, gas solubility and diffusivity, aerosol size • Wet scavenging • First order removal rate based on scavenging coefficient • Gas rates depend upon solubility and diffusivity • Aerosol rates depend upon size • Separate in-cloud and below-cloud rates (Seinfeld and Pandis, 1998)

  40. Description of CAMxTechnical Approach • Photochemistry • CBM-IV (Gery et al., 1989) • 3 variations available • SAPRC99 (Carter, 2000) • Chemically up-to-date • Tested extensively against environmental chamber data • Uses a different approach for VOC lumping • All mechanisms are balanced for nitrogen conservation • Photolysis rates derived from TUV preprocessor • Can be affected by cloud optical depth

  41. Description of CAMxTechnical Approach • Gas-phase chemistry solvers • Most “expensive” component of simulation • CAMx CMC solver • Increases efficiency and flexibility • Adaptive hybrid approach: • Radicals (fastest reactions) – in steady state • Fast state species – second-order Runge-Kutta • Slow state species – solved explicitly • “Adaptive” = number of fast species can change according to chemical regime

  42. Description of CAMxTechnical Approach • Implicit-Explicit Hybrid solver (Sun et al, 1994) • Accuracy comparable to reference methods (LSODE) • IEH vs. CMC: • Accuracy similar during the day • IEH more accurate during the night • IEH several times slower

  43. Description of CAMxTechnical Approach • Aerosol chemistry • Gas-phase mechanism (CBM-IV Mech 3) with: • Additional biogenic olefin (terpenes) • Condensible organic gas species • Chlorine and HCl chemistry • Homogeneous SO2 to sulfate • Photochemical production of nitric acid • Aerosol mechanism calculates: • Aqueous SO2 to sulfate (CMAQ/RADM-AQ) • Condensible organic gasses to organic aerosols (SOAP) • NO3/SO4/NH3/Na/Cl equilibrium (ISORROPIA) • Size spectrum is static, user-defined

  44. Description of CAMxTechnical Approach • Aerosol species treated: • Sulfate • Nitrate • Ammonium • Sodium • Chloride • Secondary Organics • Primary Organics • Elemental Carbon • Primary Fine (+dust) • Primary Coarse (+dust)

  45. Description of CAMxTechnical Approach • Plume-in-Grid (PiG) • Resolves chemistry/dispersion of large NOx plumes • Tracks plume segments (puffs) in Lagrangian frame • Each puff moved independently by local winds • Puff growth (diffusion) determined by local diffusion coefficients • GREASD PiG: fast, conceptually simple • Reduced NOx chemistry set (NO-NO, NOx/ozone equilibrium, HNO3 production) • Puffs leak mass according to growth rates and grid cell size • Puffs terminated by age or sufficiently dilute NOx

  46. Description of CAMxTechnical Approach • Probing Tools • Utilize CAMx routines for all dispersion and chemistry • Ozone Source Apportionment Technology (OSAT) • Determines source area/category contribution to ozone anywhere in the domain • Uses tracers to track NOx and VOC precursor emissions, ozone production/destruction, and initial/boundary conditions • Estimates ozone production as NOx- or VOC formed

  47. Description of CAMxTechnical Approach • HOWEVER: cannot quantify ozone response to NOx or VOC controls • Chemical allocation methodologies: • OSAT: standard approach • APCA: attributes ozone production to anthropogenic (controllable) sources only • GOAT: tracks ozone based on where it formed, not where precursors were emitted

  48. Description of CAMxTechnical Approach • Decoupled Direct Method (DDM) for sensitivity analysis • Calculates pollutant concentration sensitivity to input parameters • First-order sensitivity to emissions, initial/boundary conditions • Allows estimates of effects of emission changes • Allows ranking of source region/categories by their importance to ozone formation • Slower than OSAT, but: • Provides information for all species (not just ozone) • More flexible in selecting which parameters to track

  49. Description of CAMxTechnical Approach • Process Analysis (PA) • Designed to provide in-depth analyses of all physical and chemical processes operating in model • Operates on user-defined species and any portion of the modeling grid • Three components: • Integrated Process Rate (IPR): provides detailed process rate information for each physical process (emissions, advection, diffusion, chemistry, deposition) • Integrated Reaction Rate (IRR): provides detailed reaction rate information for all chemical reactions • Chemical Process Analysis (CPA): like IRR, but designed to be more user-friendly and accessible

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