1. Changes to the Multi-Pollutant version in the CMAQ 4.7
William T. Hutzell, Shawn J. Roselle, Annmarie G. Carlton, and O. Russell Bullock
Atmospheric Modeling Division
US EPA
November 28, 2018

2. Background: Multi-Pollutant Model
Developed to simulate criteria and hazardous air pollutants in a single modeling study
examine co-benefits of emission reductions over multiple interest
O3, Particulate Matter, and Hazardous Air Pollutants (HAPs)
Roselle et al. (2007) presented a prototype of the Multi-Pollutant Model
mechanism called CB05TXHG_AE4_AQ
Based on CB05 mechanisms for mercury and other HAPs in CMAQ version 4.6

3. Multi-Pollutant Model in version 4.7
Updates prototype in Roselle et al (2007)
photochemical production of Secondary Organic Aerosols (SOA)
aerosol physics for SOA and interactive coarse mode
cloud chemistry of organic compounds and SOA
In-calculations for dry deposition and emission processes
New mechanism called CB05TXHG_AE5_AQ
Specific settings needed to build and run the Multi-Pollutant model.
Consult release notes on building and running the model.

4. Goals of this Presentation
Describe major changes to new science options for Multi-Pollutant Model
Present differences in predictions from CMAQ using CB05CL_AE5_AQ, the standard version
Highlight differences in predictions from the prototype of the Multi-Pollutant Model

5. Gas Chemistry
Adds to CB05CL_AE5_AQ the species and reactions for Hg compounds and other HAPs from CB05HG_AE4_AQ and CB05CLTX_AE4_AQ
Two methods compute the chemical transformation of gas phase Hg and other HAPs
One participates in ozone and radical photochemistry
Other does not affect ozone and radical concentrations and serves as reactive tracers.
Changes from CB05TXHG_AE4_AQ
adds a new reaction, HG + CL → HGIIGAS
This reaction should produce Hg(I)
HGIIGAS redefined as reactive gaseous mercury
i.e., Hg(I) + Hg(II).
Hg(I) assumed to quickly convert into Hg(II)
the oxidation process not explicitly represented

6. Aerosols
Adds aerosol species representing mercury and other toxic metals
They do not affect aerosol microphysics and deposition rates
These aerosols species do coagulate and mode merge
Mercury species differ from the other metallic aerosols
photochemical source of particulate mercury
goes directly into the accumulation mode
assumes accumulation mode dominates condensation onto the three aerosol modes
unlike version 4.6 where fine modes divided condensation based on their surface area

7. In-Cloud Chemistry
Adds in-scavenging for mercury and other metallic aerosols.
Adds cloud chemistry for atmospheric mercury
Based on Bullock and Brehme (2002)
Indirectly affects other aqueous species
modifies the minimum time step used in the numerical solution when the mercury species have the fastest rate of change.
mercury chemistry requires using the gas phase HO2, HOCl and Cl2 based on Lin et al. (1998).
affects pH and ion balance in cloud droplets

8. In-Cloud Chemistry (cont.)
Possible effects from changes
change particulate sulfate
wet deposition of HO2, HOCl and Cl2
can produce HOCl from clouds with low or no participation.

9. Inline Vertical Diffusion
Aerosol emissions includes routines for particulate mercury and other metals
Emissions includes source of Cl2 over open oceans.
Set off by default
Set on with the environment variable, CTM_CL2_SEAEMIS
Source mimics implied heterogeneous production for sea salt aerosols (Spicer et al. 1998)
Knipping and Dadbub (2002 and 2003) proposed a mechanism but not used
reaction efficiencies are not well defined
CB05CL_AE5_AQ does not include ClNO2 and ClONO2.

10. Comparison to Standard CMAQ, i.e.,CB05CL_AE5_AQ
Using two weekly periods in January and July 2002
Domain covered Continental US
grid cell had 36X36 km2 horizontal dimensions
14 vertical layers in s pressure coordinates
Comparison used the weekly averages
tile plots of differences from CB05CL_AE5_AQ, i.e., standard model
scatter plots showing difference versus standard’s prediction

11. HOCl
Higher HOCl production appears higher in January

12. Ozone
Ozone higher in winter and correlate with HOCL but generally within the accuracy of the chemistry solver.

13. Impact of Ozone Difference: Benzene
January’s ozone differences alter OH thereby Benzene

14. Particulate Sulfate
January’s ozone differences also have small effect on sulfate production

15. Hg Changes between CB05TXHG version 4.6 and 4.7

16. Areas for further improvement
Gas Phase Chemistry for Mercury
explicitly representing products
Oceanic Cl2 source
representing heterogeneous production
possible with the proposed SAPRC07 mechanism
Bi-Directional Surface Flux for Mercury
addressed science model release in version 4.7
see release notes by Jesse Bash
Cloud chemistry does not include Cr(III) and Cr(VI) redox reactions
Disclaimer: Although this work has been reviewed by EPA and approved for publication, it does not necessarily reflect their policies or views.