Measurement of atmospheric aerosol

GS/CHEM 5740 3.00 Selected Topics in Atmospheric Chemistry

The chemistry and physical chemistry of suspended particulate matter in the

atmosphere

GS/CHEM 5740 3.00 Selected Topics in Atmospheric Chemistry

• Lecturers: J. Rudolph, M. Mozurkevich

• Office: J. Rudolph : Petrie Room 302

• Phone: J. Rudolph : 416 650 8117

• Email: Rudolphj@yorku.ca

• Web page (lecture notes):J. Rudolph : http://www.yorku.ca/rudolphj/

• Lecture location: SC 205, M&W at 5:30 pm

• Office hours (tentative): J. Rudolph : Wednesday 1 p.m.-3 p.m.

Important Dates:• Classes: Sept 06 - Dec 04 (24 classes)

• Final Exams: Dec 06 – Dec 20

• Tests: Sept 18, Oct.4, Oct 18, further tests may be announced

Posted course summary

• Specialized topics relevant to the chemistry of the atmosphere are presented. Course content necessarily changes from year to year. Sample topics include laboratory kinetic measurements relevant to atmospheric processes; modern methods for the determination of atmospheric composition; visualisation and analysis tools for the interpretation of field and modelled data; and the use of stable isotope analysis in atmospheric chemistry.

Expanded Course DescriptionPart I (Professor Rudolph)

• Introduction to aerosols including some descriptive information about observations: Range of concentrations as function of size, overview of chemical composition

• Typical concentrations and composition for different regions of the troposphere

• Sources of aerosols

• Relation between chemical composition, size and origin

• Bulk chemical analysis of PM including sampling

• Gas phase processes resulting in the formation of non-volatile material

• Mid-term exam

Expanded Course DescriptionPart II (Professor Mozurkewich)

Introduction to aerosol physics. Effect of pressure on the dynamics of particles (continuum, transition, and kinetic regimes).Sedimentation, impaction, Brownian diffusion.

The evolution of aerosol size distributions. Kinetics of gas-to-particle conversion. Thermodynamics of surfaces and the origin of the barrier to nucleation. Growth of particles by condensation and coagulation. Transport and deposition of particles. The General Dynamic Equation.

Aerosol instruments. Physical methods of measuring particle concentrations and size distributions (differential mobility

analysers, condensation nucleus counters).Optical methods of measuring particle concentrations and size distributions. The analysis of individual aerosol particles.

Final exam

Course Learning ObjectivesThe overall objective of the course is to provide students with the

knowledge necessary to understand the processes determining the concentrations of atmospheric particulate matter (PM) and state of the art techniques to analyze atmospheric PM.

Specifically the students will learn:

• - To interpret PM concentrations and their changes in the context of chemical and physical processes.

• - To establish relations between chemical composition of PM and the origin of PM

• - To quantitatively describe the physical processes determining the size distributions of PM

• - To understand the basic principles, potentials, and limitations of state of the art PM analysis techniques.

Grade EvaluationTests (tentative Sept 18, Oct.4)/quizzes (part I) 25%

Mid-term quiz: ( Tentative: October 18) 25%

Final exam (Exam period Dec 06 – Dec 20) 30%

Assignment (Part II) 20%

Further important information:• All students are expected to familiarize themselves with the following

information, available on the Senate Committee on Curriculum & Academic Standards webpage (see Reports, Initiatives, Documents) -http://www.yorku.ca/secretariat/senate_cte_main_pages/ccas.htm

• York’s Academic Honesty Policy and Procedures/Academic IntegrityWebsite

• Ethics Review Process for research involving human participants

• Course requirement accommodation for students with disabilities, including physical, medical, systemic, learning and psychiatric disabilities

• Student Conduct Standards

• Religious Observance Accommodation

Bibliography • B.J. Finnlayson–Pitts and J.N. Pitts, Chemistry of the upper and lower atmosphere,

Academic Press, 2000• N. A. Fuchs, The mechanics of aerosols, New York : Macmillan, 1964.• N. A. Fuchs and A. G. Sutugin, Highly dispersed aerosols, Ann Arbor: Ann Arbor

Science Publishers, 1970.• Finnlayson –Pitts and Pitts, Chemistry of the upper and lower atmosphere• Academic Press, 2000• Seinfeld and Pandis, Atmospheric Chemistry and Physics, • Wiley and Sons, 1998• Analytical Chemistry of Aerosols, K. R. Spurny, Lewis Publishers, 1999• Analytical Techniques for Atmospheric Measurements, D.E. Heard, Blackwell

Publishers, 2006.• S. Twomey, Atmospheric aerosols, New York : Elsevier, 1977.• Any modern physical chemistry text. (or, see: D. Tabor, Gases, liquids, and solids:

and other states of matter, New York : Cambridge University Press, 1991)

Today

• Why are we interested in atmospheric PM (Aerosols)

• Some definitions• Size of atmospheric PM • Possible ways to present the size

distributions of atmospheric PM

Why do we study atmospheric PM?

