1. Composition and Structure
The Atmosphere
Composition and Structure

2. Outline Intro to air pollution Atmospheric Composition
Measures of concentration
Concentration of gases
Concentration and composition of PM
Structure of the Atmosphere
Thermal stratification
Characteristic vertical and horizontal mixing times
Spatial variability of atmospheric composition
Light
Nature of light
Interaction of light and matter
Sunlight and its propagation through the atmosphere

3. Problems due to Air Pollution
Question
What are the major problems due to chemicals discharged into the atmosphere? List them.
Stratospheric ozone depletion (due to CFCs, HCFCs, etc)
Global climate change (due to GHGs, etc)
Acid deposition (SO2, NOx)
Smog (VOCs, NOx)
Particulates (PM, especially “fine PM”)
Other toxic air pollutants (eg, CO, Pb, Hg, PAHs and other toxic organics, etc)

4. Lecture Question What are the criteria pollutants?
Carbon monoxide, CO
Nitrogen dioxide, NO2
Ozone, O3
Lead, Pb
Particulates, PM10 and PM2.5
Sulfur dioxide, SO2
What are the four most abundant components of dry air at sea level?
Nitrogen (N2), oxygen (O2), argon (Ar), carbon dioxide (CO2)

5. Atmospheric Composition
Chemical Species
Concentration
Source N2
78.08%
volcanic, biogenic O2
20.95%
biogenic
H2O (gaseous)
up to 4% (avg ~2.5%)
volcanic, evaporation Ar
0.93%
radiogenic
CO2
0.037% (370 ppmv)
volcanic, biogenic, anthropogenic Ne
18 ppmv
volcanic (possibly) He
5.2 ppmv Kr
1 ppmv CO
50 – 200 ppmv
biogenic, anthropogenic, photochemical
CH4
1.7 ppmv
biogenic, anthropogenic
NMHC
5 – 20 ppbv
CH2O
0.1 ppbv
photochemical
N2O
310 ppbv
NH3
0 – 0.5 ppbv
NOx
biogenic, anthropogenic, lightning
OCS
0.5 ppbv
H2S
SO2
0.01 – 1 ppbv
volcanic, anthropogenic, photochemical
DMS
0.01 – 0.1 ppbv
all concentrations are in dry air (except, obviously, for H2O)

6. Measures of Concentration
Relative concentration: volume mixing ratio (VMR)
What is it? The mole (or volume) fraction.
Units: %, ppmv, ppbv, etc
Easy to understand
Constant with altitude for inert gases like N2

7. Measures of Concentration
Lecture Question
List the current atmospheric concentration of CO2 in air in units of (a) % and (b) ppm.
Answer in book:
375 ppm = % by volume.
According to NOAA (Jan 2007):
Current average CO2 concentration (at Mauna Loa) is 383 ppm
383 ppm = % by volume.

8. Measures of Concentration
Absolute concentration
Typical units
Mass/volume (eg mg/L)
Number density (cm-3), particularly for low concs
Pressure units (torr, atm, bar, etc)
Note: diameter of the Earth is 6400 km.

9. Particular Matter Concentration
Background PM: 300/cm3

10. PM Mass Concentration Background PM: 1 mg/m3
Typical rural conc: 5 ug/m3
Typical urban conc: 30 ug/m3
Polluted urban conc: 100 ug/m3

11. Thermal Stratification of the Atmosphere
Lecture Question
List the major regions (layers) of the atmosphere, along with the typical altitudes for each region.
Troposphere (0 – 15km)
Upper limit (the tropopause) varies between 9 – 16 km depending on lattitude and season
Stratosphere (15 – 50 km)
Contains the stratospheric ozone layer, which (mostly) shields us from harmful uv light
Mesosphere (50 – 100 km)
Thermosphere (above 100 km)
Above 60 km is the ionosphere, where there is a significant concentration of ions and electrons

12. Thermal Stratification of the Atmosphere
Troposphere is heated by the ground
Stratosphere and mesosphere are directly heated by ozone chemistry (solar energy)
Tropopause varies in height (9–16 km) depending on latitude and season
Vertical mixing in troposphere is rapid, but stratosphere is fairly stagnant
Lower 1 – 3 km called the planetary boundary layer (PBL), which is rapidly mixed and often topped by a local inversion
the tropopause is defined with the atmospheric lapse rate falls below 2 K/km.
the thermosphere is heated mostly by absorption of O2. Temps are high because few atoms/molecules to spread the thermal energy around. Not in thermodynamic equilibrium.
the homosphere includes the troposphere, stratosphere & mesosphere; it is the region where bulk mixing is complete, where molecular diffusion is not significant. Above the homosphere is the heterosphere, where there is some separation due to mass (ie heavier molecules settle more than ligher ones). The region above 500 km is the exosphere, where escape into space is significant for the lighter atmospheric consituents (eg hydrogen and helium.
Above 60 km is the ionosphere, the region of the atmosphere above which there is some significant concentration of ions and electrons. It is further subdivided into D, E and F layers. It enables bouncing radio waves around the Earth’s curved surface.

13. Time Scales of Vertical Mixing
Vertical mixing is due mostly to adiabatic heating/cooling of air parcels, as well as turbulent mixing from horizontal winds.

14. Time Scales of Horizontal Mixing
Horizontal mixing is driven by pressure differences due to uneven heating by the Sun. MIxing along latitutdes (zonal mixing) is very fast; mixing along longitudes within a hemisphere (meridional mixing) is also pretty fast. Interhemispheric exchange is much slower.

15. Mixing vs Removal Two competing processes
Effect on atmospheric composition
Depends on relative rates of mixing and removal
Species with higher rates of removal are more concentrated near their sources
Removal mechanisms
Water scavenging
Dissolution into suspended water droplet (or water bodies on ground)
Rate depends on solubility
Reaction/decomposition
Rxn with a reactive species (often OH)
Photodissociation: absorption of light, followed by breaking of bond
Adsorption to solid surface
Either PM or surface on the ground
Mixing mechanisms
Buoyant mixing (vertical)
Atmospheric circulation – ie, wind (horizontal)

16. Light: Electromagnetic Radiation
Plays a critical role
The vast majority of our energy arrives as sunlight
Drives global circulation of atmosphere and water
Many air pollution problems directly involve light
Ozone depletion
Global climate change
Photochemical smog
Drives atmospheric chemistry
Through photodissociation
Through generation of reactive species (OH, Cl, NO3, O3)

17. Light: Electromagnetic Radiation
What is light?
Light acts like a wave
With oscillating electric and magnetic fields
Can propagate through a medium or through vacuum
animation here
Light also acts like a particle
Localized energy packet: a photon
Massless...but has momentum
Lecture Question
What is the relationship between wavelength, frequency, and the speed of propagation?

18. Light: Electromagnetic Radiation
Lecture Question
What is the relationship between photon energy, wavelength, and frequency?
Photon energy
Is the minimum energy available in its interaction with matter

19. The Electromagnetic Spectrum
increasing energy
decreasing energy

20. The Electromagnetic Spectrum
Can further subdivide the UV region
UV-C is nm (most energetic, completely blocked by ozone layer)
UV-B is nm (ground-level UV-B intensity most affected by ozone depletion)
UV-A is nm (least energetic, almost all gets through the ozone layer)

21. Incident Sunlight Top & Bottom of Atmosphere
Lecture Question
What fraction of incident sunlight is in the uv, visible and ir regions?
8% ultraviolet
39% visible
53% infrared