Annmarie G. Carlton, Gerald Gipson, Shawn Roselle, Rohit Mathur

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 Earths 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
Multipollutant version of CMAQ
original simulation
Unrealistic sulfate production -problem
traced to aqueous chemistry solver technique.
-Incorporated the fix into CMAQv4.7.1
Max283.4 µg/m3
Simulation with update
Max24.3 µg/m3
Figures courtesy of P. Dolwick
5
CMAQ Aqueous Chemistry Map (aqchem.F)
partitioning
Molar conc. initial amt. amt. deposited (mol
L-1)
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 lt 0.05 of SIV oxidized since time 0,
double DT
Dont let DT gt TAUCLD
100 max. iterations
deposition
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
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 Henrys Law
• Oxidation Chemistry
• 5 sulfur family reactions S(IV) ? S(VI) via
  O3, 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 lt 34 meters
12
Differences in accumulation mode SO4
Forward Euler Method SO4 ROS3 Method SO4
µg m-3
aloft layer typical of cloud base
surface layer lt 34 meters
13
3.) Enhancing CMAQ Aqueous Processes More
Explicit Chemistry
HOx chemistry
Glyoxal oxidation chemistry

• 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 ? CO2CO2 - 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-

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-
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