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Fang-Yi Cheng 1 , Andrey V. Martynenko 2 , Daewon Byun 1 and Jiwen He 2

6th annual CMAS conference, October 1~3 , 2007 , Chapel Hill. A Comparison Study of CMAQ Aerosol Prediction by Two Thermodynamic Modules: UHAERO V.S. ISORROPIA Case study for January 2002 episode. Fang-Yi Cheng 1 , Andrey V. Martynenko 2 , Daewon Byun 1 and Jiwen He 2

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Fang-Yi Cheng 1 , Andrey V. Martynenko 2 , Daewon Byun 1 and Jiwen He 2

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  1. 6th annual CMAS conference,October1~3, 2007, Chapel Hill A Comparison Study of CMAQ Aerosol Prediction by Two Thermodynamic Modules: UHAERO V.S. ISORROPIACase study for January 2002 episode Fang-Yi Cheng1, Andrey V. Martynenko2, Daewon Byun1 and Jiwen He2 1Department of Geosciences 2Department of Mathematics IMAQS/University of Houston October 03, 2007

  2. Background • Atmospheric aerosols have direct impact on earth’s radiation balance, air pollution, fog formation, visibility and human health. • Inorganic particles typically consist of ammonium, sulfate, nitrate, sodium, chloride, calcium, etc. • Phase state of aerosols at given T and RH are determined by thermodynamic equilibrium. • A variety of thermodynamic models have been developed to predict partition of inorganic aerosols between liquid, solid and gas phases. • Predicting gas/aerosol partitioning of semi-volatile inorganic aerosol is challenging task because multiple solid, liquid and gas phases could exist. • Computational codes can be very complex and for application in 3-D air quality models, an efficient numerical algorithm must be used.

  3. Applications in 3-D air quality model • 3-D air quality models usually use pre-calculated information of phase behavior to facilitate computation, ex. ISORROPIA • A new inorganic thermodynamic module UHAERO is recently developed (Amundson et al., 2006). • UHAERO-PSC is essentially similar to AIM model (Wexler and Clegg, 2002) with same physical and chemical setup but differs in numerical algorithm • Air quality model (CMAQ) is performed with UHAERO module and benchmarked with simulation using ISORROPIA(currently used in CMAQ). • Goal is to provide thermodynamic module that is physically general and numerically efficient for 3-D air quality applications.

  4. Differences between UHAERO and ISORROPIA • Difference in activity coefficient methods • --- ISORROPIA uses Bromley’s model (Bromley, 1973) for multicompoment activity coefficient and K-M method (Kusik and Messner, 1978) for binary activity coefficient • --- UHAERO uses PSC activity coefficient model (Pitzer et al. 1986; Clegg et al. 1992; Wexler and Clegg, 2002) which is mole fraction based and considered as the state of science model • Difference in predictions of aerosol water content • ---ISORROPIA uses empirical ZSRrelation (Stokes et al. 1966)to calculate water content • --- UHAERO directly computes water content based on water activity • Difference in numerical solution methods • --- ISORROPIA incorporates pre-determined equation approach and pre-calculated tables (Nenes et al., 1999) • --- UHAERO uses numerical technique (primal-dual active-set algorithm) without any prior assumption to determine equilibrium state (Amundson et al. 2006).

  5. Configuration of the study episode • CMAQv4.6, saprc99, AERO4 • Resolution 36-km continental domain; (x,y,z) = (148, 112, 14) • Two simulations (CMAQ-ISORROPIA V.S CMAQ-UHAERO) are conducted • Metastable (only liquid in aerosol phase) assumption is used • Episode: January 1 ~ 23, 2002 • MM5, analysis nudging, KF2, RRTM, Reisner, PX PBL/LSM • NEI 2001 inventory were used for point source, biogenics were processed using BEIS 3.13, onroad mobile emissions were computed using MOBILE6 Information is provided from Bhave Prakash (EPA)

  6. Observational datasets • CASTNET (weekly sampling), IMPROVE (Two 24-hour samples are collected each week, on Wednesday and Saturday from midnight to midnight local time) • Pittsburgh Supersite, Pennsylvania (sampling time 2 hour) From http://www.epa.gov/castnet/site.htmlcc

  7. IMPROVE • Nitrate is over-predicted ISORROPIA UHAERO • Comparison is very similar between two simulations • Sulfate is fairly predicted

  8. nitrate CASTNet Jan. 8 ~15, 2002 ammonium • Model (ISORROPIA) (ring) shows over-prediction, (observation is in the core). • Sites with high bias are located in eastern region

  9. XY diagram for CASTNet datasets H2SO4 (NH4)2SO4 1 Y HNO3 X NH4NO3 1 0 • When X approaches 1, aerosol is NH4+ rich • When Y approaches 1, aerosol is SO42- rich and NO3- poor

  10. XY diagram for CASTNet Model • Generally, both models over-predict NH4+ and NO3- • ISORROPIA predicts slightly higher NH4+ than UHAERO • The tendency of the CASTNet data distributes toward off-diagonal line OBS

  11. Pittsburgh • Excess NH4+ reacts with HNO3over-produce NO3- • RH in range 50% to 100% • Temp in range –8 to +8 degree C • Sulfate is fairly predicted • Total NH4+ is over-predicted • Same bias is also observed by other scientist and attributing error to meteorological and emission uncertainty (Shaocai et al., 2005).

  12. RH and TEMP RH TEMP • Low RH (20~50 %) is over western U.S. and central continental area • High RH (>85 %) in northern part of domain, south eastern Texas, Louisiana state and ocean • Temperature is below freezing in northern part, and above 285 K in southern part of domain.

  13. ISORROPIA UHAERO Diff. Nitrate HNO3 TEMP RH • UHAERO shows less NO3- in low RH region.

  14. UHAERO Diff. ISORROPIA NH4+ NH3 • UHAERO shows less NH4+, more NH3 than ISORROPIA in low RH region

  15. At low RH region, UHAERO moves toward direction comparing to ISORROPIA 1 1 Y RH (75 ~ 95 %), 4268 grid points • When RH increase, the points moves further in direction • UHAERO shows less NH4+ than ISORROPIA X 0 ISORROPIA UHAERO XY diagram from model (corresponding to previous one snap shot) Low RH (50~ 60 %), 637 grid points ISORROPIA UHAERO

  16. Box model for NH3 1 UHAERO shows less NH4+, more NH3 than ISORROPIA 1 High RH 90% Y X 0 At high RH region, the difference is small between two modules ISORROPIA UHAERO Low RH 50%

  17. Conclusions and future work • Sulfate is fairly predicted, nitrate and ammoniumare over-predicted • CMAQ-UHAERO takes ~30% more computer running time than CMAQ-ISORROPIA • Differences of nitrate and ammonium partitioning are mostly in low RH region • XY diagram comparison for CASTnet site shows over-prediction of NH4+ from both models with slightly better agreement in UHAERO • Box model predicts less NH4+ in UHAERO than ISORROPIA at low RH region, that is consistent with our 3-D simulation result • Future work will focus on (1) deliquescence branch in which the solid phases can form; (2) collecting high time resolution data to evaluate model

  18. Acknowledgement • Thanks for Bhave Prakash(EPA) on providing the required information for model simulation, as well as the guidance and suggestions for model comparison.

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