Atmospheric Aerosols: Investigating and Characterizing the Hygroscopicity of Nanoparticles

By: Alexander GirauAdvisor: Dr. Joelle S. Underwood

What are atmospheric aerosols?•Aerosols are liquid or solid particles or combinations

suspended in gas such as dust, mist, or fumes. Atmospheric aerosol nanoparticles are defined as particles with aerodynamic diameters of 100 nm (0.1mm) and less.

10mm

2.5mm

0.1mm

Aerosol particle of interest

Width of human hair

What are atmospheric aerosols?• They are a complex and dynamic

mixture of solid and liquid particles from biogenic and anthropogenic sources.

• Biogenic aerosols: ▫ Sea foam (blue)▫ Biological debris (dark green)▫ Volcanic dust (bright red)

• Anthropogenic aerosols:▫ Industrial dust (grey)▫ Soot carbon (black)▫ Biomass combustion (dark red)

Where are they found???

They are primarily found in the troposphere which extends 16 km above sea level. Satellites have an elevation of ~ 650 km

Troposphere

Stratosphere

Mesosphere

Thermosphere

Exosphere

Ozone Shield

Atmosphere

Why to study Atmospheric Aerosols?

• Atmospheric aerosols act as micro chemical reaction vessels.

• They help in the transportation of non-volatile material; they play an important role in the formation of clouds and have a profound effect on the earth’s climate.

• Yet the ability of these particles to attract water molecules is not well understood.

Water Uptake Character = Hygroscopicity• Water uptake studies investigate the role of chemical

content and particle size on the water uptake process.• The hygroscopicity of a particle is defined as its water

uptake characterization.• This study attempts to explain water uptake phenomena

involving atmospheric aerosols.• Particles of interest then must be able to interact with

water; materials with a strong affinity for absorbing moisture from the atmosphere are called deliquescent materials.

• Deliquescent materials allow us to study a particle’s transition from the solid phase to the aqueous phase at a specific relative humidity (RH)

Water Uptake by Nanoparticles• RH is responsible for the

deposition of water molecules as a film to the surface of a crystalline particle.

• As RH is increased, more molecules of water adsorb to the surface.

• When RH reaches a threshold, the film of water becomes thick enough to promote a phase transition from solid to aqueous.

• This transition is known as deliquescence and occurs at the deliquescence relative humidity (DRH)

• Growth factor (GF) curves quantify particle hygroscopicity

• GF is a ratio measuring the particle’s mobility diameter at a specific RH to that of a dry particle.

Why do we study Deliquescence?• We are interested in particle deliquescence because we want

to understand:▫ How this interaction changes, depending on the chemical

content and the size of the nanoparticle of interest, ▫ If chemical and photochemical properties of the particle

change as more water is adsorbed on the surface of particle

▫ Ex. Does a 50 nm NaCl particle deliquesce differently than a 50 nm KCl particle ? And if it does, what effects can we generalize that this will have on things like cloud formation???

• The value of understanding elementary molecules serves as the basis for possibly understanding more complex molecules such as greenhouse emissions.

Governing Principle of Water Uptake Studies• Particles having diameters <100 nm are ubiquitous and

abundant precursors to the larger aerosols that influence global climate.

• Surface energy of nanoparticles provides a significant contribution to their overall free energy. As a result, DRH and ERH can significantly change.

• Due to the Kelvin Effect the hygroscopic growth of atmospheric nanoparticles is expected to be lower compared to that of their larger counterpart

• Thus, the Kelvin Effect accounts for size dependence of deliquescence RH observed for particles with diameter <100 nm.

Governing Principle of Water Uptake Studies

Bulk Solution, virtually infinite surface

Flat Surface, exhibited by a larger particle

Curve Surface, exhibited by smaller particle

Physical Interpretation of Kelvin Effect: larger particles exhibit flatter surface and thus are more thermodynamically favorable to adsorb water films than smaller particles that exhibit a curves surface.

>100 nm particles

< 100 nm particle

Experimental Apparatus• Our experimental

apparatus consist of 3 in-house systems:▫ 1. Electrospray▫ 2. Particle Conditioning▫ 3. Particle Sizing

1.

2.

