1. Model & Chemistry Intercomparison CMAQ with CB4, CB4-2002, SAPRC99 Ralph Morris, Steven Lau, Bongyoung Koo ENVIRON International Corporation Gail Tonnesen, Chao-Jung Chien, Zion Wang, Bo Wang, Tiegang Cao University of California at Riverside T.W. Tesche, Dennis McNally and Greg Stella Alpine Geophysics RPO Meeting, Denver, CO, May 25, 2004

2. Chemistry Versions CB4 developed in 1989, has patches but no chemical kinetics updates. CB4-2002 (Jeffries et al.) major changes include updated HNO3, PAN, alkenes, and photo-rates. SAPRC99 includes more detailed organic chemistry, explicit organic peroxy radicals. Provides better representation of peroxides for low NOx and more complete organic intermediates. CB5 – not yet evaluated.

3. N2O5 Hydrolysis Potentially important path for converting NOx to inert HNO3: O3+NO  NO2 O3+NO2  NO3 NO2+NO3  N2O5 N2O5+H2O  2 HNO3 Potentially important sink for oxidants.

4. N2O5 Hydrolysis G1: Gas Phase: N2O5+H2O  2 HNO3 G2: Gas phase: N2O5 + H2O + H2O  2 HNO3 H1: Heterogeneous: N2O5 + H2O + M  2 HNO3, various parameterizations used for reaction probability (  ). Kinetics based limited, mostly data from Julich chamber experiments.

5. Chemistry N2O5 Hydrolysis Original CB4 has G1, artificially increased. CMAQ version CB4 does not include G1. CB4-2002 has G1 and G2. SAPRC99 has G1. CMAQ includes H1, with 0.01 <  < 0.1

6. Organic Aerosol Formation Primary anthropogenic organic aerosols. Secondary Organic aerosols (SOA) –Includes anthropogenics and biogenics. CMAQ has 10 SOA precursors SAPRC99 includes paraffin SOA, CB4 does not.

7. Mechanism intercomparison for Jan02 episode

9. Summary CB4 & CB4-2002 are similar. Both CB4 and SAPRC under predicted winter O3 –SAPRC had larger negative bias for O3 Winter NO3 performance: –CB4 had positive bias –SAPRC had large negative bias. SAPRC99 had lower SOA than CB4: –SAPRC99 better in winter, negative bias summer. CB4 performance appears to be better than SAPRC. More diagnostic evaluation is needed.

10. Recommendations SAPRC99 computational cost is 2x CB4. Compromise between chemistry and grid size. Use 36-12 km grid with CB4 chemistry Need to compare model response to emissions reductions.