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Shunsuke Nakao (1, 2) , Manish Shrivastava (2, 3) ,

Influence of dilution and particle fractal dimension of diesel exhaust on measured SOA formation in a smog chamber. Shunsuke Nakao (1, 2) , Manish Shrivastava (2, 3) , Ahn Nguyen (2, 4) , Li Qi (1, 2) , Heejung Jung (2, 4) , David Cocker III* (1, 2)

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Shunsuke Nakao (1, 2) , Manish Shrivastava (2, 3) ,

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  1. Influence of dilution and particle fractal dimension of diesel exhaust on measured SOA formation in a smog chamber Shunsuke Nakao(1, 2), Manish Shrivastava (2, 3), Ahn Nguyen(2, 4), Li Qi (1, 2), Heejung Jung (2, 4), David Cocker III* (1, 2) University of California, Riverside, Department of Chemical and Environmental Engineering College of Engineering – Center for Environmental Research and Technology (CE-CERT) Currently at Pacific Northwest National Laboratory University of California, Riverside, Department of Mechanical Engineering www.cert.ucr.edu

  2. SOA from POA? • Significant fraction of POA volatilize at higher dilution ratio (~ambient condition) • Traditional SOA precursors cannot explain SOA from diesel exhaust SOA from “Intermediate/Semi”-volatility Organic Compounds (IVOCs, SVOCs) Robinson et al., 2007, Science

  3. Recent studies on SOA from diesel exhaust High SOA formation - Weitkamp et al. (2007), CMU chamber Low/No SOA formation *without addition of radical source or hydrocarbons - Samy and Zielinska (2010), EUPHORE chamber • Chirico et al. (2010), PSI chamber CMU: Carnegie Mellon University EUPHORE: European photoreactor PSI: Paul Scherrer Institute 1) Weitkamp et al., Environ. Sci. Technol., 41, 6969-6975, 2010 2) Samy and Zielinska, Atmos. Chem. Phys., 10, 609-625, 2010 3) Chirico et al., Atmos. Chem. Phys. Discuss., 10, 16055-16109, 2010 www.cert.ucr.edu

  4. SOA condensation onto agglomerates • In some studies1)2), SMPS was used to quantify SOA (ρ=1g/cm3) aging ? ? Question: • How spherical/fractal is the PM after SOA formation? • Is density uniform? SMPS: Scanning Mobility Particle Sizer 1) Weitkamp et al., Environ. Sci. Technol., 41, 6969-6975, 2010 2) Samy and Zielinska, Atmos. Chem. Phys., 10, 609-625, 2010 www.cert.ucr.edu

  5. Particle measurement methods • 1. SMPS • - volume (based on dm) • 2. APM – SMPS • - mp, dm ρeff • 3. HR-ToF-AMS • Non refractory organics SMPS: Scanning Mobility Particle Sizer APM: Aerosol Particle Mass Analyzer HR-ToF-AMS: High Resolution-Time-of-Flight Aerosol Mass Spectrometer dm: mobility diameter mp: mass of particle ρeff: effective density www.cert.ucr.edu

  6. CE-CERT mezzanine chamber FEP, 2mil, 12m3 Dilution method: Ejector

  7. Particle volume evolution Little volume increase  little SOA formation? www.cert.ucr.edu Run 1 (engine idle,short transfer line, DR: 114)

  8. Particle effective density evolution Larger particles more fractal How significant is this density increase? www.cert.ucr.edu Run 1 (engine idle,short transfer line, DR: 114)

  9. Size dependent density • V(dm) • ρ(dm) Mass concentration Power function fit Fractal dimension (Df)* www.cert.ucr.edu Run 11 (medium load, long transfer line, DR:519)) *Park et al., ES&T, 2003, Xue et al., ES&T 2009

  10. Particle volume and mass concentration Hours after lights on Mobility diameter measurement was insufficient to observe SOA formation www.cert.ucr.edu Run 1 (engine idle,short transfer line, DR: 114)

  11. Measurement of SOAby AMS SOA POA C4H9+: tracer of primary organic aerosol* www.cert.ucr.edu *Chirico et al. 2010, APCD Run 1 (engine idle,short transfer line, DR: 114)

  12. Measurement of SOA by AMS Comparable to Robinson et al. (2007) Robinson et al. (2007) Science www.cert.ucr.edu Run 1 (engine idle,short transfer line, DR: 114)

  13. CE-CERT mezzanine chamber FEP, 2mil, 12m3

  14. Injection/dilution methods and initial particle density in a smog chamber* Raw (plume injection)  possibly higher density www.cert.ucr.edu *Idle, short transfer line Dilute injection: Ejector dilutor

  15. CE-CERT mezzanine chamber FEP, 2mil, 12m3

  16. Particle volume/mass evolution (transfer line effect) Longer transfer line (Run 5~11) Increase in PM volume (Decrease in bulk ρ) More particle coagulation Idle, diluted exhaust www.cert.ucr.edu

  17. Discrepancy in diesel-SOA studies High SOA formation:CMU1) THIS STUDY Low/No SOA formation:EUPHORE2), PSI3) *without addition of radical source or hydrocarbons • In addition to the difference in engine, different injection/dilution methods may in part explain the gap • Use of SMPS2) might have lead to underestimation 1) Weitkamp et al., Environ. Sci. Technol., 41, 6969-6975, 2010 2) Samy and Zielinska, Atmos. Chem. Phys., 10, 609-625, 2010 3) Chirico et al., Atmos. Chem. Phys. Discuss., 10, 16055-16109, 2010 www.cert.ucr.edu

  18. Conclusion • Mobility diameter measurement is shown to underestimate SOA formation from diesel exhaust in a smog chamber. Hence, mass based measurement is necessary for evaluating SOA from diesel exhaust (e.g., APM, AMS, TEOM) • Experimental conditions possibly have strong impacts on particle physical evolution in a smog chamber • Raw exhaust injection enhanced particle density • Ejector dilutor speeds evaporation: lower particle density • Longer transfer line enhanced particle coagulation to such an extent that particle volume increased in dark • Difference in experimental methods (injection, dilution) may in part contribute to current discrepancy of diesel-chamber studies www.cert.ucr.edu

  19. Acknowledgements • Graduate advisor: Dr. David Cocker • Current/former students: Christopher Clark, Ping Tang, Xiaochen Tang, Dr. Quentin Malloy, Dr. Li Qi • Support staff: Kurt Bumiller, Chuck Bufalino • Funding sources: NSF, W.M. Keck Foundation, and University of California, Transportation Center www.cert.ucr.edu

  20. Expt. www.cert.ucr.edu

  21. Particle fractal dimension (Df) Df = 1: straight chain, 2: sheet, 3: sphere If a is constant, N is proportional to mass (Park et al. 2003, Xue et al. 2009) www.cert.ucr.edu

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