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

SOA from POA? Robinson et al., 2007, Science 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)

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, , ) Samy and Zielinska, Atmos. Chem. Phys., 10, , ) Chirico et al., Atmos. Chem. Phys. Discuss., 10, , 2010

4 SOA condensation onto agglomerates In some studies 1)2), SMPS was used to quantify SOA (ρ=1g/cm 3 ) ? ? How spherical/fractal is the PM after SOA formation? Is density uniform? Question: SMPS: Scanning Mobility Particle Sizer 1) Weitkamp et al., Environ. Sci. Technol., 41, , ) Samy and Zielinska, Atmos. Chem. Phys., 10, , 2010 aging

5 Particle measurement methods 1. SMPS - volume (based on d m ) 2. APM – SMPS - m p, d m  ρ eff 3. HR-ToF-AMS - Non refractory organics d m : mobility diameter m p : mass of particle ρ eff : effective density SMPS: Scanning Mobility Particle Sizer APM: Aerosol Particle Mass Analyzer HR-ToF-AMS: High Resolution-Time-of-Flight Aerosol Mass Spectrometer

CE-CERT mezzanine chamber FEP, 2mil, 12m 3 Dilution method: Ejector

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

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

9 Size dependent density V(d m ) ρ(d m ) Mass concentration Power function fitFractal dimension (D f ) * *Park et al., ES&T, 2003, Xue et al., ES&T 2009 Run 11 (medium load, long transfer line, DR:519))

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

11 Measurement of SOA by AMS Run 1 (engine idle,short transfer line, DR: 114) C 4 H 9 + : tracer of primary organic aerosol * POA SOA *Chirico et al. 2010, APCD

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

CE-CERT mezzanine chamber FEP, 2mil, 12m 3

14 Injection/dilution methods and initial particle density in a smog chamber * Raw (plume injection)  possibly higher density *Idle, short transfer lineDilute injection: Ejector dilutor

CE-CERT mezzanine chamber FEP, 2mil, 12m 3

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

17 Discrepancy in diesel-SOA studies High SOA formation: CMU 1) THIS STUDY Low/No SOA formation: EUPHORE 2), PSI 3) *without addition of radical source or hydrocarbons 1) Weitkamp et al., Environ. Sci. Technol., 41, , ) Samy and Zielinska, Atmos. Chem. Phys., 10, , ) Chirico et al., Atmos. Chem. Phys. Discuss., 10, , 2010 In addition to the difference in engine, different injection/dilution methods may in part explain the gap Use of SMPS 2) might have lead to underestimation

18 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 Conclusion

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

20 Expt.

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