Particulate Formation, Evolution, and Fate -Michelson Day 3 Part 3

Summary

The video script discusses various techniques for measuring temperature and composition in flames, such as two-line atomic fluorescence, filtered Rayleigh scattering, and laser-induced fluorescence. It emphasizes the importance of understanding soot and particle behavior in different environments, including atmospheric processing and aging. The script also covers the impact of wildfires and anthropogenic sources on atmospheric emissions, highlighting the distinction between laboratory measurements and real-world observations. The speaker encourages collaboration and communication within the scientific community to unravel the complexities of particle formation and behavior in diverse settings.

Takeaways

• 🧪 Two-Line Atomic Fluorescence is a technique used to measure the population in two different interlines of electronic states in atoms, commonly using indium as the atomic line source.
• 🔬 The Boltzmann distribution is utilized to understand and derive temperature measurements from the intensities of fluorescence observed in atomic fluorescence techniques.
• 🌡️ A comparison of Two-Line Atomic Fluorescence with thermocouple measurements shows good agreement, depending on the targeted uncertainty.
• 🔥 Filtered Rayleigh scattering is a technique used in non-sooting flames to measure temperature by observing the broadening of scattered laser light due to moving species.
• 📈 Laser-Induced Fluorescence (LIF) is a technique for measuring temperatures and concentrations of gas-phase species like NO or OH by analyzing the fluorescence from excited states.
• 🚫 Drawbacks of LIF include potential quenching effects on the nanosecond timescale due to collisions with other gas species, which can be mitigated by using shorter timescales.
• 🌟 Coherent Anti-Stokes Raman Spectroscopy (CARS) is a powerful technique for measuring temperatures and species concentrations in a single laser pulse, suitable for turbulent flames.
• 🔍 Planar Laser-Induced Fluorescence (PLIF) allows for the simultaneous acquisition of temperature and species concentration data across a plane in a flame.
• 🌿 Atmospheric scientists often refer to soot as 'black carbon' and differentiate it from other carbonaceous particles based on their measurement techniques and chemical properties.
• 🔥 Wildland fires, including wildfires and peat fires, are a significant source of black and brown carbon emissions, with tarballs from smoldering fires being of particular interest due to their climate impact.

Q & A

What is two-line atomic fluorescence and how is it used for temperature measurement?

Two-line atomic fluorescence is a technique used to measure the population in two different electronic states of an atom. It involves seeding the system with an atomic line source, such as indium, and choosing a transition to measure the population between the lower state (P one-half) and the upper state (P three-halves). By measuring the intensities of the fluorescence from the Stokes and anti-Stokes transitions, the populations of the states can be determined, and a Boltzmann distribution is used to calculate the temperature.

How do researchers compare the results from two-line atomic fluorescence with thermocouple measurements?

Researchers compare the temperature measurements obtained from the two-line atomic fluorescence technique with those from thermocouple measurements to validate the accuracy of the atomic fluorescence method. The comparison shows how well the two techniques agree, depending on the targeted uncertainty.

What is filtered Rayleigh scattering and how is it used to measure temperature in flames?

Filtered Rayleigh scattering is a technique used to measure temperature in flames without soot. It involves sending a laser into the flame and observing the Rayleigh scattering. The broadening of the scattered line due to the movement of scattering species is measured, and this broadening, similar to the Doppler effect, gives an indication of the speed of the molecules and thus the temperature.

How does laser-induced fluorescence (LIF) differ from atomic fluorescence for temperature measurement?

Laser-induced fluorescence (LIF) measures the internal rotational and vibrational states of molecules, typically diatomic ones like NO or OH, whereas atomic fluorescence focuses on different electronic states. LIF uses the Boltzmann equation to back out the temperature from the measured fluorescence intensities.

What are the drawbacks of using laser-induced fluorescence (LIF) for temperature measurement?

A drawback of using LIF is the potential for quenching of the excited states due to collisions with other gas species, which can occur if the measurement is done on a nanosecond timescale. To mitigate this, the measurement can be done on a shorter timescale or the quenching factors can be accounted for in the temperature calculation.

