An Exploration of Model Concentration Differences Between CMAQ and CAMx

1. An Exploration of Model Concentration Differences Between CMAQ and CAMx Brian Timin, Karen Wesson, Pat Dolwick, Norm Possiel, Sharon Phillips EPA/OAQPS Timin.brian@epa.gov October 3, 2007

2. Introduction • OAQPS conducted 2001 base case modeling with CMAQ and CAMx • Both models used the same “raw” emissions and meteorological data • Large differences were seen in predicted ozone concentrations as well as other precursors and species • We conducted several analyses to help examine differences in the models • Sensitivity model runs with CAMx and CMAQ • Analysis of existing information

3. 2001 Model Platforms • Both models were run with a very similar setup

4. CMAQ vs. CAMx- Ozone Concentration July 17, 2001 at 21Z CAMx (w/O’Brien) CMAQ

5. CMAQ vs. CAMx- CO Concentration July 17, 2001 at 21Z CAMx (w/O’Brien) CMAQ

6. CMAQ vs. CAMx- FORM July 17, 2001 24-hour average CAMx (w/O’Brien) CMAQ

7. CMAQ vs. CAMx- Sulfate Concentration July 19, 2001 24-hour average CAMx (w/O’Brien) CMAQ

8. Analyses • Chemical mechanism • Photolysis rates • Cloud attenuation of radiation • Vertical mixing • Dry deposition

9. Analysis of Chemistry and Clouds • CAMx (mechanism 4) uses a hybrid version of CB-IV which contains additional reactions (CB-IV+) compared to CMAQ CB-IV • Photolysis rates are generally higher in CAMx and with CB05 compared to CMAQ CB-IV • Cloud attenuation of radiation differs between the models • These differences between the models were judged not likely to cause significant regional ozone differences between the models

10. Vertical Mixing • Vertical mixing is governed by vertical diffusion coefficients (Kv) • CMAQ v4.5 used “ACM” mixing • CAMx used “O’Brien” Kv’s • There is an option in MM5CAMX to generate “CMAQ like” Kv’s • Comparison of actual CMAQ Kv’s and “CMAQ like” Kv’s confirmed similar magnitudes and spatial patterns • We conducted a CAMx sensitivity run which used “CMAQ like” Kv’s and compared the results to O’Brien • CMAQ like Kv’s (and actual CMAQ Kv’s) are generally much higher than O’Brien Kv’s • Expect higher ozone with CMAQ like Kv’s in NOx limited areas

11. CAMx Ozone Change-“CMAQ-like” Vs. O’Brien KV’s Change in CAMx hourly ozone at 15Z on July 17, 2001 Change in CAMx hourly ozone at 20Z on July 17, 2001 Blue= lower ozone with “CMAQ-like” Kv’s

12. CAMx KV’s and Ozone- Atlanta Example CMAQ-like Kv’s are (almost) always higher than O’Brien and tend to drop off at a higher layer Ozone concentrations in CAMx and CMAQ are similar at 15z, but CAMx becomes much higher at 20z

13. Maximum Daytime PBL Comparison • We compared maximum PBL heights in Atlanta from observations, predictions from MM5 (MCIP), and from CMAQ and CAMx • CMAQ tends to mix to a higher layer compared to the PBL heights from MCIP • This example for Atlanta is not representative of all days and areas Note: CAMx and CMAQ mix up to the top of discrete model layers (as defined in the table above)

14. Dry deposition • CAMx uses a Wesely based dry deposition scheme • CMAQ uses the M3Dry scheme • Closely tied to the Pleim-Xiu land surface model • Accounts for enhanced deposition to wetted surfaces (soluble species) • Contains more recent science • RADM dry deposition scheme (similar to Wesely) is optional in CMAQ (MCIP 3.2 and prior) • Examination of dry deposition velocities (Vd) revealed large differences between models

15. Dry Deposition VelocitiesCAMx vs. CMAQ- Ozone CMAQ (M3Dry) ozone Vd at 16Z on July 17, 2001 CAMx ozone Vd at 16Z on July 17, 2001

16. Dry Deposition VelocitiesCAMx vs. CMAQ- CO CMAQ (M3Dry) CO Vd at 16Z on July 17, 2001 CAMx CO Vd at 16Z on July 17, 2001

17. Dry Deposition VelocitiesCAMx vs. CMAQ- NO CAMx NO Vd at 16Z on July 17, 2001 CMAQ (M3Dry) NO Vd at 16Z on July 17, 2001