3.

Summary of Experimental Apparatus

• 1. Electrospray: polydispersed particles will be generated and then size selected depending on objective of experiment

• 2. Particle Conditioning: size selected nanoparticles will be exposed to a specified RH, promoting deliquescence.

• 3. Particle Sizing: the number size distribution of particles will be measured post conditioning. It is at this step that the growth factor can be determined to characterize the hygroscopicity of the molecule of interest.

Ultrafine Particle Generation• The ultrafine particles required to study deliquescence

are generated through an in-house electrospray.

Electrospray Setup

•A syringe pump, feeds a solution of particles of interest.•A positively charged high voltage (+HV) is applied to the syringe, causing highly charges particle to shoot off towards a static eliminator. •These particle are then introduced into a optional differential mobility analyzer (DMA) to promote a size selection step.

DMA-1

Ultrafine Particle GenerationElectrospray apparatus generates particles in a

polydispersed fashion. Thus a Boltzmann distribution is measured for the multifarious particles created.

•Particle diameters < 50 nm and geometric standard deviations < 1.2 are readily achieved.

•Use of an optional DMA narrows the distribution and provides additional stability.

Optional: DMA-1• A simple description of the

DMA is that the instrument is just two charged concentric cylinders with an inlet slit and a sampling slit.

• The DMA separates particles based on their electrical mobility.

• Aerosol particles for sizing are inserted into the annular region between the two cylinders at the inlet slit; they are carried by a constant flow of purified air to maintain a consistent laminar (non turbulent) flow.

(Aerosol Generated from Electrospray)

(Clean Air )

+HV

Optional: DMA-1• Particles with mobilities in

a certain narrow range are sampled at the sampling slit.

• the sizing chosen depends on certain parameters such as the applied voltage, the sheath flow rates, etc.

• The path of aerosol particles within the DMA is based on a stream function, thus a consistent flow is required to allow correct sizing by electrical charging.

(Aerosol Generated from Electrospray)

(Clean Air )

+HV Polydispersed aerosols

Unselected aerosol

Selected aerosol

Analysis of DMA-1 Selection• Figure 1. The result of a

properly selective DMA. As the voltage is increased the center of the distribution is shifted towards a larger diameter.

• Figure 2. Analysis of uncertainty associated with the effects of voltage applied size selection. As the voltage is ramped, the confidence interval is increased. This is physically depicted in the broadening of the peaks in Figure 1.

Figure 1.

Figure 2.

Particle ConditioningAfter the particle size of interest has been properly selected

it then proceeds to the particle conditioning process.

•Water Uptake Region: the region is kept a predetermined RH to expose the selected particle of interest.

•Sheathe Air Conditioning: the sheath air required to facilitate laminar flow through out the system is kept at a minimum RH to keep air as dry as possible.

Particle Sizing by DMA-2 and CPC• The particle sizing step consist

of the tandem DMA-2 and condensation particle counter (CPC).

• DMA -2 in conjunction with the CPC measures the number size distribution of the particles.

• The DMA-2 is responsible for measuring the change in particle diameter.

• Once monodispersed aerosols are exposed to the RH in the particle conditioning step, the number size distribution of the water uptake process was measured by the DMA-2 and the CPC.

(conditioned monodispersed aerosols)

Particle Sizing by DMA-2 and CPC• Figure 3. Analysis of the

number size distribution of particles at selected sizes.

• The CPC measures the presence of particles as a Boltzmann distribution. As the voltage is ramped to select larger particles, the CPC registers a decrease in the numbers concentration.

Figure 3.

What does all this data mean?• Soluble, crystalline particles deliquesce at well-defined

RH.

• Deliquescence RH and ensuing hygroscopic growth at higher RH are sensitive to both particle chemical content and particle size.

• Kelvin effect accounts for size dependence of deliquescence RH observed for particles with diameter <100 nm

Ongoing Work• The role of chemical content and particle size are being

studied for a variety of mixed salt and carboxylic acid particles.

• Modeling studies are underway to develop a better understanding of the relative importance of size and content

Acknowledgements• Dr. Joelle S. Underwood• Hunter Fontenot• Brian Hays• Elizabeth Gosciniak