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

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

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

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 http://www.camx.com/publ/pdfs/CB05_Final_Report_120805.pdf

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

Results - Ozone July January

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.

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

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

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

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

Results – PM2.5 and its components July January

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

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

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

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

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 -0.23 to -0.48 mg/m3, and vary from near 0 to -0.75 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.

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 DW13921548. Although it has been reviewed by EPA and NOAA and approved for publication, it does not necessarily reflect their policies or views