Particulate Formation, Evolution, and Fate - Hope Michelson Day 1 Part 2

Summary

The script discusses various aspects of soot formation in flames, including size distributions, extinction spectra, and the absorption characteristics of soot at different flame heights. It delves into the maturity of soot particles, the significance of the dispersion exponent, and the presence of aliphatic and oxygenated species within them. The role of radicals, resonance-stabilized radicals, and their association with five-membered rings in soot particle inception is also explored. The talk highlights the importance of kinetics over thermodynamics in soot formation and the unexpected presence of certain radicals and compounds, such as furans, in flame-generated particles.

Takeaways

• 🔍 The script discusses the properties and behavior of soot particles, particularly their size distribution, absorption characteristics, and composition at different heights within a flame.
• 🧐 The absorption spectrum of soot changes as it matures, with stronger absorption at shorter wavelengths at the base of the flame and a broader, flatter spectrum as the soot matures higher up.
• 📊 The dispersion exponent, also known as the Angstrom exponent, is a crucial parameter that indicates the maturity of soot particles and is used to differentiate between flaming and smoldering combustion sources in atmospheric science.
• 🌐 The optical band gap and the absorption cross-section are related to the dispersion exponent and provide insights into the structure and maturity of soot particles.
• 📚 The script mentions that as soot matures, it exhibits an increase in long-range order and conjugation length, which are associated with the stacking and ordering of graphene-like sheets within the particles.
• 🔬 High-resolution TEM and AFM imaging, along with density functional calculations, reveal the presence of spherical, waxy particles and their structural evolution within a flame.
• 🌐 The composition of soot particles is explored through various experiments, including the use of scanning mobility particle sizer and aerosol mass spectrometry, which show a significant abundance of aliphatic groups and a carbon to hydrogen ratio around 1.4.
• 🔍 The presence of oxygenated species and aliphatic side chains in soot particles was unexpected and suggests a more complex chemical composition than previously thought.
• 🧪 An experiment by High Wong's group using IR spectroscopy on extracted soot revealed the presence of oxygen, challenging the assumption that soot should be primarily carbon and hydrogen.
• 🔬 The script also touches on the use of X-ray photoelectron spectroscopy to investigate the bonding within soot particles and the unexpected discovery of furan structures in flame-generated particles.

Q & A

What are the issues with measuring extinction in the flame itself?

The script mentions that there are some technical issues with performing extinction measurements directly in the flame. While it does not specify the issues, common challenges can include flame instability, high temperatures, and the presence of other reactive species that can interfere with the measurements.

How does the absorption cross-section of soot change as the soot matures in a flame?

As soot matures in a flame, the absorption cross-section becomes broader and flatter. This is indicated by the spectrum changing from stronger absorption at shorter wavelengths at the lower parts of the flame to a more even distribution as the soot particles grow and mature.

What is the significance of the dispersion exponent or Angstrom exponent in the study of soot?

The dispersion exponent, also known as the Angstrom exponent, is a measure of how the absorption cross-section of a particle changes with wavelength. It is used to indicate the maturity of soot particles, with values close to one suggesting fully mature soot, often found in large flames. Higher values suggest less mature particles.

How does the optical band gap relate to the dispersion exponent?

The optical band gap, derived from spectral measurements, increases as the dispersion exponent increases. This suggests that as soot particles mature and their absorption cross-section broadens, the range of wavelengths they absorb also increases.

What does the term 'Rayleigh particles' refer to in the context of soot?

Rayleigh particles refer to particles that are much smaller than the wavelength of light they scatter. The script mentions that for Rayleigh particles with long wavelengths, the absorption cross-section is proportional to the particle diameter cubed over six times the wavelength of light.

What are the challenges in extracting and measuring soot particles from a flame?

The script alludes to the difficulty of making measurements on soot particles extracted from a flame. Challenges include maintaining the integrity of the particles during extraction, avoiding contamination, and accurately measuring properties such as size, structure, and composition.

What compositional information can be derived from high-resolution AFM images of soot particles?

High-resolution AFM images can reveal the structure of soot particles, including the presence of six-membered and five-membered rings, as well as bridges between aromatic structures. These images can provide insights into the chemical composition and maturity of the soot particles.

How does the carbon to hydrogen ratio in soot particles change as they mature?

As soot particles mature, the carbon to hydrogen ratio increases. This is due to the formation of additional aromatic rings, which add carbon atoms without a proportional increase in hydrogen atoms, leading to a higher carbon to hydrogen ratio.

What is the significance of the presence of aliphatic side chains in soot particles?

The presence of aliphatic side chains in soot particles is significant as it affects the carbon to hydrogen ratio and the overall composition of the particles. It also suggests that soot formation may involve different chemical pathways than previously thought, with aliphatic structures playing a more prominent role.

What role do resonance-stabilized radicals (RSRs) play in the formation of soot particles?

Resonance-stabilized radicals, or persistent radicals, are believed to play a significant role in the formation of soot particles. They are more stable than non-resonance stabilized radicals and are associated with incipient particles, suggesting they may be involved in the initial stages of soot nucleation and growth.