1. Effects Of Using The CB05 Versus the CB4 Chemical Mechanism On Model Predictions
Deborah Luecken and Golam Sarwar
U.S. EPA, ORD/NERL
CMAS annual conference
October 16, 2006

2. Objective of this work Why we’re switching from CB4 to CB05
How pollutant concentrations might change by using CB05 vs. CB4
Ozone
PM2.5
Where the largest differences tend to occur
When they might occur
Why they change

3. Why we’re changing from CB4 to CB05…
What are CB4 and CB05?
A way to represent atmospheric gas-phase chemistry in AQMs
Carbon Bond mechanisms condense the chemistry using a lumped-structure approach
One of two ways in current CMAQ (other is lumped molecule approach in SAPRC99)
Why we’re changing from CB4 to CB05…
CB4 was developed in the late 1980’s and has not had a major update since then
Computing resources have improved
Modeling domains have changed

4. Major chemical differences between the mechanisms
Better description of cycling of NOy species
Better representation of secondary products and their reactions in lower-NOx environments
Differentiation of C3+ aldehydes from acetaldehyde
Better representation of internal alkenes
Updated and more explicit cross section and quantum yields for photolysis reactions
Updated reaction rates
Complete details at

5. Modeling scenarios 36-km (continental U.S.) domain with CMAQ v4.5.1
10-day spinup, 14 vertical layers
12-km (Eastern US) domain with boundary conditions from 36-km
June and July, 2001 meteorology from MM5
Emissions from 2001 NEI
CB4, CB05 and SAPRC99 ae4 aq
This work is part of a much larger study looking at sensitivities, evaluation, comprehensive comparison with SAPRC99

6. Results - Ozone
July
January

7. July Differences in monthly avg of daily 8-hr max
36 km grid
12 km grid
Largest avg differences are centered around OK, including MI, AR, NM, north TX, although the areas of change move around from day to day, and only sometimes occur where ozone concentrations are large.

8. Monthly avg
Organic nitrate is lower where ozone differences are the largest, C3+ aldehydes contribute to higher ozone differences in the central-Southeast

9. A B C D
Differences by subregion
Mean ozone differences are about twice as high in Region A as in Region D

10. Example of avg diurnal variation of ozone and differences
Region A : MO/OK/KS
Region C : OH valley
Ozone differences are smallest in mid-morning and largest in early afternoon

11. January Differences in monthly avg of daily 8-hr max
36 km grid
12 km grid
Scale is 10x lower than for July
Ozone differences are small in January

12. Results – PM2.5 and its components
July
January

13. July
Monthly avg
36 km grid
12 km grid
The largest PM2.5 changes are negative and in the east

14. Changes in sulfate account for most of the PM2
Changes in sulfate account for most of the PM2.5 differences in the east, changes in biogenic SOA influence differences in the Northwest

15. January Monthly avg PM2.5 differences are small in January
January avg (CB05-CB4)
Monthly avg
January avg (CB05-CB4)
PM2.5 differences are small in January

16. Summary - ozone
Largest differences in ozone occur in the summer months, when CB05 predicts more ozone than CB4
Ozone differences average about 4 ppb in July, varying from 2-6 ppb by region
Ozone differences average about 0.4 ppb in January
Large ozone differences in July are centered around Oklahoma

17. Summary –PM2.5
Largest differences in PM2.5 occur in the summer months, when CB05 predicts less PM2.5 than CB4
PM2.5 differences average about to mg/m3, and vary from near 0 to depending on region
PM2.5 differences are largest in the eastern US and are influenced mainly by sulfate differences
PM2.5 differences in the western US are influenced by differences in predictions of SOA.

18. Acknowledgements CSC: CB4-O5 evaluation workgroup: Nancy Hwang
Golam Sarwar
Norm Possiel
Alice Gilliland
Wyat Appel
Sharon Phillips
Carey Jang
Rob Pinder
DISCLAIMER: The research presented here was performed under the Memorandum of Understanding between the U.S. Environmental Protection Agency (EPA) and the U.S. Department of Commerce's National Oceanic and Atmospheric Administration (NOAA) and under agreement number DW Although it has been reviewed by EPA and NOAA and approved for publication, it does not necessarily reflect their policies or views