Air Pollution- Particles in the Air

1. Air Pollution- Particles in the Air

2. Air quality and particles • Epidemiological (流行病学) evidence associates atmospheric particles with diseases and mortality. • Particles penetrate the lungs, blocking and irritating air passages. • Particles themselves could exert toxic effects. (toxic substances present in the particles) [Polycyclic Aromatic Hydrocarbons are examples of toxic substances in aerosols.]

4. The larger a particle is, the more likely it is deposited into the upper part of the respiratory system. The smaller a particle is, the deeper it can get into the respiratory system. Particle deposition as a function of particle diameters in various regions of the lung.

5. Terms • TSP: Total suspended particulate matter • RSP: Respirable suspended particulate matter • PM10: Particulate matter of less than 10 um; equivalent to RSP. • PM2.5: Particulate matter of less than 2.5 um

6. Air quality standards for particles • US • 15 ug/m3 (annual average) for PM 2.5 um • 65 ug/m3 (daily average) for PM 2.5 um • Hong Kong • RSP: 55 ug/m3 (annual), 180 ug/m3 (daily) • TSP: 80 ug/m3 (annual), 260 ug/m3 (daily)

7. Visibility and Aerosols In a clear sky without any particles, one can see as far as 100-300 km.

8. Physics of visibility • Visibility depends on the transmission of light through the atmosphere and the ability of the eye to distinguish an object because it contrasts with the background. • A change in contrast with viewing distance occurs for both dark, light and bright objects. • With dark objects, the atmosphere introduces light, called "air light", to the sight path, and the dark object appears lighter at increasing distance until it bleeds into the background at the horizon. • With light objects, light is lost from the sight path with increasing viewing distance.

9. In both cases, the result is the same— The contrast between the object and the background approaches zero when the light coming along the line of sight from the object approaches the intensity of the light from the background at the horizon. When the eye can no longer distinguish the difference, the contrast, between the object and the background, the object can not be seen, and it is said to be beyond the limit of visibility.

11. The mathematic expression for the extinction of contrast: I =Io exp[-bx] b: scattering coefficient or absorption coefficient, x: distance I: light intensity or contrast. b = bsg + bsp + bag + bap bsg and bsp are extinction coeff. caused by light scattering due to gas and particles respectively. bsp = bsdp + bsw are broken into contributions due to dry particles and water associated with particles. bag and bap are extinction coeff. caused by light absorption due to gas and particles respectively.

12. Visibility reduction due to particles is size-dependent Particle Scattering and absorption cross-section per unit volume Particles of 0.1-1.0 um size are efficient light-scattering centers.

13. Fine aerosol particles are a major contributor to visibility impairment (Blue Ridge Mountain area in the U.S.)

14. Measurement of visibility • Human observations • Optical measurements Measured by a integrating nephelometer Visual range

15. Study questions • How do particles cause adverse effect on human health? • How do particles affect visibility?