1. California Air Resources Board1 Integration of Modeling Results - the problem of double counting and possible solutions Shuming Du August 27, 2003

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3. 3 Overview Possible solutions Details of a new box model Recommendations

4. California Air Resources Board4 Problem of ‘double-counting’ Some emission sources are included in both regional and microscale modeling These emissions are counted twice when integrating regional and microscale modeling results

5. California Air Resources Board5 Possible solutions (1) Exclude duplicate emissions from regional modeling Not recommended because: * Fundamentally incorrect because chemistry mechanism requires total (or actual) concentrations *Computationally not feasible for multiple (neighborhood scale) applications

6. California Air Resources Board6 Other possible solutions Microscale modeling (usually) does not consider chemical reactions Running regional model in inert mode to calculate the impact of the double-counted sources in a regional model, then deduct that impact when calculating total concentrations

7. California Air Resources Board7 Different approaches for different applications Statewide application for risk maps –require running regional model more one time Neighborhood scale applications –Several methods are being evaluated, one of them will be discussed today

8. California Air Resources Board8 Statewide risk map - solution (2) For applications creating statewide risk maps, need to run regional model twice: –once with all emission sources and running the model in reactive mode: C reactive, –and the second time with only the emission sources that are included in the microscale modeling and running the model in inert mode: C inert

9. California Air Resources Board9 Neighborhood scale applications In principle, solution (2) could be used in neighborhood scale applications (e.g., Barrio Logan and Wilmington studies) Requires running regional model in inert mode for each and every neighborhood that needs to do cumulative impact assessment

10. California Air Resources Board10 Possible solution (3) Solution (2) is computationally demanding although much less than solution (1) Alternate solution: approximate calculation to replace regional scale modeling: develop a (new) simple box model

11. California Air Resources Board11 CALGRID modeling results indicate that emission sources in (individual) neighboring cells have minor contributions This suggests that it is possible to use the simple box model to replace CALGRID for the purpose of calculating concentrations caused by ‘local emissions’ (i.e., neglecting contributions from neighbors) Rationale for the box model

12. California Air Resources Board12 Wind Consider a grid cell (of regional model) as a box to establish mass balance of pollutants –Turbulent diffusion is neglected Emission |U|C x Area |V|C x Area W  C x Area

13. California Air Resources Board13 A New Box Model

14. California Air Resources Board14 Summary We have two recommendations to address the ‘double count’ problem: –Statewide applications: run regional scale model twice –Neighborhood scale applications  The new box model  Improvement is in progress

15. California Air Resources Board15 How does the box model perform? Sensitivity test: run CALGRID at inert mode 9 grids each with unit emission rate of different pollutant, these grids cover Wilmington area where we know double counting is a problem Concentrations are calculated at each and every grid (87 x 67)

16. California Air Resources Board16 CALGRID test case Red area: unit emission rate for pollutants A1, A2 … Blue area: zero emissions

17. California Air Resources Board17 Comparison between CALGRID and the BOX model

18. California Air Resources Board18 Comparison between CALGRID and the BOX model

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20. California Air Resources Board20 Box model overestimates? Yes. Why? Turbulent diffusion is neglected, therefore when wind speed is very low, advection will not dominate. Remedy? Impose a minimum wind speed (=0.25 m/s) -when wind speed is below this threshold, use it in the box model

21. California Air Resources Board21 Minimum wind speed = 0.25 m/s

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