18. Dry Deposition VelocitiesCAMx vs. CMAQ- NO2 CAMx NO2 Vd at 16Z on July 17, 2001 CMAQ (M3Dry) NO2 Vd at 16Z on July 17, 2001

19. Dry Deposition VelocitiesCAMx vs. CMAQ- FORM CAMx FORM Vd at 16Z on July 17, 2001 CMAQ (M3Dry) FORM Vd at 16Z on July 17, 2001

20. Dry Deposition Sensitivities • Two CMAQ sensitivity runs were conducted to examine dry deposition issues • Alternative mesophyll resistance values with M3Dry • Alternative dry deposition scheme (RADM) • The platform for these CMAQ runs was CMAQ v4.6 with CB05 chemistry • Ran CMAQ for 2 weeks in August 2002 (plus 7 day ramp-up)

21. CMAQ Dry Deposition Sensitivity No. 1 Mesophyll Resistance • M3Dry Vd values for CO, NO, and NO2 were found to be too high • Added a mesophyll resistance value in MCIP* for: • NO = 9400 S/M • NO2= 500 S/M • CO = 100,000 S/M • Ran MCIP and CMAQ with the new values • August 2002 period *The mesophyll resistance values for NO, NO2, and CO were later incorporated into MCIP 3.3

22. CMAQ CO Vd- M3Dry vs. M3Dry w/modified Mesophyll Resistance M3Dry CO Vd at 16Z on August 10, 2002 (w/mesophyll resistance) M3Dry CO Vd at 16Z on August 10, 2002

23. CMAQ CO Concentration and Change in Concentration Due to Mesophyll Resistance Change in CMAQ CO concentration on August 5, 2002 (w/mesophyll resistance) (24 hour avg.) CMAQ CO concentration on August 5, 2002 (24 hour avg.)

24. CMAQ Ozone Concentration and Change in Concentration Due to Mesophyll Resistance Change in CMAQ ozone concentration on August 5, 2002 (w/mesophyll resistance) (8 hour max.) CMAQ ozone concentration on August 5, 2002 (8 hour max.)

25. CMAQ Dry Deposition Sensitivity No. 2 RADM Dry • The RADM dry deposition routine is an option in MCIP* • Formulation is based on Wesely, 1989 • Very similar to CAMx • Ran MCIP and CMAQ with RADM dry • August 2002 period *RADM Dry was removed from MCIP v3.3

26. CMAQ Ozone Vd- M3Dry vs. RADM M3Dry ozone Vd at 16Z on August 10, 2002 RADM Dry ozone Vd at 16Z on August 10, 2002

27. CMAQ FORM Vd- M3Dry vs. RADM M3Dry FORM Vd at 16Z on August 10, 2002 RADM Dry FORM Vd at 16Z on August 10, 2002

28. CMAQ Ozone Concentration and Change in Concentration Due to RADM Dry Deposition CMAQ ozone concentration on August 5, 2002 w/M3Dry (8 hour max.) Change in CMAQ ozone concentration on August 5, 2002 (w/RADM Dry) (8 hour max.)

29. CMAQ Sulfate Concentration and Change in Concentration Due to RADM Dry Deposition Change in CMAQ sulfate concentration on August 5, 2002 (w/RADM Dry) (24 hour avg.) CMAQ sulfate concentration on August 5, 2002 w/M3Dry (24 hour avg.)

30. Conclusions • We examined numerous differences between CMAQ and CAMx • The majority of the ozone differences can be attributed to different implementations of vertical diffusion and dry deposition • Numerous other smaller differences were also identified • CO concentrations were too low in CMAQ due to high CO Vd (corrected by adding a mesophyll resistance value) • Other species (including secondary aerosols) are also affected by mixing and dry deposition

31. Recommendations • Further testing of vertical mixing is needed in both models • Need more comparisons between observed PBL and CMAQ/CAMx mixing • Does CMAQ “overmix” compared to MM5 predicted PBL? • Does O’Brien have too little mixing? • More vertical layers may be needed in the AQM boundary layer • Further examination of dry deposition velocities is needed • Evaluate diurnal pattern of Vd • Are afternoon Vd values too high in CMAQ? • Does the Wesely scheme need to be replaced?

32. Recommendations • Various combinations of chemical mechanisms (CB-IV, CB05, SAPRC), vertical diffusion (O’Brien, ACM, ACM2) and dry deposition (M3Dry, Wesely, AERMOD) can give very different results • Each process needs to be individually evaluated • Operational ozone evaluation should not be used to determine the “best” model formulation