The Atmosphere

Composition and Structure

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

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)

• 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

• Lecture Question– What are the four most abundant components of dry air at sea

level?

– Nitrogen (N2), oxygen (O2), argon (Ar), carbon dioxide (CO2)

Atmospheric CompositionChemical 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 radiogenic

Kr 1 ppmv radiogenic

CO 50 – 200 ppmv biogenic, anthropogenic, photochemical

CH4 1.7 ppmv biogenic, anthropogenic

NMHC 5 – 20 ppbv biogenic, anthropogenic, photochemical

CH2O 0.1 ppbv photochemical

N2O 310 ppbv biogenic, anthropogenic

NH3 0 – 0.5 ppbv biogenic, anthropogenic

NOx 0 – 0.5 ppbv biogenic, anthropogenic, lightning

OCS 0.5 ppbv volcanic, biogenic, anthropogenic

H2S 0 – 0.5 ppbv biogenic, anthropogenic

SO2 0.01 – 1 ppbv volcanic, anthropogenic, photochemical

DMS 0.01 – 0.1 ppbv biogenic

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

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 = 0.0375% by volume.

– According to NOAA (Jan 2007):• Current average CO2 concentration (at Mauna Loa) is 383 ppm

• 383 ppm = 0.0383% by volume.

Measures of Concentration

• Absolute concentration– Typical units

• Mass/volume (eg g/L)

• Number density (cm-3), particularly for low concs

• Pressure units (torr, atm, bar, etc)

z , km P z /P 0 (= rz / r 0 ) 1 - P z /P 0 (%)

0 1.000 0.0%10 0.259 74.1%15 0.132 86.8%25 0.034 96.6%50 0.001 99.9%100 0.000 100.0%

/0 7.4 kmat 250Kz H

za

RTP Pe H

M g

Note: diameter of the Earth is 6400 km.

Particular Matter Concentration

Background PM: 300/cm3

PM Mass Concentration

Background PM: 1 g/m3

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

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

Time Scales of Vertical Mixing

Time Scales of Horizontal Mixing

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)

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)

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?

c

Light: Electromagnetic Radiation• Lecture Question

– What is the relationship between photon energy, wavelength, and frequency?

hcE h

• Photon energy– Is the minimum energy available in its interaction with matter

The Electromagnetic Spectrum

increasing energy

decreasing energy

The Electromagnetic Spectrum

• Can further subdivide the UV region– UV-C is 200-280 nm (most energetic, completely blocked by ozone layer)– UV-B is 280-320 nm (ground-level UV-B intensity most affected by ozone depletion)– UV-A is 320-280 nm (least energetic, almost all gets through the ozone layer)

least energetic portion of solarradiation; absorbed by greenhousegases

770 nm – 100 minfrared (ir)

atmosphere is mostly transparent tovisible light

380 – 770 nmvisible (vis)

only partially blocked by theatmosphere

200 – 380 nmultraviolet (uv)

most energetic portion of solarradiation; absorbed completely by theupper atmosphere

10 – 200 nmfar ultraviolet (“vacuum uv”)

CommentsWavelength RangeSpectral Region

Incident SunlightTop & Bottom of Atmosphere

Lecture Question

What fraction of incident sunlight is in the uv, visible and ir regions?

• 8% ultraviolet• 39% visible• 53% infrared