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Revised Treatment of N 2 O 5 Hydrolysis in CMAQ

Prakash Bhave, Golam Sarwar, Wyat Appel, & Robin Dennis 2006 CMAS Conference October 16, 2006. Revised Treatment of N 2 O 5 Hydrolysis in CMAQ. Acknowledgements: Rob Pinder, Nicole Riemer, Jerry Davis. Overview. Motivation & Background Evaluation of CMAQv4.5

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Revised Treatment of N 2 O 5 Hydrolysis in CMAQ

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  1. Prakash Bhave, Golam Sarwar, Wyat Appel, & Robin Dennis 2006 CMAS Conference October 16, 2006 Revised Treatment of N2O5 Hydrolysis in CMAQ Acknowledgements: Rob Pinder, Nicole Riemer, Jerry Davis

  2. Overview • Motivation & Background • Evaluation of CMAQv4.5 • Revised treatment in CMAQv4.6 • Winter sensitivity analyses • Comparison with measurements • Summary

  3. Motivation • AQ models simulate fine NO3- poorly • For accurate NO3- simulations, we need: • Correct amount of TNO3 (≡ HNO3 + NO3-) • Thermodynamic gas/particle partitioning • Thermodynamic modules very mature due to >25 years of development (e.g., EQUISOLV, ISORROPIA, AIM, etc.). • In contrast, less attention has been paid to TNO3 predictions.

  4. NO3 HO2, VOC OH hn NO2 + OH N2O5 + H2O Wet Deposition g Dry Deposition N2O5 Background: Production & Removal TNO3

  5. Evaluation of CMAQv4.5

  6. Model exceeds observations every Winter Evaluation of CMAQv4.5 Winter model performance is sensitive to N2O5 hydrolysis. Summer is not. [Gilliland et al., Atmos.Env., 2006]

  7. NO3 HO2, VOC OH hn NO2 + OH N2O5 + H2O Wet Deposition g Dry Deposition N2O5 TNO3 Production & Removal TNO3

  8. N2O5 Hydrolysis in CMAQ • CMAQ v4.0 – 4.2.2 • Gas phase: N2O5 + H2O  2HNO3 • Heterogeneous: g = 0.1 • CMAQ v4.3 – 4.5.1 • Gas phase: turned off • Heterogeneous:

  9. N2O5 Hydrolysis: Recent Literature • Gas-phase pathway • N2O5 + H2O  2HNO3; k1 = 2.5×10-22 cm3 mol-1 s-1 • N2O5 + H2O + H2O  2HNO3 + H2O;k2 = 1.8×10-39 cm6 mol-2 s-1[IUPAC Supplement, Nov. 2003] • Heterogeneous pathway • On (NH4)2SO4 particles, g ~ fxn(T, RH)[Evans & Jacob, GRL, 2005] • Organic inhibition g 0.001 [Brown et al., Science, 2006]

  10. Relatively slow at winter nighttime conditions Gas-phase production rates dHNO3/dt @ 1ppt N2O5 [ng m-3 h-1] 1 H2O reaction 1 & 2 H2O reactions

  11. N2O5 Hydrolysis: Recent Literature • Gas-phase pathway • N2O5 + H2O  2HNO3; k1 = 2.5×10-22 cm3 mol-1 s-1 • N2O5 + H2O + H2O  2HNO3 + H2O;k2 = 1.8×10-39 cm6 mol-2 s-1[IUPAC Subcommittee, Nov. 2003] • Heterogeneous pathway • On (NH4)2SO4 particle surfaces, g ~ fxn(T, RH)[Evans & Jacob, GRL, 2005] • Organic inhibition g 0.001 [Brown et al., Science, 2006]

  12. CMAQ v4.5 • Too high during winter • Okay for summer nights g on (NH4)2SO4 particle surfaces

  13. N2O5 Hydrolysis: Recent Literature • Gas-phase pathway • N2O5 + H2O  2HNO3; k1 = 2.5×10-22 cm3 mol-1 s-1 • N2O5 + H2O + H2O  2HNO3 + H2O;k2 = 1.8×10-39 cm6 mol-2 s-1[IUPAC Subcommittee, Nov. 2003] • Heterogeneous pathway • On (NH4)2SO4 particles, g ~ fxn(T, RH)[Evans & Jacob, GRL, 2005] • Inhibition by organic surfaces, g 0.001 [Brown et al., Science, 2006]

  14. N2O5 Hydrolysis in CMAQv4.6 • Gas-phase pathway • CBIV: none • SAPRC99: turned on +1H2O reaction • CB05: turned on both H2O reactions • Heterogeneous pathway g ~fxn(SO42-,NO3-,T,RH)

  15. Winter Sensitivity Analyses • Study period: January 1 – 23, 2002 • Representative of winter performance in 2001-2003 • Ignored 1st 7 days for spin-up • CMAQv4.5, saprc99_ae4_aq, ConUS 36km, 14 layers • Sensitivity matrix • Observations: CASTNet weekly TNO3

  16. No gas production 1 H2O Reaction Both H2O Reactions Max = 10.8 Max = 11.4 Max = 11.8 4-7%↑ 2-5%↑ TNO3 Sensitivity to Gas-phase Prodxn TNO3mg m-3 16-day average: Jan.08–23, 2002

  17. g ~ fxn(SO4,NO3,T,RH) Heterogeneous Reaction Probability, g g ~ fxn(SO4,NO3) g Typical nighttime g values: Jan14 @ 0600 GMT

  18. Evaluation at Eastern CASTNet Sites 15 Jan. 8–23, 200252 sites; n = 102 10 TNO3 [mg m-3] 5 0 g = 0 g = 0.001 old g (v4.5) v4.5 + 1H2O v4.6 + 2H2O new g (v4.6) g = 0.1 (v4.2) Observations no hydrolysis

  19. Summary & Conclusions • N2O5 hydrolysis in CMAQ has been updated based on recent literature • During non-winter months, model results are fairly insensitive to N2O5 hydrolysis treatment • Addition of gas-phase hydrolysis reactions increases winter TNO3 by less than ~10% • New g parameterization decreases winter TNO3 ~8 – 16% • Modeled TNO3 is still biased high in winter • Further reduction of g will resolve bias in high-NO3 areas (e.g., Midwest), but not in low-NO3 areas (e.g., Southeast) • Other possible explanations: gas-phase hydrolysis rates are too fast, too much daytime TNO3 production, too little dry deposition, insufficient vertical mixing, ... • Disclaimer • The research presented here was performed under the Memorandum of Understanding between the U.S. Environmental Protection Agency (EPA) and the U.S. Department of Commerce's National Oceanic and Atmospheric Administration (NOAA) and under agreement number DW13921548. This work constitutes a contribution to the NOAA Air Quality Program. Although it has been reviewed by EPA and NOAA and approved for publication, it does not necessarily reflect their policies or views.

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