• Atmospheric PM is arguably one of the areas of Atmospheric Chemistry where our present understanding is far from being complete.

=> Valid and interesting research area

Atmospheric particles also have substantial impact on important issues:

• They can have a negative impact on health• They are important for the optical poperties of

the troposphere (visibility, radiation budget)• They act as cloud condensation nuclei• They contribute to the import of material into

many ecosystems (nutrients, pollutants, sediments, soil)

• They impact on surfaces exposed to air (corrosion, dust)

Why do atmospheric PM present such a complex problem:

• There is a large variety of sources emitting PM with different composition and size distribution

• The loss processes for PM consist of different physical and chemical mechanisms

• Atmospheric particles undergo chemical and physical conversion processes

• PM is formed in the atmosphere by chemical processes (gas to particle conversion).

Definitions:

• Aerosol: Suspended particles in a gas (air).

• Strictly speaking aerosol includes PM and air.

• Particle concentrations in aerosols are usually expressed in the form of number densities (particles per volume: Particles cm-3 or cm-3).

Particle size: Usually refers to the aerodynamic diameter (Da).

• Defined as size of a spherical particle with a density (ρ0) of unity (1g/cm3) which has the same aerodynamic properties (e.g. falling speed in the atmosphere) as the particle under consideration.

• The relation between the geometric diameter (Dg) and Dais:

ρp is the density of the particle and k a shape factor, which is characteristic for the particle under consideration.

0

pa gD D k

ρρ

= ⋅ ⋅

Particle numbers in the troposphere cover a wide range from less then 100 cm-3 in the free troposphere to several 105 cm-3 in urban areas.

But PM is not only characterized by the number of particles since the particles usually cover a wide size range.

Simplified description of particle size distributions.

Based on observations (which can be understood from general principles of PM generation and loss) a simple (but not perfect) mathematical description of the dependence between number density and diameter can be given using a Gaussian distribution (error function) with a logarithmic diameter axis.

( )( )

2

2

ln lnexp

ln 2 ln 2 lngNT

g g

D DNdNd D π σ σ

⎛ ⎞−⎜ ⎟= ⋅ −⎜ ⎟⋅ ⋅ ⋅⎝ ⎠

NT is the total number of particles, σg the width of the distribution (in ln diameter scale) and DgN the geometric number (weighted) mean diameter:

( )1ln ln

1ln ln

gNT

gN j jjT

D n d d dN

D n dN

= ⋅ ⋅

= ⋅ ⋅

∫

∑

• A distribution, which can be approximated by such a function is called “mode”.

• Very often the number density-size distributions of atmospheric PM (particle size distribution) consists of several modes.

• Each of these modes is characterized by three parameters: NT, DgN, and σg.

Typically three modes can be distinguished:

• Fine mode (Aitken Nuclei)

• Accumulation mode

• Coarse mode

Number, surface and mass distributions

Presenting (or measuring) PM distributions as function of different properties such as number count, surface area or mass (volume) changes the appearance of the graphs. It can change the weight given to different modes, but also changes the position of the modes. (Dg).

Problem to solve (will be discussed next week):

The following measurement was made for the dependence between number density and particle size.

1) Determine NT, DgN, and σg

2) Convert this number density distribution into a surface and a volume distribution. Assume a particle density of 1 and spherical particles.

3) DgS, and σs, DgM, and σM, as well as total surface and total mass.

Finnlayson Pitts, pages 359-361, understajnd the difference between various way of averaging (<F(x)>≠F(<x>) if F is a non-linear function)

∆N/∆lnDa, cm-3 Da, nm

3 2.5

43 5

353 10

1816 20

5772 40

11347 80

13798 160

10376 320

4826 640

1389 1280

247 2560

27 5120

2 10240

Next lecture

• Discussion of problem• Some general information on PM• Overview of processes determining PM

size distribution• Introduction to chemical composition of

PM