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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

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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

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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

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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

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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:

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E.g.,Number, surface, and volume distributions for a typical urban model aerosol (from Finlayson-Pitts & Pitts) Note the modes in these distributions!

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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

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From Finlayson-Pitts & Pitts

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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.)

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Formation of sea spray aerosol: