Changes to the Chemical Mechanisms for Hazardous Air Pollutants in CMAQ version 4.6

1. W. T. Hutzell1, G. Pouliot2, and D. J. Luecken1 1Atmospheric Modeling Division, U. S. Environmental Protection Agency2Atmospheric Sciences Modeling Division, Air Resources Laboratory, National Oceanic and Atmospheric October 16, 2006 Changes to the Chemical Mechanisms for Hazardous Air Pollutants in CMAQ version 4.6

2. Background • CMAQ version 4.5 has two different mechanisms for Hazardous Air Pollutants (HAPs). • CB4TX1P • SAPRC99TX2P • Each includes a specific type of HAPs. • identified to produce large health risks over urban areas • treated because they can be simulated as gas phase compounds. • Each omits toxic components in Particulate Matter (PM) • Also linked to large risks to health over urban areas.

3. Background (cont.) • Version 4.6 introduces two new mechanisms for several HAPs. • Each includes HAPs in CMAQ version 4.5. • They also include more gas phase HAPs, • As well as aerosol phase HAPs that represent several toxic metals and diesel emissions. • One mechanism, called SAPRC99TX3, builds upon SAPRC99TX2P. • The other, called CB05CLTX, adapts the Carbon Bond 05 (CB05) mechanism.

4. New HAPs in CMAQ version 4.6 Gas Phase HAPs

5. Aerosol Phase HAPs

6. General Treatment in Chemical Transport Model • All HAPs undergo advection and diffusion. • Wet deposition is determined by the HAP’s phase • modal scavenging rate if aerosol phase • the Henry's Law Constant if gas phase • Dry deposition also depends on the HAP’s phase • modal deposition velocity if aerosol phase • nonzero deposition velocity if gas phase • when screening implies dry deposition as important

7. Gas Phase HAP Chemistry • Two methods calculate photochemical destruction and production. • First method adds species and reactions to O3 and radical chemistry. • HCHO, CH3CHO, 1,3-butadiene, acrolein and several emission tracers • reactions can affect numerical solution for O3 and radicals • Second method estimates destruction based on O3 and radical solution. • calculation does not affect O3 and radicals • Why use this approach? • emissions are already accounted in lumped species • low reactivity or emissions imply small effect on O3 and radicals

8. Aerosol Phase HAP Processes • They track emissions of toxic components within PM. • Microphysical processing is similar to elemental carbon and unidentified coarse mode PM. • do not affect microphysical processes and deposition due to their tracking function. • They also do not affect cloud chemistry. • Last point is not valid for two HAPs. • “Cr(VI) + 3e- Cr(III)” can occur via chemistry within cloud droplets. • Shortcoming is an avenue for model developers within CMAS.

9. Simulation Details • Domain covered the continental US and spanned from surface to 100 mb. • Horizontal grid size was 36X36 km2 • Emissions came form the combined 1999 NEI and Air Toxics database. • Meteorology represented January and July 2001 based on MM5 simulations.

10. Gas Phase Destruction Differences: Summary,CB05CLTX – SAPRC99TX3 January 2001 ug/m^3, Beenzene SAPRC99TX3 ug/m^3, 1,3-Butadiene from SAPRC99TX3 July 2001 ug/m^3, Beenzene SAPRC99TX3 ug/m^3, 1,3-Butadiene from SAPRC99TX3

11. Gas Phase Destruction Differences: Examples,CB05CLTX – SAPRC99TX3 1,3-Butadiene Benzene

12. OH, 24 hour Averages: CB05CLTX – SAPRC99TX3 July 2001 January 2001 ppmV, Difference OH ppmV, Difference OH ppmV, OH from SAPRC99TX3 ppmV, OH from SAPRC99TX3

13. ug/m^3, Difference Photochemical CH3CHO ug/m^3, Difference Photochemical HCHO ug/m^3, Secondary HCHO from CB05CLTX ug/m^3, Secondary CH3CHO from CB05CLTX ug/m^3, Difference Photochemical HCHO ug/m^3, Difference Photochemical CH3CHO ug/m^3, Secondary HCHO from CB05CLTX ug/m^3, Secondary CH3CHO from CB05CLTX Gas Phase Production Differences: Summary, CB05CLTX – SAPRC99TX3 January 2001 July 2001

14. Gas Phase Production Differences: Examples,CB05CLTX – SAPRC99TX3 Formaldehyde Acetaldehyde

15. Aerosol Phase Differences:Summary,CB05CLTX – SAPRC99TX3 ng/m^3, Difference Cr(III) Accumulation Mode January 2001 ug/m^3, Difference Diesel Tracer Coarse Mode ng/m^3, Cr(III) Accumulation Mode from SAPRC99TX3 ug/m^3, Diesel Tracer Coarse Mode from SAPRC99TX3 July 2001 ug/m^3, Difference Diesel Tracer Coarse Mode ng/m^3, Difference Cr(III) Accumulation Mode ng/m^3, Cr(III) Accumulation Mode from SAPRC99TX3 ug/m^3, Diesel Tracer Coarse Mode from SAPRC99TX3

16. Aerosol Differences: Examples, CB05CLTX – SAPRC99TX3 Cr(III), Accumulation Mode Diesel PM, Coarse Mode

17. What does CL in CB05CLTX mean? • CB05CLTX contains additional species and reactions not in SAPRC99TX3. • Six species track emissions from anthropogenic and biogenic sources (see version 4.6 release notes). • Other species simulate the fate and transport of Cl2 and HCl. • Additional reactions represent how chlorine compounds affect ozone. • Reactions are based on Yarwood et al. (2005). • We added the reaction, “HCl + OH  Cl.” • NOTE, current mechanism settings omit Cl2 and HCl emissions.

18. Closing Points • New HAP mechanisms have not been evaluated against observations. • Each gives results consistent to the mechanisms from that they were derived, i.e., CB05 or SAPRC99. • The new mechanisms then allow simultaneously studying criteria and numerous toxic pollutants. Disclaimer: The research presented here was performed under the Memorandum of Understanding between the U.S. Environmental Protection Agency (EPA) and U.S. Department of Commerce's National Oceanic and Atmospheric Administration (NOAA) and under agreement number DW13921548. This work constitutes a contribution to the NOAA Air Quality Program. Although it has been reviewed by EPA and NOAA and approved for publication, it does not necessarily reflect their policies or views.