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Impact of an updated toluene mechanism on air quality in the western US

Golam Sarwar, Wyat Appel, Annamrie Carlton, Rohit Mathur, Keneth Schere, Mohammed Majeed October 11, 2010. Impact of an updated toluene mechanism on air quality in the western US. Background. Chemical mechanisms are a critical component of air quality models

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Impact of an updated toluene mechanism on air quality in the western US

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  1. Golam Sarwar, Wyat Appel, Annamrie Carlton, Rohit Mathur, Keneth Schere, Mohammed MajeedOctober 11, 2010 • Impact of an updated toluene mechanism on air quality in the western US

  2. Background • Chemical mechanisms are a critical component of air quality models • Toluene is an imporant chemical compound that can affect O3 and SOA; however its chemistry is poorly understood • Different chemical mechanisms use different approximations for toluene chemistry • Models using different chemical mechanisms produce different results • Faraji et al. (2008) noted CB-IV & SAPRC99 lead up to 40 ppbv O3 difference in Houston • A large part of the difference was attributed to differences in aromatic chemistry • Whitten et al. (2010) updated the toluene chemistry in CB05 (Whitten et al., 2010. A new condensed toluene mechanism for Carbon Bond: CB05-TU, AE)

  3. Background • Chemical mechanisms are evaluated using chamber experimental results • updated toluene chemistry performed better than base chemistry • Earlier, we evaluated impact of the updated toluene chemistry on air quality in the eastern US (Sarwar et al., 2010) • Monthly mean 8-hr O3 increased by up to 2.0 ppbv • Here, we examine impact of the updated toluene chemistry on O3 and PM in the western US

  4. TOL OH 36% 56% 8% HO2 (benzaldehyde) CRES TO2 OH NO HO2 90% 10% NTR OPEN CRO OH NO2 NO2 PAN C2O3 NTR Existing toluene chemistry in CB05 mechanismCB05-Base (simplified) Ref: Yarwood et al., 2005

  5. TOL OH 18% 65% 7% 10% CRES TO2 HO2 (benzaldehyde) OH HO2 OH NO NTR OPEN CRO 86% 14% NO2 OH CRON NO2 OPAN OPO3 NO OH CRNO O3 CRN2 NO2 OH 2 NTR CRPX Updated toluene chemistry in CB05 mechanism CB05-TU (simplified) Ref: Whitten et al., 2010

  6. Modeling details CMAQ model • CMAQ model version 4.7 was used • Horizontal grid resolution = 12-km • Number of horizontal grids = 213 X 192 • Number of vertical layers = 14 • Surface layer thickness = 36 meter • Simulation period: July 1 – 31, 2002 • Two different model simulations were performed • with CB05-Base • with CB05-TU • Used generalized Rosenbrock solver for gas-phase chemistry • Model with CB05-TU required 6% additional computational time

  7. Modeling details Meteorology and emissions • Meteorological fields • PSU/NCAR MM5 system • Anthropogenic emissions • 2002 National Emissions Inventory (NEI) • Biogenic emissions • Biogenic Emissions Inventory System (BEIS) • Model-ready emissions • Sparse Matrix Operator Kernel Emission (SMOKE)

  8. ppbv ppbv CMAQ: impact on daily maximum 8-hr O3 Monthly mean 8-hr O3 (CB05-Base) Changes in mean 8-hr O3 (CB05-TU - CB05-Base) Mean TOL/VOC (CB05-Base) O3 increases where toluene/VOC ratio is high

  9. Los Angeles Portland Seattle CMAQ: variation of daily maximum 8-hr O3 (CB05-TU – CB05-Base) While CB05-TU increases daily maximum 8-hr O3 by up to 5-6 ppbv, impact on many other days are low to moderate

  10. Monthly mean NOz with CB05-Base Changes in mean NOz (CB05-TU - CB05-Base) CMAQ: NOz with CB05-Base and increases in NOz with CB05-TU CB05-TU increases NOz compared to CB05-Base

  11. CB05-Base at Los Angeles CB05-TU at Los Angeles CMAQ: ozone production efficiency (OPE) • CB05-TU increases O3 but not OPE

  12. Comparison with observed data Daily maximum 8-hr O3at Seattle MB with CB05-TU decreases slightly at higher observed O3 and increased slightly at lower observed O3

  13. Comparison with observed data Daily maximum 8-hr O3 at Los Angeles MB decreases slightly with CB05-TU at all observed O3

  14. Monthly mean SOA with CB05-Base CB05-TU / CB05-Base Impact on SOA from toluene Predicted SOA increased slightly with CB05-TU (maximum 7%) However, impact on PM2.5 was also small

  15. Summary • Updated toluene chemistry enhances monthly mean 8-hr O3 by up to 2.8 ppbv • Updated toluene chemistry produces more NOz than CB05-Base • Updated toluene chemistry increase O3 but not OPE • Updated toluene chemistry slightly decreases MB at higher observed O3 • Updated toluene chemistry slightly increases SOA; impact on PM2.5 is smaller • Results are consistent with those reported by Sarwar et al. (2010) for eastern US • CB05-TU increased monthly mean 8-hr O3 by up to 2.0 ppbv in eastern US

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