Particles in the Atmosphere CONTENTS 1. Introduction 2. Physical properties 3. Particle formation...
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- Slide 1
- Particles in the Atmosphere CONTENTS 1. Introduction 2.
Physical properties 3. Particle formation and growth 4. Chemical
composition 5. Radiative properties and climate change 6. Analysis
and laboratory generation of particles References Finlayson-Pitts
and Pitts: Chemistry of the Upper and Lower Atmosphere Wayne,
Chemistry of Atmospheres
- Slide 2
- Influence on atmospheric chemistry Impact on health Can absorb
light (e.g., black smoke) Form cloud condensation nuclei 1.
INTRODUCTION Affect earths radiative balance What is the importance
of atmospheric particles? Reduce visibility through scattering We
want to know: Sources of particles Influence on atmospheric
chemistry Fate in the atmosphere Impact of physical and chemical
properties on health Particles are difficult to study large gaps in
our knowledge
- Slide 3
- Aerosol = relatively stable suspension of solid or liquid
particles in a gas (Often we use aerosol to imply the particles
only) Types of aerosols Size range from 0.002 100 m Primary emitted
directly to the atmosphere Either solid or liquid particles
Important properties: number concentration mass size chemical
composition aerodynamic properties optical properties Secondary
formed through reactions in the atmosphere
- Slide 4
- aerodynamic diameter, D a : Diameter of a sphere of unit
density (= 1 g cm -3 ) with the same terminal velocity in air as
the particle being considered D g :geometrical diameter of
reference particle, k:a shape factor p, 0 :density of the particle
and the reference density 2.1 PARTICLE SIZE Diameter? What about
non-spherical particles? Use an equivalent or effective diameter
depends on physical (not geometrical) properties Particle density
< bulk density (owing to cracks, voids, pores, etc.) Aerodynamic
diameter useful because related to particles residence time in air
2. PHYSICAL PROPERTIES
- Slide 5
- Parametery-axis of graphx-axis Number: N / log D log D Surface
area:S / log D log D Volume:V / log Dlog D x-axis is log D to show
large size range E.g., 10 -3 to 10 2 m y-axis is normalised by the
size interval (log D) Size distribution Particles always present in
significant numbers up to 10 8 cm -3 Size is an important property
characterise particle sizes in a size distribution Look at
distribution either by number, surface area, or volume of particles
in different size range:
- Slide 6
- E.g.,Number, surface, and volume distributions for a typical
urban model aerosol (from Finlayson-Pitts & Pitts) Note the
modes in these distributions!
- Slide 7
- coarse (> 2.5 m) fine (< 2.5 m) Particle size categories
Atmospheric aerosols occur is different size groupings that are
different in their properties and origin: Fine particles can be
further subdivided: 0.08 to 1-2 m:accumulation range 0.01 to 0.08
m:transient or Aitken nuclei range < 0.01 m:ultrafine
particles
- Slide 8
- From Finlayson-Pitts & Pitts
- Slide 9
- Large sedimentation occurs Chemical composition reflects source
mainly sand, sea salt in this range Can be transported large
distances E.g., Saharan dust storms May be important in NO x and SO
2 heterogeneous chemistry Coarse particles: Usually produced by
mechanical processes (grinding, erosion, etc.) or from biological
sources (pollen, spores, etc.)
- Slide 10
- Formation of sea spray aerosol: