Rosenbrock Approach to the Treatment of Aqueous Chemistry in CMAQ

1. Rosenbrock Approach to the Treatment of Aqueous Chemistry in CMAQ Annmarie G. Carlton, Gerald Gipson, Shawn Roselle, Rohit Mathur

2. BACKGROUND • Clouds cover ~60% of the Earth’s surface • Associated convective mixing and aqueous phase processes provide a mechanism for venting atmospheric constituents from the polluted boundary layer to the free troposphere, with substantial implications for long-range pollution transport and climate

3. INTRODUCTION Evolving knowledge indicates atmospheric aqueous phase chemistry is more complex than typical model mechanisms Current aqueous mechanism designed to predict sulfate Current CMAQ aqueous chemistry module does not easily lend itself to expansion Forward Euler solver for oxidation and bisection method for pH (note linear convergence for bisection method) Stiffness induced by timescales of different orders of magnitude (e.g., ●OH reactions) ROS3 solver is a good candidate for solving atmospheric aqueous chemistry(Sandu et al., 1997; Djouad et al., 2002)

4. original simulation Multipollutant version of CMAQ Unrealistic sulfate production: -problem traced to aqueous chemistry solver technique. -Incorporated the fix into CMAQv4.7.1 Max=283.4 µg/m3 Simulation with update Max=24.3 µg/m3 Figures courtesy of P. Dolwick

5. partitioning Molar conc. = initial amt. – amt. deposited (mol L-1) CMAQ Aqueous Chemistry Map (aqchem.F) bisection for pH, initial guesses between 0.01 – 10 pH liquid conc. (mol L-1) SO4, HSO4, SO3, HSO3, CO3, HCO3, OH, NH4, HCO2, NO3, Cl partitioning Start iteration and bisection (3000 iterations) Calc. final gas phase partial pressure of SO2, NH3, HNO3, HCOOH, CO2 liquid conc. (mol L-1) SO4, HSO4, SO3, HSO3, CO3, HCO3, OH, NH4, HCO2, NO3, Cl pH Check for convergence Check for convergence Compute ionic strength and activity coefficient (Davies Eqn.) Calculate liquid concentrations and final gas phase concs. of oxdidants oxidation Cal. Min time step – check for large time step Kinetic calcs SIV oxidized < 0.05 of SIV oxidized since time 0, double DT Don’t let DT > TAUCLD deposition 100 max. iterations Compute wet depositions and phase concentrations for each species TIME = TAUCLD (OR 100 iterations)

6. More Processes Solved Simultaneously with ROS3 Forward Euler Method Rosenbrock Method Where: J is the Jacobian are constants

7. Enhance Calculation of Aqueous Chemistry in CMAQ 1. Comparison of ROS3 solver with a GEAR solver for atmospheric aqueous chemistry tested in box model used chemical mechanism described in Barth et al., 2003 2. Implemented ROS3 solver in CCTM with same aqueous chemical mechanism currently employed to understand solver-specific effects 3. Testing: - partitioning assumptions - expansion of the chemical mechanism

8. ROS3 GEAR 1.) Comparison with Gear Solver in Box Model Test

9. 2.) Implementing ROS3 for CMAQ aqueous mechanism Current CMAQ Aqueous Processes • Gas-to-droplet partitioning Current assumption, instantaneous thermodynamic equilibrium according to Henry’s Law • Oxidation Chemistry 5 sulfur “family” reactions: S(IV)  S(VI) viaO3, H2O2, O2, MHP, PAA 2 organic reactions: GLY, MGLY + ●OH • Wet Depostion

10. 2.) Implementing ROS3 for CMAQ aqueous mechanism

11. Accumulation mode SO4 comparisons Forward Euler Method ROS3 Method μg m-3 surface layer < ~ 34 meters

12. Differences in accumulation mode SO4 Forward Euler Method SO4 – ROS3 Method SO4 μg m-3 surface layer < ~ 34 meters aloft layer typical of cloud base

13. 3.) Enhancing CMAQ Aqueous Processes: More Explicit Chemistry HOx chemistry Glyoxal oxidation chemistry 1) H2O2 + hv  2OH 2) OH+ H2O2 HO2 + H2O 3) HO2 + H2O2 OH + H2O + O2 4) HO2 + HO2 H2O2 + O2 5) OH+ HO2 H2O + O2 6) OH + O2 -  OH- + O2 7) HCO3- + OH  CO3- + H2O 8) CO3- + O2-  CO32- + O2 9) CO3- + HCO2- HCO3- + CO2- 10) CO3- + H2O2 HCO3- + HO2 11) CO2 (+H2O) ↔ H+ + HCO3- 12) HCO3-↔ H+ + CO32- 13) GLY + OH(+O2)  GLYAC + HO2 14) GLYAC + OH  OXLAC + HO2 + H2O 15) GLYAC- + OH  OXLAC- + HO2 + H2O 16) OXLAC + OH  2CO2 + 2H2O 17) OXLAC- + OH  CO2 + CO2 - + 2H2O 18) OXLAC2- + OH  CO2+CO2 - + OH- 19) GLYAC ↔ H+ + GLYAC- 20) OXLAC ↔ H+ + OXLAC 21) OXLAC- ↔ H+ + OXLAC2- 22) GLYAC + H2O2 HCO2H + CO2 + H2O 23) HCO2H + OH  CO2 + HO2 + H2O 24) HCO2- + OH  CO2- + H2O 25) HCO2H ↔ H+ + HCO2- GLY + OH  ORGC Reactions are taken from Lim et al. (2005); Carlton et al., (2008); Tan et al., (2009) and Refs. Therein.

14. 3.) Enhancing CMAQ Aqueous Processes: Partitioning Ai(g)  Ai (aq)  Current CMAQ approach  Ai(aq)  Ai (g)  volatilization sink reactions Theoretical maximum aqueous production accommodation interfacial processes by Schwartz (1986)

15. Findings and Implications • In box model testing ROS3 represents a plausible technique to solve atmospheric aqueous phase chemistry • potentially more robust method than current method • Successful implementation of the ROS3 solver to solve aqueous system in CMAQ • Beta version run time is slower but still optimizing

16. Future Directions • Put wet deposition back in • Aqchem with ROS3 as an option in FY11 CMAQ release • Test this solver for different seasons, e.g., winter • Incorporate more explicit chemistry into CMAQ • Find balance between more explicit chemistry and computational efficiency • Compare with ground-base and aloft observational data • Speciated rain, cloud, deposition measurements