Effect of Fuel to Oxygen Ratio on Physical and Chemical Properties of Soot Particles

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1 Effect of Fuel to Oxygen Ratio on Physical and Chemical Properties of Soot Particles J. G. Slowik, J. Kolucki, K. Stainken, and P. Davidovits Chemistry Department Boston College, Chestnut Hill, MA P. F. Decarlo and J. L. Jimenez University of Colorado, Boulder, CO Y. Rudich Weizmann Institute of Science, Rehovot, Israel L. R. Williams, J. T. Jayne, C. E. Kolb, and D. R. Worsnop Center for Aerosol and Cloud Chemistry Aerodyne Research, Inc., Billerica, MA AAAR 2004 Annual Conference October 4-8, 2004

2 Aerosols in the Atmosphere C.E. Kolb, Nature, , 6 Jun 2002

3 What is Soot and Why is it Important in the Atmosphere? Combustion-generated particle Very irregular shape Composition consists of black carbon (BC), and organic carbon (OC) OC includes poly-aromatic hydrocarbons (PAHs), and some aliphatic species (AL) To understand the role and processing of soot in the atmosphere, we must first understand its composition and morphology.

4 Apparatus for Soot Production, Processing, and Analysis N 2 Soot particles produced by combustion Exhaust N 2 + O 2 + Propane Diffusion Dryer Mobility diameter (d m ) d m depends on volume and shape DMA Differential Mobility Analyzer Reaction Zone Particles coated or heated SMPS Scanning Mobility Particle Sizer DMA Differential Mobility Analyzer CPC Condensation Particle Counter AMS Aerosol Mass Spectrometer Non-refractory composition Vacuum aerodynamic diameter (d va ) d va depends on volume, shape, and density

5 The Aerosol Mass Spectrometer Particle Beam Generation Aerodynamic Sizing d va = d volume * density * shape Particle Composition Ambient Pressure Sampling Orifice Aerodynamic Particle Focusing Lens Particle Beam TOF Chopper Quadrupole Mass Spectrometer Particle Vaporization and Ionization Source Turbo Pump Turbo Pump Turbo Pump

6 Two Soot Types Produced in Flame Depending on Propane/O 2 Ratio d m = 350 nm φ = Actual Propane/O2 Stoichiometric Propane/O d va depends on: volume, shape, density d m depends on: volume, shape 2

7 Two Soot Types Produced in Flame Depending on Propane/O 2 Ratio 5 Vacuum aerodynamic diameter (d va ) (nm) φ = 5.0 Type 2 Soot Propane/O 2 = 1.0 Type 2 Soot φ = 2.5 Type 1 Soot Propane/O 2 = 0.5 Type 1 Soot Compact Particles (near-spherical) Fractal Particles (irregular) φ = Actual Propane/O 2 Stoichiometric Propane/O Mobility Diameter (d m ) (nm)

8 Mass per Particle (ug/p) Soot Composition (OC) Depends on Propane/O 2 Ratio 5x Aliphatic PAHs φ = 2.5 Type 1 Soot x10-10 m/z Mass per Particle (ug/p) PAHs φ = 5.0 Type 2 Soot m/z

9 Nitrate Equivalent Mass Concentration (µg m -3 ) Biphenylene Acenaphthylene Possible PAH Assignments Anthracene Fluorene 1H-Phenalene 178 Phenanthrene Diphenylethyne 190 Benzo[ghi]fl uoranthene Napthalenenums 4H- Cyclopenta[def ]phenanthrene 202 Fluoranthene Pyrene methylp yrene 1-methylpyrene 226 2,3-Benzofluorene 1,2-Benzofluorene Cyclopenta[cd]pyrene 240 Numsdodecahyd rochrysene 252 Benzo[a]pyrene Benzo[k]fluoranthene Benzo[b]fluoranthene Perylene Benzo[e]pyrene 276 Indeno[1,2,3-cd]fluoranthene o-phenylenepyrene Dibenzo[ghi,mno]chrysene Benzo[ghi]perylene 300 Coronene m/z (Daltons)

10 Determination of Particle Mass and BC Content With some reasonable assumptions, measurement of d va and d m can be used to estimate the total particle mass to within ~10%. -Particles do not contain internal voids. -BC, PAH, and aliphatic components are immiscible. -Density of each component can be measured. -Dynamic shape factor is the same in the free-molecular and continuum regimes. AMS and CPC provide the mass of the non-refractory component (nonblack carbon) mass per particle. BC Component = Total Particle Mass Non-Refractory Component

11 Soot Properties vs. Equivalence Ratio: Composition Increase propane/o 2 Increase in condensed PAH Increase in particle mass Decrease in fractional BC content

12 Soot Properties vs. Equivalence Ratio: Morphology χ = particle drag drag on sphere of same volume Increased propane/o 2 Particles become more spherical (from increased PAH condensation)

13 Atmospheric Field Measurements AMS Mass Distribution DMA Mass Distribution Pittsburgh, 2002 Fresh Combustion Particles The mass distributions do not overlap for the smaller particle mode, indicating that these particles are fractal. Aged Particles The mass distributions overlap for the larger particle mode, indicating that these particles are compact. By using the AMS and DMA in tandem, newly-formed fractal soot particles may be identified in the field.

14 Laboratory-Simulated Atmospheric Processing of Fractal Soot Particles: Coating with Oleic Acid Increased Oleic Acid Coating Fractal Particles Compact Particles Coating causes fractal soot particles to become compact.

15 Compact Soot Particles Heated to Remove Existing PAH Coatings Compact soot particles do NOT become fractal upon removal existing coating. Compact Particles Compact Particles Increased Heating

16 Effect of Coating on Particle Shape Conclusion: Black carbon core of soot is structured differently in fractal and compact soot. Fractal Particles become nearly spherical when coated with oleic acid χ = particle drag drag on sphere of same volume Compact particles do not become fractal when preexisting coating is removed.

17 Conclusions Two types of soot (fractal and compact) are produced in flame. DMA and AMS can be used in tandem to distinguish fractal and compact particles. The black carbon structure is different in fractal soot vs. compact soot. Future Research Does the black carbon structure change when ambient soot particles are coated? Fractal Soot Coating Remove Coating Does morphology affect water uptake? Fractal Particle? Acknowledgements Hai Wang (University of Delaware) Funding: NASA, DOE, NSF, Israeli Binational Foundation

18 Estimation of Black Carbon Content, Particle Density, and Dynamic Shape Factor Assumptions: 1. No internal voids Dynamic shape factor is the same in continuum and free-molecular regimes BC, PAH, aliphatic species are immiscible Component densities estimated (ρ BC = 2.0 g/cc, ρ PAH = 1.3 g/cc, ρ AL = 0.85 g/cc) System of equations relating equivalent diameter and mass: d m = d ve Cc(d m ) Cc(d ve ) χ d va = d ve ρ p ρ 0 1 χ m p = m BC + m PAH + m AL = (π/6) d ve3 ρ p The system is underdetermined (3 equations, 4 unknowns: d ve, ρ p, χ, m BC ). However, using assumptions 3 and 4, ρ p may be expressed as: ρ p = m BC + m PAH + m AL m BC ρ BC m PAH ρ PAH + + The system has been reduced to 3 equations with 3 unknowns, and can be solved for m BC, ρ p, and χ. m AL ρ AL