Quick, Dirty and Cheap Inorganic Aerosol Thermodynamics for Regional Air Quality Modeling

1
Quick, Dirty and Cheap Inorganic Aerosol
Thermodynamics for Regional Air Quality Modeling

• K. Max Zhang, Cornell University
• Anthony S. Wexler, UC-Davis

2
What We Need

• Gas-particle partitioning is driven by the
  differences between partial pressure and the
  equilibrium vapor pressure at the particle
  surface.
• The equilibrium vapor pressures of volatile
  species (HNO3, HCl, NH3, NH4NO3 and NH4Cl) given
  aerosol composition, temperature and relative
  humidity
• Water content associated with the particulate
  composition.

3
What We Dont (Really) Need

• The detailed phase state information, while
  useful for other purposes such as interpreting
  laboratory results, may not be necessary for gas
  particle partitioning

4
Limitations
Tang et al., 1997

• We don't know the RH very well.
• We don't know if we are in equilibrium since we
  usually don't know if we are on the rising or
  falling leg of the curve.

5
The UCD Aerosol Module

• Developed by Sun, Zhang and Wexler.
• Employs a simplified thermodynamics model of the
  H-NH4-Na-SO42--NO3--Cl- system.
• The aerosol thermodynamics is calculated without
  Gibbs free energy minimization.
• Activity Coefficients
• Mutual Deliquesce Relative Humidity
• Phase States and Existence of ammonium salts
• Water Content

6
Activity Coefficients

• Low Ion Strength - Pitzers method
• High Ion Strength - Kusik-Meissener/Bromley
  method
• Intermediate Ion Strength - Weighted average
  between the two methods

7
Mutual Deliquesce Relative Humidity

• Ran the AIM model to identify MDRHs for all
  possible ionic composition and identified
  characteristic salts (which governs the MDRH)
• Compiled results into a look-up table.
• Approximate combination of solids that may exist
  under different relative compositions.
• Search the look-up table for MDRH.

8
Phase States

• The aerosol phase is assumed to be in pure solid
  state when the RH is below the MDRH.
• If the RH is higher than the DRH of any possible
  salts, the particle phase is assumed to pure
  aqueous
• MDRH is equal to zero in free acid region,
  parameterized by H / SO42- gt 1.

9
Existence of Solid Ammonium Salts (Vapor Pressure
Cap)
molality
Vapor pressure
Activity Coefficient
Constant
10
Water Content

• The water content associated with aerosol salts
  are calculated using the ZSR method.
• The single solute molality data compiled from
  numerous experiments by Clegg et al. for the AIM
  model, and interpolate for the appropriate
  relative humidity and temperature.
• Water content in a mixed phase state is taken as
  a weighted mean of dry state and wet state,
  similar to ISSOROPIA.

11
AIM vs the UCD Aerosol Module
12
Acknowledgement

• USEPA
• California Air Resources Board
• VISTAS
• Dr. Robin Dennis, Dr. Ajith Kaduwela and Dr. Gail
  Tonnesen