What is coherent anti-Stokes Raman spectroscopy (CARS) and how is it used for temperature measurement?

Coherent anti-Stokes Raman spectroscopy (CARS) is a technique that has been used for decades to measure temperatures in gases and flames. It involves crossing beams from a laser system to stimulate a Stokes and a probe beam, and the resulting CARS signal is measured. This technique can provide temperature measurements in a single pulse, making it suitable for studying turbulent flames and obtaining instantaneous temperature distributions.

How can molecular beam mass spectrometry be used to study the composition of gas-phase species in flames?

Molecular beam mass spectrometry extracts and cools gas-phase species from a flame before ionizing them and obtaining a mass spectrum. This technique allows for the selective ionization of specific species and provides detailed information about the composition of the gas phase in the flame.

What is the difference between aerosol mass spectrometry (AMS) and molecular beam mass spectrometry for studying flames?

Aerosol mass spectrometry (AMS) focuses on extracting and analyzing particles, whereas molecular beam mass spectrometry extracts and analyzes the gas phase. AMS provides information about the particle phase, including the composition of aerosols, while molecular beam mass spectrometry gives insights into the gas-phase composition.

How do atmospheric scientists categorize different types of carbon particles?

Atmospheric scientists categorize carbon particles as organic carbon (OC), refractory organic carbon, and elemental carbon (EC). Elemental carbon is similar to mature soot, while organic carbon includes less mature forms like incipient soot and brown carbon, which is colored organic carbon that is not fully carbonaceous.

What are the main sources of black carbon and how do they differ from brown carbon?

Black carbon mainly comes from anthropogenic sources like diesel engines, deforestation, and heating/cooking with solid biomass, especially in developing countries. Brown carbon, on the other hand, is often associated with wildfires and peat bog fires, which are not considered anthropogenic. Brown carbon is less mature and has a brownish color.

What is the significance of tar balls in the context of wildfires and their impact on the atmosphere?

Tar balls are large organic spheres that condense from the emissions of biomass during wildfires. They are significant because they are thought to have a substantial climate effect due to their strong absorption capabilities. Tar balls are also considered less mature particles, which can affect how they interact with the atmosphere.

What is the difference between flaming and smoldering fires in terms of particle generation?

Flaming fires involve a flame front and high temperatures, leading to the generation of mature soot particles. Smoldering fires, on the other hand, involve surface oxidation without a flame front and do not generate mature soot. Instead, they produce a large amount of less mature particles and pollutants.

Why are inverted burners not ideal for simulating natural flames in atmospheric studies?

Inverted burners create an environment where particles are exposed to high temperatures for extended periods, leading to the formation of very mature and large particles. This is not representative of natural flames, such as wildfires, where particles are not typically as mature or large. Therefore, using regular diffusion flames is recommended for more accurate simulations.

What is the term used to describe the interface between wildland areas and urban areas, and why is it a concern?

The term is 'Wildland Urban Interface' (WUI). It is a concern because it represents areas where wildfires can easily spread from wildland areas into urban areas, posing a significant risk to property and lives due to the potential for rapid and uncontrollable fire spread.

How have trends in wildfires in the western U.S. been changing over the years?

Trends in the western U.S. show a dramatic increase in both the frequency and size of wildfires over the years. This is largely attributed to climate change, which results in drier conditions that fuel larger and more intense fires.

What is the significance of the dispersion exponent or angstrom exponent in the study of atmospheric aerosols?

The dispersion exponent, or angstrom exponent, is used to understand the size distribution and evolution of particles in the atmosphere. A high dispersion exponent can indicate the presence of less mature particles, often associated with smoldering fires, and is used as a tracer for biomass emissions.

What is the difference between aggregates and agglomerates in the context of particle formation in flames?

Aggregates refer to particles that are tightly bound together, while agglomerates are particles that are not tightly bound and can fall apart. Aggregates are typically formed in high-concentration environments where particles clump together, such as in forest fires, leading to the formation of super-aggregates.