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

Influence on atmospheric chemistry Impact on health Can absorb light (e.g., black smoke) Form cloud condensation nuclei 1. INTRODUCTION Affect earth’s 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

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 – 100 μm 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

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 ρ p, ρ 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 particle’s residence time in air 2. PHYSICAL PROPERTIES

Parametery-axis of graphx-axis Number: ΔN / Δlog D log D Surface area:ΔS / Δlog D log D Volume:ΔV / Δlog Dlog D x-axis is log D to show large size range E.g., to 10 2 μm y-axis is normalised by the size interval (Δlog 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:

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

coarse (> 2.5 μm) fine (< 2.5 μm) 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 μm:accumulation range 0.01 to 0.08 μm:transient or Aitken nuclei range < 0.01 μm:ultrafine particles

From Finlayson-Pitts & Pitts

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.)

Formation of sea spray aerosol: